A TEXT-BOOK OF PAPER-MAKING; by C. F. CROSS AND E. J. BEVAN (1888)

INTRODUCTORY NOTE.

The raw materials of the paper-maker are primarily the vegetable fibrous substances; in addition to these there are various articles which are employed as auxiliaries, either in the preparatory or finishing processes to which these fibrous materials, or the web of paper are subjected. The latter class are of subsidiary importance, more especially from our present point of view.

In insisting upon the recognition of first principles, we cannot overrate the importance of a thorough grasp of the constitution of the plant fibres, as the necessary foundation for the intelligent conduct of paper-making, and to this subject we will at once proceed.

Careful study of a mature plant will show that it is made up of structural elements of two kinds, viz. fibres and cells, which, to use a rough parallel, we may liken in function to the bricks and mortar of a house. It is the former which admit of the many extended uses, with which we are familiar, in the arts of spinning and weaving, and which constitute the fabrics which are the most indispensable to our civilised life. For the most part, as we know, fibres and cells are aggregated together into compound tissues, and a process of separation is therefore a necessary preliminary to the utilisation of the former. The cotton fibre is the only important exception to this general condition of distribution. Here we have the seed envelope or perisperm, converted into a mass {2} of fibres, and these by a spontaneous process accompanying the ripening, so isolated as to be immediately available. Next in order in point of simplicity of isolation, are those fibrous masses, or tissues, which, although components of complex structures, exhibit a greater cohesion of their constituent fibres than adhesion to the contiguous cellular tissues with which they go to build up the plant. Into such a tissue the “bast,” or inner bark layer of shrubs and trees, more especially those of tropical and sub-tropical regions, frequently develops, and it is, in fact, this bast tissue, graduating in respect of cohesion of its constituent fibres, from a close network such as we have spoken of, to a collection of individual fibres or fibre-bundles disposed in parallel series, which supplies the greater part of the more valuable of the textile and paper-making fibres; we may instance flax, hemp, and jute, each of which is the basis of an enormous industry. According to the degree of adhesion of the bast to the contiguous tissues, or, in another aspect, according to its lesser aggregate development, so is the difficulty of isolation and the necessity of using processes auxiliary to the mechanical separation of the tissue.

It is worthy of note here that the Japanese paper with which we are in these times so familiar, is prepared by the most primitive means from the bast of a mulberry (Broussonetia papyrifera); the isolated tissue, consisting of a close network of fibres, is simply cut and hammered to produce a surface of the requisite evenness, and the production of a web of paper is complete. In isolating the bast fibres employed in the textile industries, a preliminary partial disintegration of the plant stem is brought about by the process of steeping or retting, by which the separation of fibre from flesh or cellular tissue is much facilitated.

Last in order of simplicity of distribution, we have the fibres known to the botanist as the fibro-vascular bundles of leaves and mono­cot­y­le­don­ous stems, these bundles being irregularly distributed through the main cellular mass, and consequently, by reason of adhesion thereto, much more {3} difficult of isolation. For this and other reasons, more or less in correlation with natural function, we shall find this class of raw material lowest in value to the paper-maker.

It is necessary at this stage to point out that the work of the paper-maker and that of the textile manufacturer are complementary one to the other, and the supply of fibrous raw material is correspondingly divided: it may be said, indeed, that the paper industry subsists upon the rejecta of the textile manufactures. The working up of discontinuous fibre elements into thread, which is the purpose of the complicated operations of the spinner, is conditioned by the length and strength of these ultimate fibres. Paper-making, on the other hand, requires that the raw material shall be previously reduced to the condition of minute subdivision of the constituent fibres, and therefore can avail itself of fibrous raw material altogether valueless to the spinner, and of textile materials which from any cause have become of no value as such. To the raw materials of the paper-maker, which we have briefly outlined above, we must therefore add, as a supplementary class, textiles of all kinds, such as rags, rope, and thread.

Having thus acquired a general idea of the sources of our raw materials, we must study more closely the substances themselves, and first of all we must investigate them as we should any other chemical substance, i.e. we must get to understand the nature and properties of the matter of which the vegetable fibres are composed. While these exhibit certain variations, which are considerable, the substances present a sufficient chemical uniformity to warrant their being designated under a class name: this name is cellulose. The prototype of the celluloses is the cotton fibre.

CHAPTER I. CELLULOSE: THE CHEMICAL PROPERTIES OF TYPICAL MEMBERS OF THE CELLULOSE GROUP, WITH REFERENCE TO THEIR NATURAL HISTORY.

Plants are so far built up of cellulose that it may be called the material basis of the vegetable world. Plant tissues, however, seldom, if ever, consist of pure cellulose, but contain besides, other products of growth either chemically combined with the cellulose or mechanically bound up with the tissue, which are, according to the nature of their union, removable either by means of fundamental chemical resolution or by the application of simple solvents. A general method for the isolation of cellulose consists in exposing the moist tissue to the action of chlorine gas or of bromine water in the cold, and subsequently boiling in dilute ammonia; repeating this treatment until the alkaline solution no longer dissolves anything from the tissue or fibre. The cellulose is then washed with water, alcohol, and ether, and dried. Obtained in this way, or in the form of bleached cotton, or of Swedish filter paper, it is a white substance, more or less opaque, retaining the microscopic features of the tissue or fibre from which it has been isolated. Its sp. gr. is 1·25–1·45. Its elementary composition is expressed by the percentage numbers (Schulze)

or by the corresponding empirical formula, viz. C₆H₁₀O₅.

These numbers represent the composition of the ash-free cellulose. Nearly all celluloses contain a certain proportion, {5} however small, of mineral constituents, and the union of these with the organic portion of the fibre or tissue is of such a nature that the ash left on ignition preserves the form of the original. It is only in the growing point of certain young shoots that the cellulose tissue is free from mineral constituents. (Hofmeister.)

As already indicated, cellulose is insoluble in all simple solvents; it is, however, dissolved by certain reagents, but only by virtue of a preceding chemical modification. An exception to this is to be found, perhaps, in the ammoniacal solution of cupric oxide (Schweitzer’s reagent), in which it dissolves without essential modification, being recovered by precipitation, in a form which is chemically identical with the original, though differing, of course, in being structureless, or amorphous. This reagent may be employed in a variety of forms, but the following method of using it is to be recommended as the most certain in its results. The substance to be operated upon is intimately mixed with copper turnings in a tube which is narrowed below and provided with a stopcock. Strong ammonia is poured upon the contents of the tube and, after being allowed to stand for some minutes, is drawn off and returned to the tube; the operation is several times repeated until the solution of the substance is effected. In order to facilitate the oxidation of the copper by the atmospheric oxygen, a current of air may be aspirated through the apparatus. The solution of the oxide prepared in this way is more effective in its action on cellulose than that obtained by dissolving the precipitated hydrate in ammonia. Cellulosic tissues in contact with this reagent are seen to undergo a disaggregation of their fibres, which swell up, become gelatinous, and disappear in solution. On adding an acid to the viscous solution, a precipitate of the amorphous cellulose is obtained in the form of a jelly resembling hydrated alumina; after washing and drying, it forms a brownish, brittle, horny mass. The cellulose is also precipitated upon simply diluting the viscous solution with water and allowing it to stand {6} 8–10 days in a closed vessel. From this observation it was inferred by Erdmann that the cellulose could not be considered as dissolved in the strict sense of the word, but the experiments of Cramer upon the osmotic properties of the solution proved this inference to be unfounded, and that cellulose is actually dissolved by the ammoniacal solution of copper oxide.

On treating the ammonio-cupric solution of cellulose with metallic zinc, this metal precipitates the copper, replacing it in the solution, and producing the corresponding ammonio-zincic solution of cellulose, which is colourless. Some of these solutions are lævo-gyrate.

Cellulose, in those forms to which the application of the term has been hitherto restricted, is a comparatively inert substance, and its reactions are consequently few. One of these is available for the identification of cellulose, and is chiefly used in the microscopical examinations of tissues: this is its reaction with iodine. Cellulose is not coloured blue by a solution of iodine excepting under the simultaneous influence of hydriodic acid, potassium iodide, sulphuric acid, phosphoric acid, or zinc iodide or chloride. The solution is prepared in the following way: zinc is dissolved to saturation in hydrochloric acid, and the solution is evaporated to sp. gr. 2·0; to 90 parts of this solution are added 6 parts potassium iodide dissolved in 10 parts of water; and in this solution iodine is dissolved to saturation. By this solution cellulose is coloured instantly a deep-blue or violet. For the identification of cellulose in the gross, mere inspection is usually sufficient; confirmatory evidence is afforded by an observation of the action of the ammonio-copper reagent, and of the absence of reaction with chlorine water. (See p. 18.)

Cellulose in its earlier stages of elaboration has no action upon light; but with age it acquires the property of double refraction, not, as has been shown by experiment, by virtue of its state of aggregation, but of its molecular constitution (Sachs). {7}

The molecular ratio of the absorption, in the two latter, is 3 HCl : 10 NaOH, and it is noteworthy that the same ratio was observed for silk.

Cellulose removes barium hydrate from its solution in wafer to form with it an insoluble compound. On adding lead acetate to the solution of cellulose in the ammonio-copper reagent, so prepared as to contain no carbonate, a {8} precipitate is obtained consisting of a compound of cellulose with lead oxide, but in variable proportions. The compound C₆H₁₀O₅PbO is formed by the action of finely-divided lead oxide upon the above solution. Quite recently it has been shown (O’Shea, Chem. News, May 28th, 1886) that when dilute solutions of lead are passed through ordinary filter paper, a certain amount is retained which cannot be removed by washing.

Cellulose does not combine with metallic salts, a fact which has been established incidentally to researches upon the mode of action of mordants.

The combinations of cellulose with acid radicles (ethereal salts) are both definite and stable.

Triacetyl Cellulose [C₆H₇ (C₂H₃O)₃ O₅] is formed by treating cellulose with six times its weight of acetic anhydride at 180° C. (356° F.). The product of the reaction is a syrupy solution from which the compound in question separates on dilution with water as a white flocculent precipitate.

Triacetyl cellulose is insoluble in alcohol and in ether, but soluble in glacial acetic acid. It is easily saponified by boiling with a solution of potassium hydrate, the cellulose being regenerated. No derivative containing more than three acetyl groups has been obtained; but a mixture of the mono-and di-acetyl cellulose is formed in treating cellulose with only twice its weight of acetic anhydride, the formation of these bodies being unattended by their solution.

Whenever cellulose, in any form, is brought into contact with strong nitric acid at a low temperature, a nitro product, or a nitrate, is formed. The extent of the nitration depends upon the concentration of the acid, on the time of contact of the cellulose with it, and on the state of the physical division of the cellulose itself.

Knop, and also Kamarsch, and Heeren, found that a mixture of sulphuric acid and nitric acid also formed nitrates of cellulose; and still later (1847), Millon and Gaudin employed a mixture of sulphuric acid and nitrates of soda or potash, which they found to have the same effect. {9}

Several well characterised nitrates have been formed, but it is a very difficult matter to prepare any one in a state of purity, and without admixture of a higher or lower nitrated body.

The following are known:—

• Hexa-nitrate, C₁₂H₁₄O₄(NO₃)₆,* gun cotton. In the formation of this body, nitric acid of sp. gr. 1·5, and sulphuric acid of sp. gr. 1·84 are mixed, in varying proportions, about 3 of nitric to 1 of sulphuric (sometimes this proportion is reversed), and cotton is immersed in this at a temperature not exceeding 10° C. (50° F.) for 24 hours: 100 parts of cellulose yield about 175 of cellulose nitrate. The hexa-nitrate so prepared is insoluble in alcohol, ether, or mixtures of both, in glacial acetic acid or in methyl alcohol. Acetone dissolves it very slowly. This is the most explosive gun-cotton. It ignites at 160°–170° C. (320°–338° F.). According to Eder the mixtures of nitre and sulphuric acid do not give this nitrate. Ordinary gun cotton may contain as much as 12 per cent. of nitrates soluble in ether-alcohol. The hexa-nitrate seems to be the only one quite insoluble in ether-alcohol.

  * To represent the series of cellulose nitrates so as to avoid fractional proportions, the ordinary empirical formula is doubled and the nomenclature has reference to this double molecule.

• Penta-nitrate, C₁₂H₁₅O₅(NO₃)₅. This com­po­si­tion has been very com­monly ascribed to gun-cotton. It is dif­fi­cult, if not im­pos­sible, to prepare it in a state of purity by the direct action of the acid on cellulose. The best method is the one devised by Eder, making use of the property discovered by de Vrij, that gun-cotton (hexa-nitrate) dissolves in nitric acid at about 80°–90° C. (176°–194° F.) and is precipitated, as the pentanitrate, by concentrated sulphuric acid after cooling to 0° C. (32° F.); after mixing with a larger volume of water, and washing the precipitate with water and then with alcohol, it is dissolved in ether-alcohol, and again precipitated with water, when it is obtained pure.

  This nitrate is insoluble in alcohol, but dissolves readily {10} in ether-alcohol, and slightly in acetic acid. Strong potash solution converts this nitrate into the di-nitrate, C₁₂H₁₈O₈ (NO₃)₂.

• The tetra- and tri-nitrates (collodion pyroxyline) are generally formed together when cellulose is treated with a more dilute nitric acid, and at a higher temperature, and for a much shorter time (13 to 20 minutes), than in the formation of the hexa-nitrate. It is not possible to separate them, as they are soluble to the same extent in ether-alcohol, acetic ether, acetic acid or wood spirit.

  On treatment with concentrated nitric and sulphuric acids, both the tri-and tetra-nitrates are converted into penta-nitrate and hexa-nitrate. Potash and ammonia convert them into di-nitrate.

• Cellulose di-nitrate, C₁₈H₁₃O₈ (NO₃)₂ always results as the final product of the action of alkalis on the other nitrates, and also from the action of hot, somewhat dilute, nitric acid on cellulose. The di-nitrate is very soluble in ether-alcohol, acetic ether, and in absolute alcohol. Further action of alkalis on the di-nitrate results in a complete de­comp­o­si­tion of the molecule, some organic acids and tarry matters being formed. The reactions and resolution products of this body have, however, been but slightly studied, and apparently not at all with the view to elucidate anything respecting the constitution of cellulose itself.

Other oxidising agents produce similar results; even by exposure to air and light, cellulose is slowly converted into these oxidised derivatives.2 From their mode of formation, they have been termed oxy­cel­lu­loses, and to distinguish them from a series of more highly oxidised derivatives, produced by the action of nitric acid upon cellulose, which they nevertheless resemble in many of their characteristics, the prefix α is employed. The following are the distinguishing features of the α oxycelluloses as represented by the more extreme of the above mentioned products. It reduces Fehling’s solution at the boiling temperature, and the cuprous oxide is deposited upon the fibre in a state of intimate union, producing the effect of an orange dye. It attracts the basic colouring matters from their solutions and is dyed to a full shade, the depth of colour being proportionate to the amount of oxidation to which the cellulose has been subjected. See also p. 43. Treated with a warm solution of phenylhydrazine salts in water, it is coloured a bright lemon-yellow. Its most remarkable property is its attraction for the vanadium compounds, which is so powerful that combination may be proved to take place when this element in the form of chloride is presented to the oxycellulose in an aqueous solution containing not more than 1 in 1,000,000,000,000 parts.

The β oxycellulose resulting, as already indicated, from the action of dilute nitric acid upon cellulose, will be subsequently considered, under the head of the de­comp­o­si­tions of cellulose, to which the reaction which we have been considering may be regarded as transitional. {12}

The modification of cellulose, which occurs on the conversion of unsized paper into the so-called parchment paper, by exposure for a short time to the action of strong sulphuric acid, and subsequent washing and drying, consists doubtless in a superficial conversion of the cellulose into amyloid, or a body closely resembling it.

The action of zinc chloride solution upon cellulose is similar to that of sulphuric acid.

Cellulose in contact with iron, and in presence of air and moisture, is converted into a sugar and a gummy substance, which latter is converted into a sugar on boiling with dilute acids. Oxidised by potassium permanganate or bichromate in presence of acetic acid, it is converted into glucose, dextrin, and formic acid.

The most important part which ulmic substances play in the economy of nature, is in the composition of soils. It is doubtful whether they act directly as fertilisers, but by their action upon the mineral constituents of soils they contribute to the supply of these necessary elements of growth to the plant.

The process of cellulosic fermentation may be represented by the equation:

A similar transformation takes place under the influence of certain fatty seeds, e. g. those of rape and colza; and it is probable that the formation of cellulose in living plants may take place at the expense of saccharose and under the influence of ferments. In support of this it has been established that in the sugar-cane, the formation of wood—i.e. cellulose—is accompanied pari passu by a decrease in saccharose. More recently, A. Brown (Chem. Soc. Journ., 432, 1886) has investigated the formation of cellulose by the “vinegar plant” growing in solutions of the carbo-hydrates, e. g. dextrose in yeast-water. The cells elaborate an extra-cellular fibrin, which acts as a “cell-collecting medium,” and they possess therefore a two-sided activity, i.e. the property above mentioned, in addition to their strictly fermentative activity. The cellulose film in question was found to contain 50 to 60 per cent. of pure cellulose. It is noteworthy that in a solution of levulose the growth of the “plant” is unattended by fermentative action, 33 per cent. of the substance being, on the other hand, transformed into cellulose.

Like cellulose, jute dissolves in cuprammonia, and is similarly acted upon by the concentrated acids. By nitric acids it is converted into nitric ethers, which are yellow coloured, but in other respects closely resemble the pyroxylins. They are entirely soluble in acetone.

Jute differs from cellulose in the following respects: its percentage composition (excluding ash) is

It is harsher to the touch, and its colour varies from grey to brown; it combines directly with the greater number of the organic colouring matters, removing them from solution, i.e., becoming dyed with them; it is coloured deep yellow by immersion in a solution of aniline sulphate; moistened with a solution of phloroglucol and afterwards with hydrochloric acid, it gives a deep red coloration; with pyrrol also in presence of hydrochloric acid it gives a deep carmine colour; it is attacked and partially converted into soluble products by a number of reagents which have no action, under similar conditions, upon cellulose. Certain of these we must consider more in detail.

After dissolving away this compound by exhaustive treatment with alcohol, the fibre still gives a brilliant reaction with sodium sulphite, showing that a portion is still held back in combination with the cellulose, or cellulose residue. On treating the residual fibre with boiling nitric acid, a considerable quantity of chloropicrin CCl₃NO₂ is formed, and it is probable that the union of the molecule C₁₉H₁₈Cl₄O₉ with the cellulose may be effected by the aldehyde5 molecule CCl₃COH; each group being contained in the original lignocellulose, the action of the chlorine tending to disturb the atomic equilibrium and to rearrange the atoms into groups, which in their modified form have less mutual coherence. If the chlorinated fibre be directly boiled with the sodium sulphite solution, its resolution into cellulose and soluble non-cellulose derivatives is complete: and this treatment constitutes the most simple and rapid method of estimating the cellulose in lignified tissues. It is only necessary finally to wash the cellulose with hot water containing a little acetic acid (when placed in a funnel it acts as its own filter), and then with, alcohol, when it may be dried and weighed. In order to ensure the resolution of the fibre by a single chlorination (in the case of jute, and the like), it must, previously to exposure to the gas, be boiled in a dilute (1 per cent.) solution of potassic hydrate. In the case of wood and other more resistant structures, it will be necessary to repeat the chlorination.

The percentage of cellulose yielded by this method is, in the case of jute, usually 2–3 per cent. higher than by the bromine method. Moreover, if the temperature be maintained at 0° C. (32° F.), by placing the fibre, which is to be exposed to the action of the gas, in contact with pounded ice, the percentage may be still further increased, amounting {20} in some cases to 80–82. Corresponding to this increased yield, the cellulose is obtained in long filaments. It will be seen, therefore, that the cellulose isolated by chemical treatment from a compound cellulose is affected both in character and quantity by the process employed, and it is affected in a much greater degree than the cellulose itself exposed to the same treatment, after isolation. The composition of the cellulose obtained in this way differs from that of celluloses, such as cotton, which exist in the plant in an isolated and more fully formed condition; it contains 43 per cent. C and 6 per cent. H, corresponding to the formula n [3 C₆H₁₀O₅H₂O.] The composition of this cellulose will be seen to be identical with that of certain of the oxycelluloses previously described; and its properties are, moreover, those of an oxycellulose. These facts go to show that the jute fibre substance, and the substances allied to it, are compounds of cellulose with other molecules, i.e. they are compound celluloses. They may be conveniently grouped under the term ligno-cellulose.

Schulze’s method of isolating cellulose from wood, and from the ligno-celluloses generally, consisted in macerating them for 12 to 14 days with ⁸⁄₁₀ their weight of potassium chlorate dissolved in 12 parts by weight of nitric acid (sp. gr. 1·10), and completing the resolution by afterwards boiling with dilute ammonia. By the action of a more concentrated acid, 50 per cent. HNO₃, at 80° C. (176° F.), a soluble derivative is obtained which has the composition (C₂₅H₄₀NO₂₅). It is a syrupy, highly acid body, dyeing animal fibres a deep yellow shade, and giving with the earthy bases salts of the formula (C₂₅H₃₂NO₂₅M₄), which are precipitated by alcohol from their aqueous solution in the form of bright yellow flocks that dry to a yellow powder.

The solution yields on distillation furfural and acetic acid.

The increase in weight resulting from the fixation of the {23} nitric acid residue is approximately equal to that of cotton “nitrated” under the same conditions.

They are freely soluble in acetone. On examination they prove to be homogeneous. If the fibre be warmed with the mixture after the first reaction is completed, it dissolves. The solution is found to contain oxalic, succinic, and suberic acids, but no aromatic nitro-derivatives, whereas if ligno-cellulose contain a benzene nucleus, as has been supposed, such derivatives could not fail to be formed under these conditions.

The coals themselves may be regarded as pseudo-carbon derivatives of celluloses, formed by a process of molecular condensation, the true nature of which remains a matter of speculation. In this view, the whole of our vast series of aromatic or benzene compounds, derived as they are from the products of the destructive distillation of coals, may be traced back to a cellulose origin.

Pseudo-carbons are obtained as products of the action of various reagents upon the celluloses, and other of the so-called carbo-hydrates. These reagents, such as sulphuric acid, act in virtue of their dehydrating power; and the recognition of this fact, together with the supposed “carbonaceous” {25} character of the product, led to the erroneous conclusion that the carbohydrates are in such decompositions simply resolved into carbon and water; a conclusion which seems to derive additional warrant from the peculiar numerical relationship which exists between the C, H, O atoms of all the members of the group. Their relation is expressed in the general formula C_n H_2(n−m) O_(n−m), and in the somewhat misleading term carbohydrate, which is applied to the whole group. We now know that the removal of water from these bodies by the action of dehydrating agents—including heat—follows the ascertained laws of chemical dehydration, involving molecular con­den­sa­tions and rear­range­ment, and that the pseudo-carbons are the extreme terms of a series of such con­den­sa­tions or cum­u­la­tive res­o­lu­tions. The matter, however, is not as yet sufficiently investigated to enable us to state with any preciseness the mechanism of these changes. Still this general statement will enable us to avoid many of the erroneous views which have existed on the subject, and in a measure to anticipate the results of future investigation.6

Cork, however, unlike the jute fibre, is by no means chemically simple, but can be resolved by the action of mere solvents into a number of proximate constituents, such as acids, a variety of fatty bodies, nitrogeneous bodies, &c.

Cuticular tissues, such as constitute the covering of fruits, are more simple in composition; the cuticle of the apple after purification, has the following percentage composition:

The cuticular substance of cotton, straw, esparto, &c., are doubtless similar bodies. These numbers point to a {27} remarkable similarity in composition to the fats, and, indeed, from the results of his study of this tissue, Frémy concluded that it was a fat in everything but its physical properties, of which we may mention insolubility in alcohol and in ether, and infusibility. This tissue, however, as also cork, contains a cellulose residue, which may be isolated by any of the methods of treatment given for ligno-cellulose. Under the action of boiling nitric acid these tissues are resolved into cellulose on the one hand, and a series of fatty acids or products of their decomposition, suberic and adipic acids, &c., on the other; the latter amounting in the case of cork to 40 per cent. of the weight of the substance treated. The cellulose obtained by this treatment is but 2–3 per cent.; this number, however, represents only the amount which has survived a treatment which we know to be destructive to cellulose. If, on the other hand, cork be resolved by treatment with sodium sulphite solution at 100lb. pressure, or 166° C. (331° F.), a soft mass is obtained, preserving the structural features of the original cork, until subjected to slight pressure, when it falls to a cellular mass. From this, cellulose is isolated by any of the less drastic processes above described, and is found to amount to 9–10% of the original cork. As in ligno-cellulose, we have evidence of a transition from cellulose to the tannins, so in cork and cuticular tissue we have evidence of the metamorphosis of cellulose into fats, a fact indicated in the term adipo-cellulose, which we have applied to the compound celluloses constituting these protective plant tissues. This metamorphosis is doubtless a very complex process, and would appear to involve the formation of tannins also, at least as a subsidiary result. Still, the essential feature of the change is the production of the peculiar fat-like substances which have been described; and with due regard to the limitation pointed out, the views here advanced represent the results of the investigations of the subject as far as they have proceeded.

It may be observed that the general characteristics of the {28} adipo-celluloses as of the ligno-celluloses are those of a complex aldehyde.

On boiling with an alkali it loses in weight about 20 per cent., the substance dissolved being identical in properties with Frémy’s pectic acid, a substance containing 42 per cent. C and 4.8 per cent. H = C₁₆H₂₂O₁₅. Many other bast fibres, as well as cellulose tissues, exhibit similar properties, and although this branch of the subject has been but little investigated, there is sufficient evidence for constituting a special class of compound celluloses under this term, pecto-cellulose, having properties indicated in the type selected above.

In addition to pectic acid there exists in, or can readily be obtained from certain plants, a series of bodies of a similar nature, such as pectose, pectin, parapectic acid and metapectic acid. The most important of these are pectic acid and metapectic acid.

Pectic acid, though it rarely exists ready-formed in the plant, can readily be obtained from it by the action of weak alkalis. It is best procured by boiling the pulp of turnips with a 10 per cent. solution of sodium carbonate for about half an hour. If an acid be added to the filtered liquid, pectic acid is precipitated as a transparent colourless jelly, which {29} dries up to a transparent horny mass. By the prolonged action of alkalis or acids, pectic acid is converted into metapectic acid (C₈H₁₄O₉), which may be obtained as a syrupy, strongly acid liquid.

From what we know of the properties of the members of the pectic group, and therefore of the pecto-celluloses, it will be readily seen that the latter are very liable under the action of alkalis to undergo conversion into soluble derivatives.

Conversely, a study of the action of alkalis upon plant substances, enables us to determine to what extent the latter partake of the nature of the pecto-cellulose, the importance of which to the practical paper maker we need hardly point out. For a further development of this idea (see p. 43).

CHAPTER II.

PHYSICAL STRUCTURE OF FIBRES.

We now have to treat of the fibrous raw materials from the point of view of form or structure, which is, of course, a very important factor in determining the quality of the paper or other fabric into which they are manufactured. It is sufficiently evident that the strength of paper is primarily due to the cohesion of its constituent fibres. When paper is torn, the edges present a fibrous appearance, and observation teaches us that, other things being equal, the greater the manifestation of fibrous structure the stronger will be the paper. If a paper be thoroughly wetted, its tensile strength is reduced to a minimum, and if subjected to a slight strain we get, not a tearing, but a pulling asunder of the fibres. If this be performed under a lens, the structure of the paper is more clearly seen, and it will be appreciated to what extent the qualities of a paper are the aggregate of the qualities of its constituent fibres. A more careful dissection of the paper shows that these fibres, which are the ultimate fibres of the plant, as distinguished from the bundles of these, or filaments, which the spinner employs, are interlaced in all directions. To produce this effect of interweaving, and to insure that uniformity which is an essential feature of good paper, we have among others the following contributory causes: (1) the deposition of the fibres from suspension in water; (2) the composition of the pulp with regard to the reduction of the fibrous bundles, and the isolation of the individual fibres; and (3) the structure of these ultimate fibres. It is with the last that we are chiefly concerned at present.

To convey a general notion of the influence of the structure {31} of fibres upon fabrics, we shall with advantage travel outside our immediate province to consider briefly the woollen and silk manufactures in relation to this point. Wool is, as we know, a discontinuous fibre, and its structure is that represented in Fig. 1, the most conspicuous feature being its broken surface, consisting apparently of imbricated scales.

FIG. 1.

The silk fibre, on the other hand, is a dual cylinder, spun by the worm in a continuous length, and with a perfectly smooth surface. Now, it would not be to our purpose to point out that in starting from a discontinuous simple fibre, to produce a continuous, therefore necessarily compound one, a very different treatment or process of spinning is required from that which the opposite condition renders practicable. We will rather consider the influence of structure upon materials manufactured from these fibres. It is obvious that the wool fibres, brought into contact with one another, tend to interlock; whereas silk fibres if rubbed or pressed together, simply slide over one another; the result in the fabric is by multiplication of the effect, a shrinking or contracting in length and breadth. This interaction of the fibres, and the phenomena to which it gives rise, is known as the felting of wool and woollen goods; this tendency, for the contrary reason, is not seen in silk fabrics. The production of paper from a disintegrated fibrous mass or pulp introduces similar considerations. That paper will be the stronger in which the constituent fibres are the better felted, and the degree in which felting takes place will depend to a great extent upon the form or microscopic peculiarities of the fibres. This is only one of the more obvious inferences to be drawn from the structure of fibres to the qualities of the papers which they compose. Other {32} equally important practical bearings will be seen to attach to the microscopic study of our fibrous raw materials, and to the consideration of this branch of the subject, we now ask the careful attention of the student.

It is impossible for us to deal specially with the subject of the microscope and its manipulation. The microscope, as a revealer of natural wonders is one thing; as an instrument of scientific discovery, quite another. For the latter, the student must train himself by systematic work, and should especially concentrate his attention upon some one branch of natural history, however restricted.

We shall assume, in our treatment of the subject, a knowledge of the microscope as an instrument of research, such as can be easily acquired in a few weeks of work under the guidance of a teacher or of one of the excellent manuals which now abound. We also assume a certain acquaintance with the elements of vegetable physiology, which it will be seen is necessary for a full grasp of the subject. Such an acquaintance, also, may be easily acquired, under direction, in a few weeks of work.

FIG. 2.

FIG. 3.

We have before alluded to the differences presented by {33} mono and di­cot­y­le­don­ous stems in regard to the distribution of their fibrous constituents. In illustration of this, we may cite Figs. 2, 3, which represent, (2) a section of the aloe, (3) a section of the jute plant. The available fibres are in (2) the fibro-vascular bundles (f), which are irregularly distributed throughout the main mass of cellular tissue, and {34} in (3) the bast fibres (f), which constitute a definite and separate tissue. We have already alluded to the practical consequences of this typical difference of distribution, in regard to processes of separating these fibres on the large scale.

This process we have explained is necessarily simpler in the case of a fibrous tissue, definitely localised; and this may be demonstrated by a superficial examination of a young branch of an exogen. As we know, the bark tissues are easily stripped from the underlying wood. If now we work up the former in a mortar, with a little water, we soon perceive the separation of the compound tissue into cellular matter on the one hand and fibres, the latter being more or less long and silky, according to the plant from which isolated. They vary in length from one millimetre to several centimetres, and are aggregated together in the plant in such a way as to constitute bundles, often of very considerable length; the general arrangement being comparable with that of the tiles in the roof of a house. It is important to distinguish the fibre-bundles from the elementary or normal fibres, and to this end they are designated by the term filament. Bast fibres are flexible and fusiform, terminating gradually in a point at either end, as represented in Fig. 4; bast filaments, built up of these fibres, containing often as many as twelve in the bundle, are usually cylindrical, but exhibit the widest differences in regard to the aggregation, in degree as well as number of their constituents. It is obvious that while the spinner has to do with these filaments, the paper maker works up the ultimate fibre constituents or fibres. It is also an obvious corollary from this distinction that a fibrous material which from “weakness” is unavailable for textile application, may yet be perfectly “strong” from the paper maker’s point of view; in other words, the individual fibres may be strong, but have little cohesion in the filaments. As we proceed, the student will see more and more the practical bearings of this branch of the study, and will perceive the inferences to be drawn from the investigation of {35} minute relationships to manufacturing processes and their products.

FIG. 4.

We shall say but little as to the necessary equipment. (1) A dissecting microscope, for dissecting under a lens, magnifying the object to 40 or 50 diameters. (2) An ordinary student’s microscope with lenses for magnifying to 100 and 300 diameters. This is adequate to the work, though of course, it may be an advantage in certain cases, to be provided with higher powers. (3) A glass slide, carrying an engraved scale of centimetres and millimetres for measuring the lengths of objects, and a micrometer, divided into ¹⁄₁₀ mm. for measuring diameters. It is also important to be able to determine the degree of enlargement under any particular combination of lenses, and for this purpose to possess a micrometer eye-piece, with a millimetre scale divided into hundredths. (4) An effective microtome and the usual mounting accessories.

A very important feature in the diagnosis of fibres, more especially in regard to the composition of the fibre substances, is the effect produced by treatment with various reagents. Certain of these reactions we have already {36} indicated. We shall now give the details of composition of the several solutions which will be required.

The accessory solution, dilute sulphuric acid, which is employed to determine the reaction between the cellulose and the iodine, is prepared as follows:—2 volumes of concentrated glycerin are mixed with 1 volume water, and to the mixture an equal volume of oil of vitriol (1·78 sp. gr.), is slowly added, so as to prevent as far as possible a rise of temperature. The effect of the glycerin is very remarkable in preventing the distortion of the objects under the action of the acid, which in other respects remains uninfluenced.

By way of verification of this iodine test, which is somewhat capricious, it is advisable to test the reagents with a standard substance. The best for the purpose is a linen yarn which has been partially bleached. Under the action of the reagents the fibres composing this yarn, which must, of course, be suitably “teazed out” for mounting, are coloured a light blue, the centre, however, showing a yellow line, {37} marking the distribution in the interior canal of a non-cellulose fibre constituent. Should these effects not appear, it may be concluded that the acid requires to be strengthened. On the other hand, too great a concentration is equally to be avoided; it is evidenced by causing a distortion of the fibre, easily recognised by comparison with the fibre mounted in a neutral medium.

The employment of the aniline sulphate solution, as well as of the solutions of the aniline colours, presents no difficulties, and therefore needs no detailed de­scrip­tion. The former strikes a more or less deep yellow with lignose; the aniline colours stain or dye the tissue or fibre more or less deeply, according to its composition, and, as it is a reciprocal action, {38} according also to the composition of the colouring matter. In following up this subject, the student will require to consult works on vegetable histology.

Sections of the filaments are prepared by cutting in a microtome, the filaments being previously agglutinated into a stiff bundle by means of any of the usual stiffening solutions, and after drying, embedded in wax in the usual way. Sections of fresh stems and tissues are cut with a “section” razor.

Having prepared the objects, their examination under the microscope necessarily divides itself into:—(1) the determination of external features; (2) the diagnosis of chemical composition. The fibres themselves will be individually considered in regard to microscopic features.

There is one aspect of these structural features, however, which admits of more general treatment, and in respect to this we may anticipate with advantage, viz., the dimensions or simple elements of form. The importance of the determination of the length and diameter of both filaments and fibres will be readily appreciated by an inspection of the following table, in which the numbers are given for several of the more important.

A careful study of this table in relation to the application of these several fibres, will show that the correlation of the latter with these ultimate dimensions is close and essential. {39}

CHAPTER III. SCHEME FOR THE DIAGNOSIS AND CHEMICAL ANALYSIS OF PLANT SUBSTANCES.

It will be convenient at this point to present to the reader a general scheme for the chemical analysis of fibrous raw material.

We have already discussed (p. 4) the methods for the determination of cellulose in fibrous substances. This is to the paper-maker the most important constituent, and is that upon which his calculations of the value of a raw material are based. Not only, however, is it of the greatest importance for him to be able accurately to determine the amount of cellulose, but it is necessary to be able to form some idea of the nature of the cellulose and also the ease with which it can be obtained from the fibre. This latter point can only be properly established by direct trial of the fibre under the conditions existing in the factory. It should also be borne in mind that these conditions influence to a considerable degree the amount of cellulose, actually obtained in practice, and that therefore the percentage shown by a careful determination in the laboratory should only be taken as an indication of the amount likely to be realized. The student will be able, from what has been said before of the properties of the different celluloses, to apply his knowledge to a determination of any particular specimen. In order to supplement this knowledge, it may be well at this particular point to cite the chief results of an investigation bearing upon this point by Webster (Chem. Soc. Journ., 1883, p. 23).

His method of investigation consisted in (1) estimating {43} the cellulose; (2) determining the loss sustained by the fibre when boiled for 5 minutes, and also for 60 minutes, with an alkaline solution (caustic soda 1 per cent. Na₂O), the former being taken to represent the purely solvent action, the difference between the two amounts being an indication of the degrading action of the soda; (3) determining the ultimate composition (C H and O) of the fibre substance. It should be observed that the specimens were carefully averaged and purified by boiling in glacial acetic acid and in alcohol. On carefully considering the following table it will be seen that the chief points brought out by Webster’s investigation are (1) the association of a low carbon percentage with pectous properties and considerable degrading action on long boiling in alkali. The bearing of this will be better understood in connection with a discussion of the properties of pectic acid (p. 28) and the oxycelluloses (p. 13). (2.) The influence of the composition of the original fibre on the isolated cellulose.

The solutions of aniline sulphate and of the aniline colours give valuable aid in a diagnosis of plant substances. Webster observed, as was to be expected, that the yellow colour produced could be taken as an indication of the amount of lignification, those fibres which were of a pectous nature showing little or no colour. On the other hand, when the fibres were treated with a neutral solution of the so-called alkali-blue it was found that the converse took place, highly pectous fibres being dyed much more deeply than those which were lignified. (Compare the action of aniline colours on oxycellulose, p. 11.)

CHAPTER IV. AN ACCOUNT OF THE CHEMICAL AND PHYSICAL CHARACTERISTICS OF THE PRINCIPAL RAW MATERIALS.

This chapter contains the results obtained from an investigation of the different plant fibres by the methods indicated in the two previous chapters.

In the following table the fibres are classified according to the reactions which they give with iodine solution.

Microscopic features.—Fibres simple, opaque, flattened, always more or less twisted upon itself, side wall membranous, showing striæ. {47}

Sections.—Simple, oval, irregular, central cavity often containing granules.

Micro-chemical reaction.—Blue with iodine solution.

FIG. 5.

General chemical characteristic.—Pure cellulose basis with slight admixture of colouring matters, &c.

E. Schunck in his investigation of cotton bleaching (Chem. News, 17, p. 11) finds that in boiling with sodium carbonate solution the fibre loses 5 per cent. in weight. Some portion of the dissolved substances is precipitated on acidifying and is found to contain a wax, brown resinous and colouring matters, and pectic acid. {48}

Forms in which employed.—Fibrous refuse from the decortication of the seeds; spinning waste, threads, rags, new and old.

FIG. 6.

Microscopic features.—Fibres transparent, regular, with tapered ends, side walls thick, consequently central canal small, smooth externally, sometimes slightly striated. Filaments easily split up into fibres.

Sections.—Polygonal, regular, angles more or less acute, lumen represented by point; slight indications of concentric arrangement of fibre substance.

Micro-chemical reaction.—Blue with iodine solution.

General chemical characteristic.—Pecto-cellulose.

Composition of raw fibre.—This varies with the different species, and is doubtless also influenced by variations in the {49} processes of retting. The following are the analyses of two samples of Belgian flax (heckled):—

The chemistry of the flax fibre has been investigated by Kolb.

Forms in which employed.—Scutching refuse, spinning waste, threads, rags, new and old.

FIG. 7.

Microscopic features.—Compact bundles. Fibres show striæ and fissures, and often fibrillæ, detached or adherent. The central canal almost obliterated, ends of fibres large and flattened. The bundles show fine transverse markings. {50}

Section.—Well marked concentric zones of fibre substance, irregular in outline.

Micro-chemical reaction.—Blue and yellow reaction with iodine solution, the joint result showing green colouration.

General chemical characteristic.—Pecto-cellulose.

Forms in which employed.—Scutching refuse, spinning waste, threads, cuttings and rope ends.

Microscopic features.—Generally similar to those of hemp.

Micro-chemical reactions.—With iodine various, from blue to yellow. With aniline sulphate, slight yellow.

General chemical characteristics.—Pecto-cellulose (with some ligno-cellulose).

The following are the results of analysis of the raw fibre (H. Müller):—

This fibre, exported from India and the Sunda Islands, has found employment in Spain and Portugal, but up to the present has not been adopted to any extent in other parts of Europe.

FIG. 8.

Microscopic characteristics.—Irregular in form and length; often conspicuous in respect of latter (see Table, p. 39); fibre sometimes cylindrical, either smooth or striated, sometimes flattened; central canal well developed, often containing granules; extremities of fibres rounded, spatulated or lance-shaped. Sections marked by numerous concentric layers, often showing radiating striæ.

Micro-chemical reaction.—Blue to violet, with iodine solution.

General chemical characteristics.—Pecto-cellulose.

Composition of raw fibre (H. Müller):—

Forms in which employed.—Up to the present the price has been prohibitive. Has been the subject of successful {52} experiments; imparts great tensile strength to paper. The portions available will be as in flax.

The fibres of this plant were, in the olden times, separated in the same way as flax, and worked up into cloth. At present it has no practical interest to the paper-maker, but deserves attention at the hand of the student, as the most easily accessible for the purpose of study. It is, moreover, possible that, under cultivation, it may yet become a source of raw material for paper-making.

FIG. 9.

Microscopic features.—Compact bundles; fibres smooth.

Micro-chemical reactions.—Yellow brown, with iodine; yellow, with aniline sulphate; bright yellow, with chlorine water; changed to carmine by treatment with sodium sulphite solution. {53}

General chemical characteristic.—Ligno-cellulose. The chemistry of jute as the type of ligno-cellulose has been treated on p. 17.

Composition of raw fibre (Müller):—

Forms in which employed.—Threads, butts, bagging.

The fibres give a yellow colouration with aniline sulphate, but they contain only a small proportion of lignose. The following are the results of analyses of this bast:—

FIG. 10.

Order, Gramineæ. Bast fibres of fibro-vascular bundles. Length, 1·5 mm.; diameter, 0·012 mm.; ratio, 125. Illustrated in Figs. 10, 11. In Fig. 10, the fibro-vascular {56} bundles f are seen spread throughout the interior of the leaf, but the intervals, instead of being occupied by parenchyma, with large cells and thin walls, are filled with a compact mass of fine solid fibres f ′ ; e, external epidermis; e′, internal epidermis; mag. 100. In Fig. 11, a is a section of a group of fibres; b, fibres seen longitudinally; c, ends; mag. 300.

FIG. 11.

Microscopic features.—Short, smooth, cylindrical, uniform in diameter, central canal very small, extremities rounded, truncated and bifurcated.

Section.—Minute, generally oval, sometimes polygonal; central cavity represented by a point.

Micro-chemical reaction.—Both blue and yellow with iodine solution.

In examining a paper containing esparto under the microscope, the pulp will be found to contain, in addition to the fibres of the fibro-vascular bundles, a certain number of the cuticular cells (see Fig. 12), together with some of the fine hairs which are seen in the section (Fig. 10). These are very characteristic.

General chemical characteristics.—Pecto-cellulose mixed with some ligno-cellulose. {57}

Composition (Hugo Müller):—

Microscopic features.—Generally similar to those of esparto. There are, however, differences of shape and dimensions of the serrated cuticular cells, which differentiate the various kinds of straw from each other and from esparto.

FIG. 12.

In Fig. 12 are shown these cells, from maize-straw (a and b); from rye-straw (c) and from esparto (d). The {58} following table gives the dimensions of the cells from different kinds of straw:—

Another distinctive feature of straw-fibre is the presence in it of a number of oval cells, derived from the pithy matter attached to the inside of the stem. These are clearly shown at b (Fig. 13), which represents the general appearance of straw pulp.

FIG. 13.

General chemical characteristics.—Both ligno-cellulose and pecto-cellulose.

The following are the results of analyses of straws (Hugo Müller):— {59}

From the close botanical relationship of these products to the stems of the gramineæ of our own climate, their microscopic features are, as might be expected, similar to those of straw. The similarity is further shown by the chemical composition (Hugo Müller):—

Microscopic characteristics.—Length of fibres, 9 mm., diameter 0·016 mm., ratio, 560. Fibres are fine, regular and smooth; the walls are uniform, central canal small, extremities vesicular. The fibres have little cohesion in the bundle. Sections round or polygonal. Illustrated in Fig. 4, p. 35.

Micro-chemical reaction.—Yellow, with iodine solution. Characteristic deep red colouration with concentrated nitric acid (Müller).

General chemical characteristic.—Ligno-cellulose. It contains 86·3 per cent. of cellulose.

Microscopic characteristics.—Length of fibres, 6 mm.; diameter, ·024 mm.; ratio, 250. Fibres, white lustrous; the walls are uniform; central cavity large and very apparent. Fibres easily detached. {60}

Sections round or polygonal. Illustrated in Fig. 14: a, section of bundle of fibres; b, fibres seen longitudinally; c, ends; mag. 300.

FIG. 14.

Micro-chemical reaction.—Yellow with iodine solution.

Composition (Hugo Müller):—

In America, however, poplar and other woods are largely employed. The coniferæ yield a larger proportion of pulp than most other woods, the individual fibres, moreover, are longer, and for these reasons it is generally preferred. On the other hand, however, poplar is more readily acted upon by reagents. Fig. 15 gives the microscopic appearance of {61} the fibre of the common white fir. It is characterised by the presence of numerous pitted vessels (Fig. a).

FIG. 15.

Pine wood consists essentially of a compound cellulose, resembling in most of its properties the jute fibre (see p. 16). With iodine solution it gives a deep yellow colour. The chemical composition of some of the more important woods will be seen from the following analyses. (Müller.)

The above results are calculated on the ash-free wood. The ash varies from about 0·3 to 0·7 per cent.

CHAPTER V. PROCESSES FOR ISOLATING CELLULOSE FROM PLANT SUBSTANCES.

We are now in a position to discuss generally the various methods by which the paper-maker obtains cellulose from the different raw materials. The special treatments necessary for each of the more important of these will be given more fully subsequently. These methods, though they vary considerably with the different materials to be treated, may be roughly divided into two groups—the alkaline and the acid processes. The former, which we will call a, comprises all processes in which caustic soda, carbonate of soda and lime, or mixtures of these are employed; the latter, b, the various processes which have been introduced of late years, involving the use of sulphurous acid, either alone or in combination with a base, such as lime or magnesia.

The processes in group a may be applied to the treatment of every known fibrous vegetable material; those in group b are at the present time applied exclusively to the preparation of pulp from wood.

Group a may be conveniently subdivided according to the nature of the materials operated upon.

It is obvious that those materials, such as bleached cotton and linen threads and rags, which have already undergone treatment by the textile manufacturer, and are therefore more or less already in the state of pure cellulose, require but little chemical treatment at the hands of the paper-maker, whose attention is therefore chiefly directed to the removal of such adventitious matters as size, grease, &c. This can generally be effected by the employment of weak solutions of caustic soda, or even of lime at a low pressure. {63}

We now have to consider the treatment of those materials which consist of the compound celluloses.

All methods for the isolation of cellulose from the compound celluloses depend upon a hydrolytic resolution of their constituents, i.e. a splitting up by combination with water into cellulose on the one hand, and a series of soluble derivatives on the other. This, as will be shown, may to a certain extent be ac­com­plished by the action of water itself at a high temperature. As, however, the products of such an action are acid bodies, which, if allowed to remain in contact with the cellulose, would injuriously affect it, and would induce the production of complicated bodies, the removal of which from the cellulose would become increasingly difficult, it is necessary to have present a body such as caustic soda, which by combining with these acid bodies, removes them as such from the sphere of action.

In certain raw materials, such as straw and esparto, we have, in addition to the compound celluloses which form, so to speak, the groundwork, a certain proportion of fatty and resinous bodies, whose removal is brought about by the action of the caustic soda converting them into soaps.

The compound celluloses may be divided, as we have seen, into three classes—pecto-celluloses, ligno-celluloses, and adipo-celluloses. The last-named being present in only a few paper-making materials, and then only in very small proportion, is without any practical interest. We are concerned therefore with the two former only.

(1) Pecto-celluloses.—From what we have seen of the nature of the pecto-celluloses, it follows that for their resolution a tolerably simple treatment with soda will be sufficient.

The chief members of this group in which the paper-maker is interested, are esparto and straw. The former being but very slightly lignified, its resolution can be effected at a very low pressure; in fact it is even possible by using tolerably strong solutions to do it by boiling in open vessels.

Straw, on the other hand, though it possesses certain features in common with esparto, is more lignified, and in {64} consequence, necessitates a much more energetic action to complete its resolution. It is therefore usual to boil it at somewhat high pressures (40–70 lb.), and with larger quantities of soda than are demanded in the case of esparto. It is, moreover, essential, so to regulate the treatment, that even the most resistent portions of the straw, such, for example, as the knots, shall be completely resolved, otherwise unbleached portions are liable to find their way into the finished paper. By increasing the quantity of soda, it is possible to boil under reduced pressure, and vice versâ; this may be taken as a general principle applicable in all other cases.

We now come to the second class of celluloses, that is to say, the ligno-celluloses. Of these, perhaps the most important are jute, manilla, and wood.

For their complete resolution, it is imperative to employ very strong solutions at very high temperatures and pressures; thus in the case of wood it is necessary to boil at temperatures corresponding to a pressure of 100–120 lb. per square inch.

The use of strong caustic soda solutions at high temperatures is attended with very serious objections, apart from the question of cost. Among these may be mentioned the destructive action of the soda upon the cellulose itself, involving a considerable loss of pulp; the de-hydration, condensation, and oxidation of the non-cellulose portion of the fibre (which, as we have seen (p. 25) is of the nature of an aldehyde, and therefore peculiarly susceptible of such changes), and the formation of dark coloured products which remain intimately combined with the residual cellulose; the inconvenience of working at very high temperatures and pressures.

With a view to removing the difficulties attending the use of soda, various processes have from time to time been proposed, in which the hydrolytic agent is a solution of sulphurous acid in water, either alone or in combination with a base. These processes are comprised under group B. {65} Several of them have lately acquired great commercial and industrial importance, and a clear understanding of them is therefore essential to the student of paper-making.

Though applicable to lignified fibres generally, they have only been applied up to the present time to wood, and it is in this connection, therefore, that we propose to consider them.

In order to bring out more clearly the fundamental differences underlying the two classes of processes, it will be well to consider, somewhat in detail, these processes, which, although they have now a merely historical interest, will assist the student in thoroughly grasping the rationale of the modern and improved methods.

Water Process.—Wood is to a certain extent resolved by treatment with water at a high temperature, the degree of disintegration being, within certain limits, proportional to the temperature at which it is digested. The volatile products of the resolution are chiefly acids of the acetic series, furfurol and terpenes. The non-volatile soluble products are for the most part acids. This process was studied by Fry, who, in 1866, made it the subject of a patent. He found that under the most favourable conditions of treatment, the yield of brown pulp amounted to 70 per cent. of the wood employed. The objections to the process were sufficiently obvious: the pulp produced was of low quality, and the soluble products acted powerfully on the iron digesters employed. The chemical causes of its failure lie in the prominence of the conditions which determine re-combinations of the products of resolution with one another, viz. oxidation, high temperature, and the presence of acids. It is worthy of note that in one experiment the amount of acids produced from 100 parts wood were equivalent to 3·4 parts of soda (Na₂O). This was by far the most serious defect in the process, and it eventually caused its abandonment in favour of the older method of treatment with caustic alkali.9

Alkali Processes.—The first to develop the alkali process {66} were Watt and Burgess, in America (1853–1857), and Houghton in this country. To follow up the several modifications introduced is unnecessary; they differ from one another in the economical balance which they strike between temperature and quantity of chemicals: thus Watt and Burgess employed a lye of 12° Baumé, and digested at 60 lb. pressure (equal to 152° C.), whereas Houghton used a lye of 4° B., at 100–180 lb. One great defect of these processes was the gradual neutralisation of the alkali, necessitating as a compensating condition the employment of a very high temperature towards the end of the operation. To overcome this objection, Ungerer devised a process which aimed at securing a properly graduated action of the lye. This process, being based on an eminently rational view of the action of the alkali, is worthy of a more extended notice, especially as the principle on which it is founded is capable of wide application. A system of nine vertical digesters, connected together in series by means of pipes and cocks, so that the liquid contents can be transferred from one to the other in succession, is the main feature of the method employed. In connection with this system there is a double service of pipes, by which the hot soda lye at 90 lb. pressure, and cold water, may be respectively introduced. The operation is so conducted that in five of the digesters wood is being treated at a high temperature, in three the pulp is being washed, while the remaining one is being charged with fresh wood.

The whole round of operations may be described thus, numbering the digesters according to their position in the series:—Nos. 1, 2, and 3, Washing the finished pulp; 4, Final treatment with lye at the extreme pressure of 90 lb.; 5, Digesting at lower pressure with lye previously employed in 4; 6, Treatment at still lower pressure with lye which has already done duty in 4 and 5; and so on to 8, where the liquor, having passed through the series, is employed at 15–20 lb. pressure on a fresh charge of wood. The washings are conducted in such a way that the water which is introduced into 1 displaces the solution in each digester without {67} the last being allowed to escape. The introduction of the fresh liquor at 4 is similarly ac­com­plished, 1, 2, and 3 being cut off by closing the cocks which control the ingress and egress of liquid.

The pulp produced by this method is of a sufficiently pale colour to be capable of application for many purposes without bleaching. It is converted into a perfectly white pulp by treatment in the ordinary way with from 3 to 4 per cent. of bleaching powder, calculated in the dry pulp.

As far as it goes the method is practically perfect, and it appears to have given sound results, at least as regards the main product. The principal disadvantages were the complications attending such an elaborate system of working which would doubtless make it costly.

We have next to consider a class of processes which are based primarily upon the action of an alkali, but which introduce the simultaneous action of a soluble sulphide, generally that of sodium. As far back as 1855 Jullion patented in this country a process of this nature, which, however, does not appear to have been applied to wood. The results obtained with such materials as straw and esparto appear to have been very unsatisfactory, and to have led to the early abandonment of the method.

Recently the increased attention which has been given to wood as a raw material has led to a revival of these sulphide processes for its treatment. We believe that at least two processes of this class are being worked on the continent, viz. those of Dahl and Blitz.

That of the former consists in heating wood, at pressures varying from 60–120 lb. per square inch, with a solution of the following composition:—

This solution is obtained only after the process has been some time at work, and it then remains tolerably constant.

The wood, to commence with, is boiled with a solution containing a large quantity of sodium sulphate, with a small quantity of sodium hydrate, obtained by boiling sodium sulphate with lime. The liquor from this operation is then evaporated and calcined, the result being a fused mass which, when dissolved in water, yields a liquor containing the above-mentioned ingredients. The loss of chemicals is made up by the addition of fresh quantities of sodium sulphate.

From experiments which the authors have made with this process, it appears, that while it is perfectly true that pulp can be obtained, it is necessary to employ either very strong solution or very high pressures. This is equivalent to saying that either the hydrolytic action is weak, or the tendencies to reversal of the hydrolysis are strong, and indeed both are to be predicted from the nature of the solution. One great objection to the process is that during the boiling a quantity of very foully-smelling compounds are produced.

The process of Blitz consists in heating wood to from 50–60 lb. pressure, with a solution of sodium sulphide, containing an infinitesimal proportion of vanadate of ammonia.

It is difficult to see what influence the presence of the costly vanadate of ammonia can have on the result. Actual experiment on this point has failed to convince us of its efficacy.

Although in these sulphide processes a strongly reducing agent is present, its effect in preventing oxidation is more than outweighed by the weakening of the hydrolytic action of the soda, both directly by its combination with sulphur, and indirectly by the complication which it induces by combination with the soluble bye-products. Further, as regards the final product, they offer no advantage over the caustic alkali processes. There is, however, a considerable advantage in point of economy, which should lead to their adoption in districts where the production of malodorous gases is permitted. {69}

Acid Processes.—We now come to consider the processes which are based upon the hydrolytic action of acids as a primary condition. We have seen that in the water process this condition obtains as a secondary result, and that while it aids in resolving the wood, it also determines the limit of the action, in consequence of its promoting secondary combination amongst the soluble products. The use of powerful oxidising acids, such as nitric and nitro-hydrochloric, induces an action which is sufficiently drastic to cause a complete resolution of the non-cellulose constituents of the wood with the formation of soluble products. The use of these acids has been made the subject of numerous patents.

The process of Barre and Blondel consisted in digesting the wood in the cold for twenty-four hours in 50 per cent. nitric acid, by which it was converted into a soft fibrous mass. This was boiled for some hours in water, afterwards in a solution of sodium carbonate, and finally bleached in the ordinary way. Many other patents have been taken out in this direction, but none have met with practical success, owing doubtless, to the high cost of nitric acid, and the difficulty of recovery. A process has lately been patented by Young and Pettigrew (No. 14,988, 1884) in which they claim the use of either nitric or nitrous acids; the acid fumes which are evolved being condensed and reconverted into nitric acid. The process has not yet, however, been tried on a practical scale.

The resolution of wood by the hydrolytic action of non-oxidising acids, is for reasons already stated, very imperfect. An important feature, however, of this action is the production of glucose as a bye-product, and a process having the double object of producing this, together with a brown pulp, was patented by Bachet and Machard in 1864, and worked for some time in Switzerland. The process consisted in boiling the wood for twelve hours in dilute hydrochloric acid; the solution, which contained glucose to the amount of 20 per cent. of the weight of the wood, was drawn off, neutralised with chalk, and fermented by the addition of {70} yeast. Alcohol was separated from the fermented liquor in the usual way. The residual pulp, after being ground with stones, was made into coarse packing paper. The process failed commercially.

The next processes in the order of the development of our subject, but not chronologically, are those involving the use of reducing acids, i.e. those which combine with their hydrolytic function a de-oxidising or anti-oxidising action. Of these the only one which has been practically investigated is sulphurous acid. A full account of the earlier discoveries in regard to the use of this reagent, for the special purpose under consideration, will be found in the Papier Zeitung, 1884, No. 51, p. 1927; this should be consulted by those who are interested in the history of a subject which has now grown to be of the first importance.

The first patented process claiming the use of sulphurous acid, in the form of its aqueous solution, for the purpose of disintegrating wood, was that of Tilghmann (1866). So far as is known, however, the results obtained by the inventor with the acid alone were such as to lead him to abandon it for solutions containing a certain proportion of base. The reasons, doubtless, were the greater facility of preparation and simplicity of application of such solutions.

More recently Professor Pictet, of Geneva, on the basis of the experience acquired by him in the preparation and employment of sulphurous acid in his patent freezing processes, has revived and perfected a process which in other hands had proved impracticable.

The method and apparatus employed will be found fully described in the author’s specification. The process consists in digesting the wood in closed vessels with an aqueous solution of the acid (10–12 per cent. SO₂) at 176°–194° F. (80°–90° C.), and under the corresponding pressure (60–75 lb. per square inch). A complete disintegration of pine wood is effected in about twenty-four hours’ treatment. The yield of pulp is, we are informed, from 40–50 per cent. of the wood (Pinus sylvestris); it is also stated to be readily bleached {71} with a moderate amount of bleaching powder, and to yield a pulp of the highest quality. Further, the recovery of the sulphurous acid, for which special apparatus is provided, is practically complete (95–98 per cent.). In view of the probable utilisation of the bye-products, it is interesting to observe that they are obtained in a form in which they can very readily be treated.

The process being as yet undeveloped on a large scale, we cannot do more than recommend it to the consideration of paper-makers. At the same time we would draw attention to the features of interest which it possesses in regard to the general aspect of our subject. The resolution which it effects is obviously one of simple hydrolysis, and it affords an indirect proof of the chemical complications which attend all other processes for disintegrating wood which employ temperature much above the boiling-point of water. The soluble bye-products are obtained in the simplest form, and may be isolated by mere evaporation. As regards the chemical features of the pulp, they will presumably be different from those of any other, whether on the side of advantage or not it remains to decide. Provided the process fulfils the condition of economical working, we have no hesitation in predicting for it a considerable development.

The difficulties attending the employment of a gaseous acid have doubtless operated in deterring many, in addition to the originator, from a complete investigation of its action, and have led them to adopt in preference one of its several compounds with bases, the employment of which defines a group of processes to which we now desire to call attention. There is a variety of processes agreeing in the essential feature of employing a bisulphite as the disintegrating reagent, and differing from one another only in the less essential details of method.

The general principle of these processes will be readily understood from what has gone before. The chief agency is the hydrolytic action of sulphurous acid, aided by the conditions of high temperature and pressure: and the subsidiary {72} agencies are—(1) The prevention of oxidation; (2) The removal from the sphere of action of the soluble products of resolution in combination with the sulphite as a true double compound. This mode of union is well known to be peculiarly characteristic of all aldehydes, to which class we have shown that the non-cellulose constituents of wood belong. The combination is expressed by the general equation—

(3) The removal of a portion of the constituents in combination with the base, i.e. with expulsion of sulphurous acid.

The verification of these views has been afforded in the course of an investigation of the bye-products from the Ekman process, undertaken by the authors for Messrs. Thomson, Bonar & Co., the proprietors of the patent.

As the various bisulphite processes differ chiefly in their working details, the essential feature of the employment of a solution of sulphurous acid, combined with a certain amount of base, being common to them all, it will not be necessary to do more than examine one or two typical cases. We shall select those of Ekman and Partington, the latter being based upon Francke’s patent, with certain modifications and additions.

(1) The Chemicals employed.—In both cases sulphur is used as the source of the sulphurous acid. The Ekman process starts from magnesite, a native carbonate of magnesia. This is converted by calcination into magnesia, and in this state is introduced into lead-lined towers, through which is passed sulphurous acid gas, obtained by burning sulphur in suitable furnaces, while a stream of water trickles down from the top. The supply of the gas being properly adjusted, a continuous production of a solution of magnesium bisulphite of uniform strength is obtained. To secure this uniformity, constant attention is necessary, and further, the loss of sulphurous acid by oxidation to sulphuric acid, and by diffusion, is, in consequence of its distribution over so large a surface, {73} and with free exposure to the air, necessarily very large. It is unnecessary, however, to point out that the preparation of the magnesium bisulphite is capable of very considerable modification; in fact, we understand that numerous improvements in this direction have already been introduced by the proprietors of the process.

Partington’s method, in addition to the advantage of starting from lime, the cheapest of all basic substances, is free from the disadvantages referred to as attending the use of a tower. It consists in passing the sulphurous acid gas through a series of three vessels connected together with pipes. Into these vessels a charge of milk of lime is introduced, and the two first being closed air-tight, the gas is passed in. The third remains open to the air, and from it escapes a continuous stream of nitrogen, resulting from the removal of the oxygen by the burning sulphur, from the stream of air which is blown through the sulphur retort at a pressure of 5 lb. to the square inch. The expenditure for chemicals is under 1l. per ton, whereas, by the Ekman process, it has been variously estimated at from 2l. to 3l. 10s. per ton. Provided therefore, that as good pulp can be obtained by Partington as by Ekman, the former offers considerable advantages both from point of view of simplicity and economy. Doubtless the cost of chemicals could be somewhat reduced in both systems by the adoption of a cheaper form of sulphur, such as pyrites.

Preparation of the Wood.—Although the acid sulphite processes can, by suitable modifications, be made available for the preparation of cellulose from almost all kinds of wood, yet it is necessary, in order to insure uniformity of result, not only to take care that the wood is carefully selected, but that it is freed from bark and roots, which, if allowed to remain, would be imperfectly pulped, and would be exceedingly difficult to bleach. The most suitable kind of wood is Pinus sylvestris.

After the bark has been carefully removed the logs are sawn into boards of ordinary sizes. This treatment {74} exposes the knots, which are then removed either by boring or cutting out with an axe. The pieces are then passed through a machine, which cuts them or rather breaks them into fragments about an inch long. In order that the acid sulphite may as quickly as possible enter into the wood, these fragments must be crushed and opened out by passing through heavy rollers: moreover, the wood is cut across the grain, by which means it is more or less split up into laminæ. A machine for this purpose has been patented by Fry, Ransome, and Wilkie (No. 11,389, 1884).

The Boiling Process.—Ekman uses a lead-lined, jacketed cylindrical boiler, suspended on trunnions, so that it can be turned upside down for discharging. The pressure in the outer jacket is 70 lb. per square inch, while inside the boiler it is 90 lb., the difference being due to the tension of the sulphurous acid gas. The duration of boiling is from ten to twelve hours. Partington, on the other hand, boils with live steam, and therefore is enabled to use a boiler of simpler construction. His boilers are spherical, lead-lined and rotary. The pressure employed does not exceed 60 lb. per square inch, the time of boiling being about sixteen hours.

Various other sulphite processes have been from time to time devised, which differ but little either in principle or in working detail from those described, and therefore a brief mention of them will suffice. Mitscherlich employs a bisulphite of lime in somewhat weaker solution than that employed by Francke or Partington, and in order to produce the same result he is obliged to extend the time of boiling to from 40 to 60 hours.

Graham, after treating the wood with a mono-sulphite, injects into the boiler either a solution of a bisulphite or gaseous or liquid sulphurous acid.

Flodgvist uses a solution prepared by dissolving bones in sulphurous acid solution.

The cellulosic products of the various processes present certain slight differences, but so far as has been investigated, not such as to constitute a distinct advantage for one or {75} another. One feature of these processes, to which but little attention has hitherto been paid, is that of the utilisation of their bye-products. These bodies are, as already indicated, obtained in a condition not materially different from that of the parent substance, the non-cellulose constituents of the wood. One important property of these bodies, and one capable of considerable application, has been made the basis of a patent by the authors. When a solution of gelatin is added to the liquors in which wood has been treated by any of the bisulphite or sulphurous acid processes, a body is precipitated in the form of a caoutchouc-like mass which can be dissolved in neutral sodium sulphite solution, or in weakly alkaline solutions, and can be reprecipitated on addition of alum to the solution. The substance consists of a compound of the gelatin with the non-cellulose constituents resembling, but not identical with, as has been supposed, the precipitate obtained by adding tannic acid to a solution of gelatin. The solution of this substance in sodium sulphite can be applied to many of the purposes to which gelatin itself is ordinarily applied.

A serious objection to all processes involving the use of sulphurous acid or acid sulphite, and one which has to some extent deterred paper-makers from adopting them, is the necessity of using a boiler lined with lead. In addition to the fact that such boilers are heavy and costly, they are objectionable on account of the difficulty and expense of keeping them in thorough repair. This difficulty arises from the fact that, owing to the different co-efficients of expansion of lead and iron, the lead lining originally placed in contact with the iron or steel shell of the boiler, becoming detached on heating, swells out in various places, which are thus rendered weaker and liable to fracture. This is especially the case if, in the course of lining the boiler, any air has been allowed to remain between the two linings. Various expedients have been proposed to obviate these defects but with only partial success. Graham (Eng. Pat. No. 5367, 1883) coats the plate of his boiler by pouring molten lead upon the perfectly clean surface which has been {76} previously prepared with zinc chloride. Sometimes minute holes are drilled in the iron plate to allow the escape of air.

Notwithstanding all the attempts that have been made, the use of a lead-lined boiler is a serious objection, and any process which will fulfil the conditions on which the bi-sulphite processes depend, and which at the same time admits of the use of the ordinary iron boilers, is an obvious desideratum. Such a process is that patented by one of the authors (Cross, No. 4984, 1880), claiming the use of solutions of the neutral sulphites with or without the further addition of an alkali. This process is very suitable for the treatment of fibres such as straw, jute, &c., but not economically applicable to the preparation of wood pulp on account of the cost of chemicals. The preservative effect of the sulphite is shown by the largely increased yield of pulp.

The chief advantages of the various bi-sulphite processes for the preparation of wood pulp over the old soda processes are, (1) the increased yield of fibre; (2) the preservation of its original strength, and (3), the economy in chemicals.

It is well known that cellulose, even in its most stable forms, is attacked more or less by hot solutions of the caustic alkalis, and is more or less profoundly modified according to the strength of the solution and the temperature at which it is employed; while on the other hand the solutions of the bisulphites affect it but little. Thus, the authors have exposed jute cellulose, which is of the same nature as that obtained from wood, to the action of magnesium bisulphite under the conditions which obtain in the Ekman process, without its undergoing sensible loss of weight, and further, they have determined the amount of cellulose in freshly cut grasses, in which it is but imperfectly elaborated, and therefore very susceptible of modification, both before and after treatment by the process, and in each case the percentage obtained was the same.

There is therefore no doubt that in the bisulphite processes, even the structurally delicate forms of cellulose are conserved. As before indicated, the influence of this upon the yield of cellulose is of course very marked; the yield from white pine, {77} by the soda process, being approximately 33 per cent., whereas by the Ekman process it is from 45–50 per cent. It is important to remember that this increased yield of cellulose cannot fail to affect the quality, and therefore the paper-making properties of the pulp. Papers made entirely by the bisulphite processes have a strongly marked individuality, being distinguished by their hardness and transparency. In regard to the bleaching of the pulp, it will be found that some practical experience is required for the control of the operation, so as to produce uniform results.

Although the pulp produced by these processes is of a very light-grey colour, so light indeed that they may be used for certain low classes of white paper without further treatment, yet to obtain a pure white colour almost as much bleaching powder is required as in the case of soda pulps, although the latter are much darker in colour. The chief points to be observed are (1) the complete removal of the sulphites from the pulp before the addition of bleaching liquor, which would otherwise be ineffectively consumed; (2) the avoidance, in washing, of waters containing iron, which would combine with certain constituents of the pulp to form dark-coloured bodies.

The following Table, compiled from Dr. Hugo Müller’s ‘Pflanzenfaser,’ is of interest, as indicating the probable yield of pulp from different woods, compared with the amounts obtained in actual practice by the Watt and Burgess (soda) process:—

The following Table, which gives a concise account of the various typical processes that have been employed for the preparation of wood pulp, will be readily understood in connection with the theoretical considerations previously discussed:— {78}

CHAPTER VI. SPECIAL TREATMENT OF VARIOUS FIBRES; BOILERS, BOILING PROCESSES, ETC.

We have already, in Chapter V., discussed in a general way the principles upon which the treatment of the various fibres should be based; we now proceed to consider each fibre in detail, giving at the same time such information as is necessary regarding the preliminary treatment of the various fibres and describing the forms of apparatus in which these operations are conducted. We shall consider the fibres in the order of their simplicity of treatment.

The two former consist of the cuttings produced in the manufacture of various garments, &c.; not having been worn, they are usually free from dirt, and are, in fact, if bleached, nearly pure cellulose, containing only the starch and other sizing material which has been added in the process of finishing the goods. They may, of course, contain considerable quantities of china clay or other loading material. In purchasing rags, therefore, regard should be had to the probable presence of these bodies. Such rags require only a very slight treatment; in fact, for certain classes of paper they might be used without any preparation. If, as is sometimes the case, the rags are unbleached, a rather more severe boiling is necessary. {80}

FIG. 16.

The greater part of the rags used in paper-making, however, consist of the residual portions of garments, household linen, &c., which vary in quality from clean, almost unused portions, to the very foulest sorts; the latter require a very drastic treatment. The first thing to be done with the rags is to “sort” and cut them into convenient pieces. This is usually done by women, who stand at tables furnished with broad knives firmly fixed into them, with the backs towards the worker, and inclined at a slight angle. Near to the women are placed a number of boxes, corresponding with the number of qualities of rags, lined at the bottom with coarse wire gauze, into which the different sorts are put. The distinctions made are purely arbitrary, but as a general rule the rags are sorted with special reference to their colour and the material of which they are composed. The coloured rags may be allowed to accumulate, and then made into a coloured paper. During the process of cutting, all hard substances, {81} such as buttons, pieces of iron, &c., are carefully removed. The rags are cut into pieces of from two to five inches square. In some mills machines are used for cutting.

Fig. 16 shows the construction of a machine that may be used either for rags or rope. The material is passed into the machine along the table A, where it passes between the stationary knife C and the knives B, fixed in the revolving drum D. The cut rags fall into a receptacle placed underneath the drum.

Notwithstanding the extra expense of cutting by hand, it is nevertheless preferred by many, especially for the finer grades of paper. One reason for this is to be found in the fact that more perfect sorting and removal of impurities can be effected. It is moreover said that less waste of fibre occurs in the subsequent operations.

FIG. 17.

The next process which the rags undergo is that of removing all loose extraneous matter. This may be done in a machine such as is shown in Fig. 17. The rags are fed continuously by the endless travelling platform A into the willow. This consists of two wrought-iron drums, B B, furnished with wrought-iron teeth C, which when the drums revolve pass rapidly near to the stationary teeth fixed into the cast-iron framework of the willow. The sides are covered in with cast-iron doors, and the top is covered over {82} with sheet iron. Underneath the drums is a grating for the escape of dust.

The rags, having been thoroughly beaten by the teeth of the drums, pass into the duster D, consisting of a kind of hollow cylinder E, made of strong iron bars securely fastened to the circular ends F, the bars being covered with wire cloth or perforated zinc. It is made to revolve almost horizontally, a slight dip being given to it in order that the rags may be carried forward to the lower end. The bars or skeleton of the cylinder are furnished with a number of teeth securely bolted on. The whole is enclosed in a strong wooden box in which it revolves, and which serves to collect the dust passing through the wire cloth. The cylinder E is driven by the gearing G. This combined willow and duster may be used for dusting and cleaning not only rags, but almost any other kind of fibre. The willow and duster sometimes form two separate machines.

The cleaned and dusted rags are now ready to be boiled, although it is the practice in some mills to give them a preliminary washing with water.

The boiling may be conducted either in spherical or cylindrical boilers, or in the ordinary vomiting boilers described under Esparto. (See p. 92.) In Fig. 18 is shown, part in section and part in elevation, a spherical rag-boiler, as manufactured by Messrs. G. & W. Bertram.

This boiler, of from 8 to 9 feet diameter, is supported by means of the hollow journals A on the standards B, and is made to revolve by means of the gearing C.

Steam enters by the pipe D, which is fitted with a safety-valve E, and vacuum valve F. The steam on entering the boiler is distributed by means of the “baffle-plate” G. Lye enters by the pipe H. The boiler is fitted with two doors I, wherewith to fill and empty; the waste lye is run off by the cock J. L is a small blow-through cock. The false bottom K prevents the rags from choking up the cock and also serves to drain off the waste liquor.

CHAPTER VII. BLEACHING.

The methods of bleaching being essentially the same for all pulps, it will only be necessary to describe one in detail.

The process depends upon the use of a solution of calcium hypochlorite, prepared by dissolving bleaching powder in water. This solution is best prepared in a vessel provided with stirrers or agitators, whereby a thorough mixture of the bleaching powder and water is obtained. Bleaching powder always contains a certain amount of free lime and calcium carbonate, which remain undissolved in the water. This should be allowed to settle to the bottom of the vessel, and the clear liquor run off. The residue may then be again treated with water, or with weak liquor obtained by washing the residue. If the washings are too weak to be conveniently used for bleaching, they may be stored in a separate tank and used for dissolving fresh powder. In this way a strong stock solution can be readily prepared, the powder at the same time being thoroughly exhausted. The washing should be repeated as frequently as possible, after which the residue may be allowed to drain. This operation is best conducted on a filter bed provided with an air-pump similar to that described in Chapter XII. The residue is obtained in this way as a hard cake, containing about 60 per cent. of water. If the above operations have been properly conducted, it should not contain more than about 0·25 per cent. of chlorine.

The original powder should contain from 33·0 to 35·0 per cent. of “available chlorine,” that is, chlorine which is effective in the bleaching process. {111}

Owing to the ease with which it decomposes it should be kept cool and not exposed to the air.

If possible, one or more large store tanks should be provided for the strong bleach liquor, so that time may be given for the complete separation of the insoluble portions. The liquor can be drawn off with a “siphon” pipe without disturbing the residue.

It is of the utmost importance that only clear liquor be used, as a milky solution of calcium hypochlorite is much more sluggish in its action than one which is perfectly clear and bright.

A convenient strength for the stock solution is 6° Twaddle: this corresponds to about half a pound of bleaching powder to the gallon.

The method of testing the powder and its solution will be described in Chapter XIV.

The washed and broken pulp is placed in the “potcher” together with the necessary quantity of bleaching liquor and as much water as is required to produce complete circulation of the mass. In many mills the breaker itself answers the purpose of a potcher also. The quantity of water should be kept as low as possible, as it is found that by the use of strong solutions less bleaching powder is required than with weak solutions; moreover, less time is required to produce the effect. On the other hand, with very strong solutions, the pulp is liable to be injuriously affected.

Straw and esparto pulps are sometimes bleached in large potchers made of brick covered with cement. The circulation of the pulp is produced by the action of paddles made of wood or, preferably, of iron.

Many “potchers” are provided with steam pipes, whereby the mass of pulp and liquor is heated. This should be done with great care, so as to avoid superheating in any part, as this is certain to cause destruction of the fibre. The better plan, where it can be adopted, is to uniformly heat the pulp before running in the bleach.

In certain cases the action of the calcium hypochlorite is {112} assisted and accelerated by the addition of either sulphuric or hydrochloric acids; these combine with the lime and liberate hypochlorous acid, which has a more rapid action than its calcium salt. If the acid be added in too large quantity, or of too great a strength, it sometimes happens that instead of hypochlorous acid, chlorine gas is given off, part of which escapes into the air, thus causing loss. Moreover, the chlorine is liable to enter into combination with the fibre substance forming the yellow chlorination products described on p. 18.

This liability is greater in the case of highly lignified fibres, such as wood or jute: in these cases, therefore, the use of acids should be avoided. When used the acid should be largely diluted with water and added gradually. The best plan is to allow the action of the bleach to continue for some time, only adding the acid when it is nearly exhausted. In this way risk of damage to the pulp is avoided.

The diluted acid should be conveyed by a leaden pipe passing down to the floor of the potcher, and perforated at its lower extremity.

Other means of accelerating the action of the bleaching powder have been suggested, such, for example, as the use of bicarbonate of soda, which by double decomposition forms with the bleaching powder carbonate of lime and free hypochlorous acid. It is, however, much too expensive an agent.

The amount of bleaching powder necessary to produce a good white colour differs materially with the fibre to be bleached, and of course with the degree of resolution of the fibre substance in the preliminary treatment with caustic soda.

The following numbers may be taken to be approximations of the amount necessary to bleach well-boiled pulps:—

A well-boiled pulp should contain from 90 to 95 per cent. of cellulose.

Very excellent results are sometimes obtained by substituting sodium hypochlorite for the calcium compound. This is done by decomposing a solution of bleaching powder with an equivalent quantity of either sodium sulphate or sodium carbonate. The precipitate of calcium sulphate or carbonate is allowed to settle and the clear liquor run off. Such a solution is more sluggish in its action than one of calcium hypochlorite, but it is more easily controlled, and is less liable to injure any material treated with it.

The time necessary to produce a perfect bleach depends on the nature and condition of the pulp, on the degree of concentration of the liquor, and on the temperature at which the operation is conducted. Some paper-makers prefer to extend the bleaching over a considerable period of time, but this involves the employment of a larger system of “potchers,” which in many instances is an objection. If this method is adopted, especially in the case of straw or esparto, the action of the paddles or roll should be stopped, as otherwise the fibres are liable to felt together in the form of small grains, which sometimes escape the action of the beating roll, and occasionally find their way into the paper, causing an unevenness of surface. By giving longer time to the bleaching there is less risk of damage to the fibre, and consequently a greater yield of bleached pulp.

The action of the bleaching solution should, as far as possible, be confined to the non-cellulose portions of the fibre. A certain action on the cellulose itself invariably takes place, but it should be reduced to a minimum. The action of bleach upon cellulose has been indicated in Chapter I., p. 10. For further information on this subject the reader is referred to the ‘Journal of the Society of Chemical Industry,’ 1884, April 29 and May 29.

A very good method of bleaching consists in a preliminary treatment in the ordinary way in the potcher, followed by a prolonged steeping in tanks. {114}

In the case of pulps which are very difficult to bleach, the action can be greatly assisted by washing out the products of the bleaching action, treating the pulp for a short time with a weak alkaline solution, again washing, and then repeating the bleaching process. In this way very refractory pulps can easily be bleached. Even an intermediate washing with water is useful.

In most cases, where a fairly good colour has been obtained by the use of bleaching powder alone, the colour is greatly improved by a souring or treatment with weak hydrochloric acid, or, better still, with solution of sulphurous acid.

Or the bleaching may be conducted in two or more operations with intermediate treatments with acid.

All these modifications, however, are more or less troublesome, and should only be resorted to when the ordinary method fails.

After bleaching, a certain quantity of calcium hypochlorite always remains in the pulp. It may be removed in the potcher itself, or when the pulp reaches the beater. In any case, the methods adopted are the same. (See p. 127.)

In some mills it is the practice to free the bleached pulp from the products of the decomposition of the bleach by pressing the pulp in hydraulic presses, or by draining in vats provided with perforated bottoms. Or the pulp may be made to pass through a pulp-saver (see Fig. 52, p. 155.)

Chlorine gas as a bleaching agent has been almost entirely superseded by the more manageable calcium hypochlorite. Its employment is open to the serious objection indicated before, viz. the liability to form difficultly removable chlorine compounds. Its use as a disintegrating agent has already been alluded to (see p. 102).

A process has lately been invented (English patent, No. 11,333, 1884), by Prof. Lunge, which involves the use of acetic acid. The quantity required is, however, very small, as during the process of bleaching it becomes regenerated. The free lime in the bleaching powder solution should first be {115} nearly neutralised with a cheaper acid, such as hydrochloric or sulphuric, followed then by the addition of the acetic acid. This process, we are informed, gives excellent results with high-class material, such as the best cotton and linen rags; it is, however, not to be recommended for such materials as straw or esparto.

For the bleaching of rags the process invented by Thompson (English patent, No. 595, 1883) has proved very effective. The method consists in saturating the material with a weak solution of bleaching powder, and then exposing them to the action of carbonic acid gas. In this way the bleaching action is made very rapid and effective.

The electrolysing vat consists of a rectangular vessel of slate or other suitable material. The negative electrode may be constructed of zinc; for the positive it is necessary to employ platinum.

The electrolysed solution has been found to possess very remarkable properties, which have considerable bearing upon the economy of the process. If a solution be taken of equal {116} oxidising efficiency with one of calcium hypochlorite, as indicated by the arsenious acid test (see p. 206), it is found that the former possesses greater bleaching efficiency than the latter in the proportion of 5 to 3. Moreover, the bleaching is much more rapid, and the loss of weight which the substances undergo is less, for equal degrees of whiteness obtained.

It has been shown that by the electrolytic method the bleaching of paper pulp can be effected at nearly one-half the cost of bleaching powder. The process is now being industrially carried out. Those interested in the subject are referred to a paper by the authors in the ‘Journal of the Society of Chemical Industry’ for April 1887.

CHAPTER VIII. BEATING.

The bleached pulp is now ready for the final treatment. If it were attempted to make paper from the pulp in the state in which it leaves the “potchers” or “steeping” chests, it would be found to be wanting in evenness of texture and uniformity of surface. This result can only be secured by an elaborate process of comminution or disintegration. This is in effect the result produced by the action of the “beaters.”

They resemble in general appearance the breaking engines previously described; the roll, however, carries more knives, and it is usually let down much nearer to the bed-plate. In the case of fibrous substances, whose ultimate fibres are relatively short (see table, p. 39), it is only necessary to split up the filaments into their constituent fibres: esparto, straw, and wood are of this class. In the case of straw, the disintegration is for the most part ac­com­plished in the boiling and bleaching processes, and therefore but little work devolves upon the beater. Esparto and wood require a certain amount of beating, but this should be regulated to the drawing asunder of the individual fibres, any cutting of the fibres being carefully avoided. This is ac­com­plished by adjusting the distance of the roll from the bed-plate, so that by the friction of the fibres upon themselves, when passing over the plate, a kind of rubbing or “brushing” is produced. If a carefully-made paper of esparto or wood be examined by the microscope, it will be found that the majority of the fibres preserve the pointed or slightly rounded ends {118} characteristic of bast cells. On the other hand, it is obvious that cotton, whose ultimate fibres have a length of 20 to 40 mm., and flax (25–30 mm.), with other similar fibres, will require to be broken up into short fragments in order to develop to the fullest the felting property of the pulp. Not only are the fibres reduced to the most favourable dimensions, but in cotton and linen a further contributory advantage accrues; for on account of the internal structure of the ultimate fibres they tend to split up at the point of rupture into a number of fibrillæ, which, in the case of cotton, take the form of a network; and in case of linen, are seen as a bundle of distinct fibres parallel to and continuous with the fibre. This gives to ends of the fragments a ragged contour, which has considerable influence on the felting power of the pulp, and therefore on the strength of the paper into which it is made.

With these fibres, therefore, the “cutting” as distinguished from the “breaking” action should be avoided as much as possible, otherwise the effect described above will not be produced, and the fibres will show instead a clean cut. The appearance shown by cotton and linen pulp, when thoroughly “beaten” and ready to be made into paper, is given in the frontispiece, which is taken from the author’s micro-photographs.

The “half-stuff” is furnished to the “beater,” or “beating” engine as it is sometimes called, previously partially filled with water. The furnishing is done in successive portions, the first being allowed to mix thoroughly with the water before another lot is added. This is continued until the mass is so thick that it will only just turn round in the beater under the action of the “roll.” Owing to the construction of the beater it frequently happens that a portion of the pulp lodges in the corners, from which the beaterman removes it by means of a wooden paddle, which also serves to push forward the pulp to the roll in case the motion is inclined to be sluggish. The proportion of water to pulp should not be too high, otherwise the beating is not so effective; at the same time, {119} if the mass is allowed to get too thick, imperfect circulation results.

The operation of “beating” occupies a considerable time, and consumes a large amount of power. Cotton and linen rags naturally take longer—in some cases as much as ten hours is given to the operation.

Esparto, on the other hand, can be sufficiently disintegrated in from two to four hours.

Wood pulp requires very gentle beating; it is therefore necessary to prolong the time to about six hours.

These differences in the duration and method of beating should be borne in mind when pulps of different natures are mixed together in the same beater, as is frequently the case. It is better, with very dissimilar fibres, to “beat” each separately, and only to mix them in the stuff-chests. This, however, is open to the objection that the pulps may be insufficiently mixed.

The length of fibre to which pulps should be reduced depends to some extent upon the kind of paper to which it is to be applied. The authors have examined a number of papers by well-known makers, and find the dimensions in millimetres of various pulps to be as follows:—

Wood and straw pulp, when imported in the form of dry sheets, may, before being beaten, be conveniently disintegrated and thoroughly mixed with water by means of the edge-runner described under “Broke Paper,” p. 104, Fig. 28.

In making the finer kinds of paper, the roller bars or knives, instead of being made of steel are made of bronze, thus any contamination with oxide of iron is avoided. This is especially liable to take place in case of steel knives when the beater has been allowed to stand for some time. {120}

The inside of the heater itself is often lined with lead, a material which is not liable to oxidize, and which can very readily be cleaned.

When a beater has been running for some time the knives of the roll and the bed-plate become worn and so far reduced that they must be taken out and re-cut. The bed-plate is removed, firmly fixed in the bed of a planing machine, and the edges trimmed by means of a chisel, so as to cut the knives at the proper angle.

The roll and bed-plate are shown in section in Fig. 32. Fig. 33 is a plan of a bed-plate, and Fig. 34 illustrates the manner in which the knives are fixed.

FIG. 32.

It will be seen that the knives in the bed-plate are placed so that they do not lie parallel with those of the roll. This arrangement imitates to some extent the action of a pair of scissors. Occasionally the knives are slightly bent, so as to form a very obtuse angle. Bed-plates so fitted are called {121} “knee-plates.” They are largely used in America, but not much in this country.

FIG. 33.

FIG. 34.

To obviate the necessity of removing the roll, a small machine has been devised whereby the knives can be cut in situ. This machine, which can be firmly fixed between the mid-feather and the side of the beater, consists of a small steam-engine which actuates a movable cutter, which is made to pass to and fro horizontally along the edge of each knife in succession. The engine is supplied with steam by a piece of strong flexible rubber tubing.

The ordinary form of beater contains only one roll, though some have been constructed containing two or even four rolls. In America, beaters of a totally different construction are much in vogue. The most important of these are the Jordan and Kingsland beaters, so called from the names of the inventors. The former consists essentially of a roll in the shape of a truncated cone, fitted with knives in the usual way, revolving in an iron box of corresponding shape, furnished with knives placed in the direction of its length, but at slightly different angles. The half-stuff enters the beater at the narrow end through a box provided with an {122} arrangement for regulating the flow, and is discharged by two or more openings in the cover at the wider end.

The Kingsland engine consists of a circular chamber, the sides of which are covered with knives, and between which a circular plate, also furnished with knives, revolves. The pulp enters through a pipe in the centre of one of the sides of the chamber, and flows out through an opening in the opposite side.

Another form of beater is that invented by S. L. Gould. The only essential difference between it and the Kingsland beater is that, instead of having a plate which revolves vertically against two stationary ones, the plate, which is placed horizontally, is furnished with knives on one side only, and revolves upon but one fixed plate, much in the same way as a pair of millstones.

The half-stuff supplied to these forms of beater is generally disintegrated to a greater extent in the breaker than is the case with those of the ordinary construction, as it is necessary to make it flow easily through them, and this could not be done if the fibres were kept long. A single passage of the pulp through the beater is sufficient to beat it up ready for making into paper. The chief advantages claimed for them is that they are more economical, both of time and power; also that the pulp is more regularly beaten.

Certain new forms of beaters have lately been introduced in this country, and are rapidly gaining in favour, chiefly on account of the saving of driving power, and the space occupied, compared with the amount of “stuff” they are capable of beating. Fig. 35 shows the construction of Forbes’ patent engine, as manufactured by Messrs. G. & W. Bertram, of Edinburgh. This beater is divided into three channels. The rolls, one of which is shown uncovered, are placed in the outer channels, while in the centre one the mixing and circulating wheel a is placed, the peculiar construction of which enables it to throw the pulp alternately into the two outer channels, from whence, after passing under the rolls again, it enters the central channel at the other end of the beater. {123}

In Umpherston’s patent engine great economy of space is effected by causing the pulp to travel over and under the backfall (a, Fig. 36). Another advantage is that the stuff circulates freely with less water than in the ordinary forms, thus increasing its output. It is also claimed that the pulp is beaten with less power, and as it is less liable to lodge in corners it is more evenly beaten.

FIG. 35.

The construction of the engine will be readily understood by reference to Fig. 36. The direction which the stuff takes is indicated by the arrows.

Cooke and Hibbert’s beater, as manufactured by Messrs. Masson, Scott and Bertram, resembles, to a certain extent, Gould’s engine, previously described; it differs from it, however, in that the discs are placed vertically, instead of horizontally. Moreover, the general construction of the beater more nearly resembles those in ordinary use. Fig. 37, which shows it in plan and in section at the line D E, illustrates its construction.

CHAPTER IX. LOADING, SIZING, COLOURING, ETC.

The bleached half-stuff as it leaves the steeping chests usually contains an excess of bleaching liquor, which can be removed in two ways, viz. by washing or by decomposition with an “antichlor.” The first method has the advantage of not only removing the bleach, but of also eliminating the chloride of calcium, partly existing ready formed, and also that resulting from the decomposition of the calcium hypochlorite originally present in the bleach. On the other hand, this method takes some time, and consumes a large amount of water, which in some mills is a matter of considerable importance. For this purpose, many beaters are provided with one or more drum washers (see Fig. 36). An additional objection to this method lies in the fact that a certain quantity of fibre passes through the meshes of the wire-cloth covering the washers, and is thus lost.

The more usual plan is to remove the bleach by decomposing it with an “antichlor.” The substance generally employed for this purpose is sodium hyposulphite, or thiosulphate as it is now called, which, in presence of calcium hypochlorite, is oxidized to sodium sulphate, the latter being reduced to calcium chloride. Double decomposition then takes place between these salts, with the formation of calcium sulphate and sodium chloride. The reactions which take place may be expressed by the following equation:—

The above decomposition does not accurately represent the action of bleach upon sodium thiosulphate. If the solutions employed are very dilute, the decomposition may take place in another direction, viz.:—

At the particular degree of dilution which occurs in a beater, the bleach is decomposed almost entirely according to the first of the equations; from which, on calculating, it will be seen that 158 parts of sodium thiosulphate are equivalent to 286 parts of calcium hypochlorite. As commercial sodium thiosulphate contains 36·3 per cent. of water, and bleaching powder 70 per cent. of calcium hypochlorite, on the basis of 35 per cent. available chlorine it follows that 248 parts of the former are required to neutralize 409 parts of the latter.

Within the last few years other forms of “antichlor” have been introduced, such, for example, as the various sulphites. The most important of these is sodium sulphite, which has been manufactured by a patent process at a cheap rate, by Gaskell, Deacon, & Co., Widnes. Their product contained as much as 75 per cent. of Na₂SO₃, ordinary crystallized sodium sulphite containing only 50 per cent.

Sulphites are converted by the action of bleach into sulphates, thus:—

From this equation, it will be seen that 252 parts of sodium sulphite will neutralize 143 parts of calcium hypochlorite, or 204·3 parts of bleaching powder. Assuming that crystallized sodium sulphite contains 50 per cent. Na₂SO₃, the same amount of bleach would require 504 parts. As Messrs. Gaskell, Deacon & Co.’s sulphite contains 75 per cent. Na₂SO₃, only 336 parts are needed. Comparing these numbers with {129} those given above for sodium hyposulphite, it will be seen that 204·5 parts of bleach require for neutralisation 129 parts of sodium thiosulphate and 504 parts of crystallised sodium sulphite, or 336 parts of the stronger product.

Sodium sulphite is preferred to sodium thiosulphate by some paper makers, notwithstanding the fact that even in its most concentrated form, nearly three times as much is required to produce a certain result. It is said that when it is used the wire cloth of the machine is preserved for a longer time than if sodium thiosulphate is employed. This may be due to the fact that with the latter a certain amount of free acid is always formed, which of course would act injuriously on the wire; whereas, when sodium sulphite is used, the products of decomposition are neutral salts without any action upon metals. (See the above equations.)

A very cheap “antichlor” may be prepared by boiling together lime and sulphur. One hundred and sixty-eight parts of lime, made into a milk with water, are heated to boiling in an iron vessel. Three hundred and eighty-four parts of flour of sulphur or ground sulphur are then added in small quantities at a time, and the boiling continued until the whole is dissolved. The liquid which is now of a deep yellow colour, is allowed to settle and cool and is then ready for use. It contains a mixture of calcium thiosulphate and calcium pentasulphide, the latter compound giving to it its deep yellow colour. The following equation represents the action which takes place between the lime and the sulphur:—

The decomposition which takes place when the mixture is acted upon by calcium hypochlorite may be represented as follows:—

From this it is seen that a large quantity of free sulphur is formed, which is precipitated as a fine yellowish powder in the fibre. This not only affects the colour of the pulp, but is objectionable on account of its liability, in contact with the hot cylinders of the paper machine, or by the action of moisture over a long period of time to become oxidised to sulphuric acid, which undoubtedly, even if present only in very small quantities, has an injurious effect upon the finished paper. (See hydracellulose.) This property of sulphur and its action upon cellulose may be shown by the following experiment. Take a piece of water-leaf paper, rub into it a mixture of flour of sulphur and water. If the paper thus treated be rapidly dried, it will be found to be weakened or even slightly charred in those portions which have been in contact with the sulphur. (See also Chem. Soc. Journ., p. 249, 1879.)

Whichever variety of “antichlor” is used an excess should be carefully avoided, as all act more or less upon the size and colouring matter added to the pulp subsequently. The proper method is to run in small quantities of a solution of the antichlor at a time, testing the pulps, after allowing a few minutes for complete admixture after each addition. The test is made by immersing a piece of iodide of potassium and starch paper, which will be turned blue so long as any calcium hypochlorite is present. These papers are made as follows:—

Three grms. of starch are ground up with a small quantity of water and poured into 700 cc. of boiling water, and to the solution 1 grm. of iodide of potassium and 0·5 grm. carbonate of soda are added. Sheets of white paper, water-leaf in preference, are now soaked in the solution and dried. In presence of calcium hypochlorite, iodine is liberated from the potassium iodide, and acting upon the starch forms with it a characteristic blue colour.

China clay or kaolin is sold in the form of large lumps of a white or yellowish-white colour. It is formed by the gradual disintegration of felspar by means of the action of air and water, and consists essentially of a silicate of aluminium. Its quality depends upon its whiteness and its freedom from coarse micaceous particles. It is usually prepared for admixture with the pulp by making it into a fine cream with water in a vessel provided with stirrers; it is then passed through a fine sieve in order to remove any impurities it may contain, and is then run into the beater. The clay or other filling material is usually run into the beater as soon as the latter is charged with pulp, so that by the time the beating operation is concluded, a perfect admixture of pulp and clay is effected.

Sulphate of lime, or “pearl-hardening,” is usually sufficiently pure to put direct into the engine. It is made by decomposing a solution of calcium chloride, with sulphate of soda, and is precipitated as a fine brilliantly white powder, consisting of CaSO₄ + 2 H₂O.

Two distinct forms of precipitated calcium sulphate are met with in commerce, differing from each other by their microscopical features, the one consisting of flat tabular crystals (Fig. 41), the other of fine needles (Fig. 42). Another form, erroneously called precipitated pearl-hardening, is also sold: it consists of the finely ground native mineral.

FIG. 40.

FIG. 41.

Some of the finer qualities of paper are made without addition of any loading material whatever, though such papers are of course the exception. The proportion of clay or {132} other material that can be put into a fibre depends to a certain extent upon the nature of the fibre, and upon the degree of fineness to which it is reduced in the beater. The amount added by different makers varies considerably, from {133} two or three per cent. to twenty, and even in rare cases to thirty per cent.

FIG. 42.

FIG. 43.

A new loading material called “agalite” has been lately introduced, possessing certain advantages over china clay, {134} or calcium sulphate. Agalite is a mineral of the nature and chemical properties of asbestos: it consists of nearly pure magnesium silicate. Its structure is more or less fibrous, like that of asbestos, which, as is well known, can be spun and woven and even made into paper, and it therefore, when added to a paper, forms a part of the fabric itself. It is even claimed that it assists in keeping back some of the finer fibres that invariably find their way through the meshes of the wire cloth, and it is said that 90 per cent. of the amount added to the engine is found in the paper. In the case of china clay it is well known that only from 40 to 60 per cent. is actually “carried” by the pulp. Figs. 40, 41, 42, and 43 show the appearance of china clay, pearl-hardening, and agalite when viewed under the microscope, magnified 200 times. The nature of agalite is such that it assists the paper in taking a high finish. This is probably due to its “soapy” nature, a feature which is characteristic of asbestos, French chalk, “soap-stone,” and other magnesium silicates.

FIG. 44.

When papers contain such excessive quantities as 15 or 20 per cent. of clay, it cannot be to the advantage of the consumer, and should be looked upon as an adulteration. It is a matter of some importance to be able to determine rapidly and accurately the amount of mineral matter in a paper. The usual method is to ignite a weighed quantity of the paper in a platinum crucible until the ash so obtained is either white or a very pale grey. From the weight of the ash, the percentage of mineral matter is easily calculated. The following is a very convenient plan in cases where a platinum crucible or dish is not obtainable:—Take a weighed piece of the paper to be examined, from 2 to 4 in. square, according to the thickness, roll it into a narrow hollow cylinder. Round this wind a weighed piece of platinum wire about ¹⁄₅₀ in. thick, as in Fig. 44. Hold this by means of a pair of crucible tongs in the flame of a Bunsen burner until it is completely burned. If the wire is carefully wound round, and especially if the roll of paper is made conical, the ash will be securely held in {135} position. Those who do not possess a chemical balance of the ordinary form will find a convenient substitute, which will answer the purpose of weighing the paper and ash with sufficient accuracy, in the spiral balance invented by Prof. Jolly11 illustrated in Fig. 44. It consists of a spiral of hard wire A, which is suspended in front of a mirror B, upon which millimetre divisions are marked. A small float D, dipping under the surface of the water in the vessel E, is provided for the purpose of steadying the spiral and allowing it to come quickly to rest. The balance is provided with a light pan made of a thin plate of mica, and suspended by very thin platinum wires. For the present purpose, however, the pan is not necessary, and it can be replaced by the roll of paper and platinum coil as shown in the drawing.

The method of using is exceedingly simple, as the increase in the length of the spiral is in direct ratio to the increment of weight. The position of the spiral is ascertained by placing the eye in a direct line with the small glass bead C and its image in the mirror, and noting the corresponding division on the scale. The position of the bead can be altered so as to bring it to any desired point on the scale by raising or lowering the upright rod F, which is kept in position by the screw G. The balance stands on a foot provided with levelling screws.

It is evident that where proportional weights only are required it is not necessary to know the value of the spiral, but if the balance is to be used to ascertain actual weights, the coefficient of the spiral must first be determined. This is done by noting the increase in length after the addition of a one gram weight. The spirals are made of different thicknesses of wire, which of course give varying degrees {136} of sensibility: the most useful is one which gives with one gram an extension of 100 millimetres: one mm. being therefore equivalent to one mgrm.

The following experiment will illustrate the method of using the balance and of calculating the results of a determination of the amount of mineral matter in a paper:—

In order to ascertain the percentage of mineral matter actually added to a paper it is necessary to deduct from the amount of ash obtained a certain quantity due to the mineral matter in the fibre of which the paper is composed. This amount varies with each particular fibre, and with the method by which it has been prepared. The following table gives the percentage of ash yielded by the various pulps in a perfectly bleached state. In all cases the fibres were previously treated with a dilute solution of hydrochloric acid, in order to remove any carbonate of lime or other bodies which might have been introduced in the boiling and bleaching processes: the percentages are calculated on the dry substances:—

If the paper contains calcium sulphate the ash obtained may consist partly of calcium sulphide, due to the reducing action of the carbon found on ignition, and the amount will {137} therefore not represent the true amount added. The ash should be moistened with a few drops of sulphuric acid, and again ignited, in order to reconvert it into calcium sulphate. It should also be borne in mind that the sulphate of lime as present in the paper is combined with two atoms of water CaSO₄ + 2 H₂O, and therefore that every part of calcium sulphate obtained represents 1·26 parts of “pearl-hardening” actually added.

Rosin (colophony) consists of a mixture of pinic and sylvic acids, which, when heated with a solution of carbonate of soda, combine with the alkali to form a soap, carbonic acid being evolved. The soap is prepared in the following manner:—

Ordinary rosin, the quality depending upon the quality of paper, is boiled in a jacketed pan for from two to three hours with a solution of carbonate of soda, until a sample of the soap formed is completely soluble in water. The quantity of carbonate of soda (Na₂CO₃ 10 H₂O) required is about 30 per cent. of the weight of the rosin. It is usual to employ crystallised carbonate of soda (soda-crystals), but {138} soda-ash of good quality, or even caustic soda, might very well be employed, in which case of course the proportion would be different.

A very convenient form of carbonate of soda has lately been introduced by Messrs. Gaskell, Deacon & Co., which goes by the name of “Crystal Carbonate.” It is obtained in the form of minute crystals of the mono-hydrate, Na₂CO₃ + H₂O. It contains 50 per cent. of alkali (Na₂O), and possesses this advantage over soda-ash, that is dissolves readily in water without forming a hard cake. It is much cheaper in proportion than soda crystals, as the expense of crystallisation and carriage is saved. The latter contain only 21·68 per cent. Na₂O, 62·93 per cent. of their weight consisting of water of crystallisation. One hundred parts of soda crystals are equivalent to 43·36 parts of the “Crystal Carbonate.”

An excess of soda should be carefully avoided, as it consumes an equivalent quantity of alum (see below): on the other hand, it is very essential that the rosin should be completely dissolved, otherwise small particles are sure to find their way into the pulp, and would form clear transparent specks in the finished paper.

The boiling being completed, the charge is run off into iron tanks and allowed to settle; the soap forms a semi-solid mass, while a dark-coloured liquor, containing the impurities of the rosin, rises to the surface, and can thus be removed. The soap so purified is next dissolved in hot water containing a small quantity of carbonate of soda, in case complete solubility has not been attained, and is then mixed with a quantity of starch paste prepared in a separate vessel by dissolving starch in boiling water. The mixture is then carefully sieved and is ready for use.

The proportion of starch to rosin differs in nearly every mill, and also the quantity of size to be added to the beater varies according as the paper is required to be soft or hard-sized. About 3 parts of starch to 1 of rosin, and between 3 or 4 lbs. of the mixture to 100 lbs. of pulp may be considered an average quantity. Some manufacturers prefer to add the {139} starch and rosin size to the engine separately; others again do not dissolve the starch in water, but merely make it up to a thin paste, in which state it is added to the pulp. This method is employed by makers of very fine papers, as it is said to give a certain feel to the paper which cannot be obtained in any other way. The method is costly, as only a small proportion of the starch added is actually found in the paper.

Some papers, which are not intended to be sized, such as blotting and filter-papers, are made with the addition of starch only, this being used to bind the fibres together to some extent. The presence of the starch does not prevent the paper from being absorbent. Papers which are intended to be tub-sized only are usually made up with a small quantity of starch, and are sometimes sized to a small extent with rosin also. The addition of a certain amount of ordinary soap to the rosin soap is said to give good results; it enables the paper to take a higher finish when calendered. Sizing with animal size (gelatine), or Tub-sizing as it is called, will be described in Chapter X.

The mixture of size and starch may be added directly to the engine, or it may be previously dissolved in water; the latter method is perhaps preferable. After allowing it to mix thoroughly with the pulp, a solution of alum is run in. It is made up with boiling water in either lead or copper tanks. Iron or zinc vessels must be avoided, as the solution acts rapidly on these metals.

Alum consists of a double sulphate of aluminium and potassium or ammonium, Al₂ 3 SO₄, K₂SO₄ + 24 H₂O; its function being to form with the rosin acids insoluble soaps which are precipitated in intimate mixture with the pulp, the sulphate of potash taking no part in the reaction. The choice of a suitable alum is a matter of very great importance; it should be free from excess of sulphuric acid and from iron. The former is deleterious on account of its action upon the colouring matter used to tone the paper, some colours being completely discharged by it; and because {140} of its effect upon any metal work with which it may come in contact, especially upon the brass wire-cloth on which the paper is made. The iron is objectionable as it forms a dark red precipitate of oxide of iron.

Alum as supplied by makers of repute is generally sufficiently pure for even the best classes of paper.

Many years ago a substitute for alum was introduced, called Pochin’s aluminous cake. It consisted principally of sulphate of alumina Al₂ 3 SO₄, and was, provided it was pure, as suitable for the papermaker as the more expensive potash alum. It was more economical at equal price, as it contained about 14 per cent. of alumina, whereas potash alum, even if absolutely pure, could only contain 10·85 per cent. Pochin’s aluminous cake however, was liable to contain a considerable amount of free sulphuric acid and soluble sulphate of iron. Of late years, a number of different varieties of sulphates of alumina have been introduced into commerce, of so pure a nature, and at such a low price, that it is a matter of wonder that they have not entirely superseded the more expensive alum. They are prepared either from very pure native alumina (bauxite), or from China clay of good quality. Methods have been discovered of eliminating almost the whole of the iron and free sulphuric acid. The following analyses of different specimens sufficiently indicate their purity.12

Sulphate of alumina possesses this advantage over alum, that it is more soluble in water, and thus a stock solution of considerable strength can be prepared. It is soluble in {141} 2 parts of cold water, alum requiring 18 parts for complete solution.

The amount of alum or sulphate of alumina added to a pulp is largely in excess of the quantity necessary to precipitate the rosin soap; as a matter of fact, in the case of esparto or straw pulps, for the bleaching of which considerable quantities of bleach have been employed, and which therefore contain a certain amount of basic lime, together with calcium chloride, complete precipitation of the size is effected without the addition of alum. A certain amount is also required to precipitate the starch. The excess of alum appears to be necessary, however, not only to brighten the colour of the paper, but also to render it capable of resisting the action of ink. From experiments made by the authors, it appears that one part of rosin requires 2·9 parts of alum for complete precipitation from its solution in soda. One part of starch requires 0·40 part alum.

In choosing an ultramarine, attention should be directed not only to its tinctorial power but to its behaviour with a solution of alum, inferior qualities being entirely discharged by it. All ultramarines are readily decomposed by free acids, hence the necessity of employing an alum as free as possible from this impurity.

Smalts on the other hand resists the action of acids, but it is only used for the finer qualities of paper, owing to its high price. The aniline blues are perhaps scarcely so brilliant in colour as the above-mentioned colours, and they {142} are moreover somewhat readily discharged by the action of light; they have however the advantage of cheapness.

The pink colouring matters are sold either as lakes, that is, compounds of the colouring matter with alumina or as solutions. They are also frequently to be met with mixed with starch in varying proportions. Those made from cochineal are superior to those containing aniline colours.

Ultramarine can be detected by means of its reaction with hydrochloric acid, the colour being discharged and sulphuretted hydrogen evolved, which can be recognised by its odour. Smalts is not acted upon by acids. The latter contains cobalt, the presence of which can be determined by its giving a blue bead with borax in a blowpipe flame.

The aniline blues are soluble either in hot water or hot alcohol, the liquid being intensely coloured. Most of them are discharged by treatment either with strong acids or alkalis, the colour being restored by the reverse treatment.

Paper of any desired colour may be made either by using rags already dyed the necessary colour or by adding to the bleached pulp in the beater such dyes or pigments as will produce it. Any colouring matter which can be obtained in the form of a fine powder or as a solution can be used. Blue papers are usually coloured with ultramarine; the dark blue papers used for wrapping sugar and other purposes are coloured with Prussian blue, either added directly to the beater or produced in it by the action of potassium ferrocyanide on ferric sulphate.

The so-called toned paper is produced by adding to the pulp a solution of pernitrate of iron, from which a fine precipitate of oxide of iron is deposited on the fibres; thus the slightly brown shade characteristic of this kind of paper is produced. The same effect may be produced by the addition of yellow ochre or some similar pigment. The following are a few of the materials which are used for producing coloured papers:—Methyl violet, eosine, chrome yellow, venetian red, catechu. Useful receipts for the preparation of coloured papers will be found in Dunbar’s ‘Practical Paper-Maker.’ {143}

During the time that the loading, sizing and colouring processes have been going on, the pulp has been continually acted upon by the roll, and if these operations have extended over a considerable time, it is probably in a proper condition for making into paper. The amount of “beating” depends, as has been stated before, upon the nature of the fibre, and also to some extent on the nature of the paper for which it is intended. The “beaterman” examines the pulp from time to time by taking a portion from the engine and placing it in a hand-bowl containing water: from its appearance when so diluted he is able to judge of the time during which it may be necessary to continue the disintegration. As soon as this is completed, the pulp is ready to be let down to the stuff-chests, usually placed at a lower level than the beaters, so that the pulp can flow into them by gravity. For this purpose the valve at the bottom of the engine is opened: to remove the last portion of pulp it is necessary to rinse out the engine with water.

CHAPTER X. PAPER MACHINES; HAND-MADE PAPER.

The pulp as it comes from the beaters is now ready to be made into paper. We will first consider briefly the manufacture of hand-made paper.

It is made on a mould of wire-cloth, with a movable frame of wood, called the “deckle,” fitting on to the outside of the mould and extending slightly above its surface.

The wire-cloth is generally supported by a much coarser wire-cloth, or by pieces of thick wire, and these again by wedge-shaped pieces of wood, the thin end being next to the wire.

To form a sheet of paper the workman dips the mould, with the “deckle” in position, into a vat containing the prepared pulp diluted with water, lifting up just so much as will make a sheet of the necessary thickness. As soon as the mould is removed from the vat, the water begins to drain through the wire-cloth, and to leave the fibres on the surface in the form of a coherent sheet of paper. The felting or intertwining is assisted by lateral motion in every direction given to the frame by the workman. The movable deckle is then removed, and the mould, with the sheet of paper, given to another workman, called the “coucher,” who turns it over and presses it against the felt, by this means transferring the sheet from the wire to the felt. In the meantime the “vat-man” is engaged in the formation of another sheet with a second mould.

The Fourdrinier Paper Machine. Plate I.

A number of the sheets thus formed are piled together, alternately with pieces of felt, and when a sufficient number {145} have been obtained, the whole is subjected to strong pressure, to expel the water. The felts are then removed, and the sheets again pressed.

They are then sized, if required, by dipping them into a solution of gelatine: again slightly pressed, and hung up on lines or poles to dry. Such paper is called loft-dried.

When dry the sheets of paper are calendered. (See Chapter XI.)

The making of paper by hand involves considerable dexterity on the part of the workman; on account of the expensive labour necessary, in comparison with paper-machines, it is comparatively little practised in the present day; certain kinds of paper, however, such as bank-notes, various drawing papers, and printing papers intended for the production of very elaborate editions are always made in this way.

Any pattern or name required on the paper is obtained by means of a raised pattern on the wire-cloth mould: consequently, less pulp lodges there, and the paper is proportionally thinner, thus showing the exact counterpart of the pattern. Such devices are known as “water-marks.” (See also p. 156.)

A modern Fourdrinier paper machine is shown in plan and side elevation in Plates I. and II.

The pulp, after leaving the beaters, passes into a large vessel called the stuff-chest, of which there are one or more to each machine. As soon as the beater is empty, water is run in to thoroughly rinse out the remaining pulp, the washings also going into the stuff-chests. These may be {146} made either of wood or iron, and should be provided with arms fixed on a vertical shaft, made to revolve by suitable gearing. The arms are for the purpose of keeping the pulp thoroughly mixed, and should only work at a moderate speed, otherwise they are liable to cause the fibres to form into small knots or lumps. The pulp is drawn from the stuff-chests by means of the pump A, and is discharged into a regulating-box (not shown). The object of this box is to keep a regular and constant supply of pulp on the machine. It consists of a cylindrical vessel, having two overflow-pipes near the top, and a discharge-pipe near the bottom. The pulp is pumped in through a ball-valve in the bottom, in larger quantity than is actually needed, the excess flowing away back into the stuff-chests, through the two overflow-pipes. By this means, the box is always kept full, and therefore the stream of pulp issuing out of the bottom pipe is always under the same pressure. It flows from this pipe, the quantity being regulated by means of a cock, according to the thickness of paper required, directly on to the sand-tables. These may be of various sizes and shapes, but should be so large that the pulp takes some little time to travel over them. They consist of long shallow troughs, generally of a sinuous form. The bottoms are sometimes covered with woollen felt, or with thin strips of wood placed across the direction of the flow of the pulp, and at a slight angle. These serve to retain any particles, such as sand and dirt, that may have escaped removal in the previous treatment of the pulp, and that are heavy enough to have sunk down during the passage of the pulp along the sand-tables. In some mills, where great care is exercised, the pulp is caused to flow over sand-tables 200 yards in length. As the pulp, when it leaves the stuff-chests, does not contain sufficient water for the purpose of making paper, it is mixed, where it enters the sand-tables, with a quantity of water from the “save-all” (see p. 154), flowing from the box B placed at a higher level.

In some mills, instead of being pumped into the {147} regulating-box, the pulp flows into a small vessel below the stuff-chest, and is lifted on to the sand-tables by means of buckets fastened on the circumference of a wheel.

The pulp, after leaving the sand-tables, passes on to the strainers. These consist of strong brass or bronze plates, having a large number of very fine -shaped slits cut in them, the narrowest end being on the outside.

FIG. 45.

Various patents have been taken out from time to time for {148} flat strainers worked by means of a vacuum underneath the plates caused by the motion of discs of indiarubber or thin metal. Fig. 45 shows in plan a set of strainers, as manufactured by Messrs. G. & W. Bertram, similar to those in Plate II., but illustrated somewhat more in detail.

The pulp first passes through the flat strainer B, and from thence to the two revolving strainers A. From these it flows along the shoots placed at the side on to the paper-machine at E.

FIG. 46.

Fig. 46 gives a view of a patent flat vacuum strainer made by the same firm, which may also be used for cleaning straw or esparto previous to its passage over a presse-pâte. The pulp flows on to the strainer at a, and passes away by {149} the cast-iron pipes f. The valve g is for running off waste pulp. The plates are placed at a slope of about 1 in. in their length; those nearest the supply of pulp are provided with coarser slits, as the impetus carries the knots forward. The vacuum pumps are worked by the rods d from the shaft e. By means of the tubes c water can be directed on to the plates, whereby the coarser particles of fibre are carried forward, and the slits are kept clean. The plates can be removed in a few minutes.

Figs. 47 and 48 show in side and end elevation, Messrs. Masson, Scott, & Bertram’s patent self-cleaning strainer. The novelty consists in an arrangement by means of which the upper surface of the plates are continually freed from those portions of the pulp which cannot pass through the slits.

The scrapers d are made of vulcanised indiarubber, and are continually carried forward by an endless chain; the knots, &c., collect in a heap at the end of a strainer. The {150} pulp flows on at a, and passes away through the pipe b. The pumps are driven from the shaft e.

CHAPTER XI. CALENDERING, CUTTING, ETC.

The paper, as it leaves the machine, is for many purposes not sufficiently highly glazed, and it is therefore necessary for it to undergo a further process of calendering. This may be done in various ways.

FIG. 58.

One method, called “web glazing,” is to pass the paper between a number of rolls, alternately of polished iron and very highly compressed paper or cotton. The construction of such a calender will be understood by reference to Figs. 58 (end elevation) and 59 (front elevation). The reel of paper, as taken from the machine, is shown at A (Fig. 58), its course over the rolls being indicated by arrows. After passing over the bottom roll, it is wound off on a wooden or hollow iron cylinder B (Fig. 58), driven by the toothed wheel shown by the dotted line C, on the same shaft as the wheel D, which is driven by E, keyed upon the bottom roll. The whole machinery is driven by the large toothed wheel F (Figs. 58 and 59), which is itself driven by the small wheel G on the main shaft H. The paper rolls are marked P, and the iron rolls I. It will be seen that there are two paper rolls in the middle, for the purpose of, as it were, reversing the paper, and so making both sides alike. Pressure is applied to the rolls by means of the screws K, and by the weight L (Fig. 58) acting on the compound lever M. The brake, which consists of a strap of leather, pressing, by means of the weight and lever N, on the circumference of the wheel O, connected by toothed wheels with the cylinder A on which the paper is wound, is used for the purpose of preventing the paper from leaving {168} the cylinder too rapidly. But for this appliance, the paper would be apt to crease. The paper rolls have an inner core of iron, the paper only extending to a depth of about 5 in. The iron rolls are hollow, and are connected with steam-pipes, by which they can be heated. {169}

FIG. 59.

Another method, known as “friction-glazing,” employed for giving a very high finish to paper, generally on one side only, is to pass it between a large paper roll and a smaller iron one, the latter revolving at a much greater speed than the {170} former. By this means a very smooth surface can be obtained. It is sometimes assisted by rubbing a small quantity of bees’-wax on the small iron roll. The above-mentioned methods apply to the glazing of paper in the web. Paper cut into sheets may also be treated in the same way. Various modifications of these calenders have been devised; they do not, however, involve the application of any special principle. Plate-glazing, a method that is adapted to hand-made and the better qualities of paper, consists in applying heavy pressure to sheets placed between polished plates of copper or zinc. The metallic plates and the sheets of paper are made into bundles, and the whole is passed between two strong rolls, heavy pressure being communicated to them by means of screws or levers and weights applied to the ends of the upper roll.

FIG. 60.

A calender for this purpose is shown in Fig. 60. The bundle of plates and paper is passed along the table a; after {171} passing through the rolls c, it slides along the inclined table d, where it is received by a workman. The pressure on the rolls is regulated by the handle e and the weights b.

By passing paper between rolls on which devices have been cut, the “repped” and other similar papers are produced.

With calender rolls of the ordinary construction, as the pressure is applied at the extreme ends, the roll is liable to assume a slightly curved shape, the effect of which is to produce an uneven surface on the paper, the outer portion of the web being more highly finished than the centre.

FIG. 61.

This defect is obviated to a very large extent by Schurmann’s Patent Anti-Deflection Rolls (Fig. 61). The roll proper consists of an outer shell a, through the centre of which and securely wedged in at b is the centre core c, the ends of which run in journals, and to which pressure is applied in the usual way. The pressure is communicated to the outer shell at the point of contact b, the result being that the parallelism of the surfaces of the rolls is maintained, and in consequence, the paper tends to be equally finished in every direction.

CHAPTER XII. CAUSTIC SODA, RECOVERED SODA, ETC.

As we have already pointed out, caustic soda and lime are the only alkalis employed by the paper-maker for boiling fibres; the special cases in which carbonate of soda is used have been mentioned in their proper place.

The preparation of milk of lime requires little or no de­scrip­tion, as the operation is a simple and tolerably familiar one. Before using, it should be passed through a fine wire sieve, to keep back sand, coal, and similar impurities which the lime invariably contains.

Caustic soda is met with in commerce in four forms, differing from each other in alkaline strength and colour, and of course in price. The lowest quality is what is known as caustic “bottoms”: it consists of that portion remaining at the bottom of the caustic-pot after the clear fused caustic soda has been ladled out, and it contains a considerable quantity of ferric oxide and other insoluble impurities. It is of a dark reddish brown colour, and contains 50–60 per cent. of alkali (Na₂O). Its use in paper-making cannot be recommended, except for the preparation of the very lowest grade of pulp. The solution should be allowed to remain at rest, in order that the insoluble matter may subside.

The next in quality is known as cream caustic, so called from its slightly brown colour. It is usually sold containing 60 per cent. of alkali, in the form of a tolerably friable mass, having a crystalline structure. It is a very suitable form of alkali for the paper-maker.

The next in order is what is called 60 per cent. white. It {178} consists of a hard white mass, requiring considerable force to break it. Though whiter in appearance than cream caustic, it is in reality less pure, as it contains a considerable quantity of salt. It is made by continuing the evaporation of the caustic liquor to a further point than is the case with cream caustic, the result being that the whole of the water is driven off. A small quantity of nitre is then added to oxidise the sulphides and other compounds which impart the colour to cream caustic. Common salt is then added to reduce its strength to 60 per cent., in order to satisfy the whim of the consumer, who insists upon an article of a given definite strength. On this account it is somewhat inferior to cream caustic, and is, moreover, more expensive.

The highest quality of all is white 70 per cent. This resembles white 60 per cent. in appearance, but is much purer, and more expensive.

A very pure form of caustic soda has within the last few years been introduced by the Greenbank Alkali Company. It consists of small fragments, more or less crystalline, of almost pure sodium hydrate. It can be packed in wooden casks, and it dissolves very readily in water. It contains 76 per cent. of alkali. It is, however, much too costly for boiling purposes; it might, however, be used for the preparation of rosin size.

The following analyses of different forms of caustic soda and their relative prices will possibly be of interest:—

Their relative prices per ton f.o.b Liverpool, in December 1886, were:—70 per cent. white, 8l. 5s.; 60 per cent. white, 7l. 5s.; 60 per cent. cream, 7l.

All the above-mentioned forms of caustic soda occur in the form of more or less hard masses, and are contained in thin wrought-iron drums, which are filled with the caustic in a fused state. The soda is removed by breaking the drum with a chisel and hammer. The lumps of caustic may be put direct into the boiler, though it is better to dissolve them previously in water and allow any insoluble impurities to settle to the bottom of the solution.

Instead of buying caustic soda direct, it is the custom of some paper-makers to prepare it for themselves from carbonate of soda, which can now be obtained in various forms, some of great purity.

It occurs as caustic ash, containing about 40 per cent. of alkali as carbonate, and about 10 per cent. as caustic soda; soda-ash containing about 52 per cent. total alkali, nearly all as carbonate; and refined soda-ash containing 57–58 per cent. of alkali as carbonate. Some forms of carbonate of soda, such, for example, as those made by the ammonia process, are of even greater purity. The method of converting sodium carbonate into caustic soda will be described subsequently.

Unless the arrangements for washing and draining the lime-mud are very perfect, it is probably cheaper for the paper-maker to buy his caustic direct from the maker.

FIG. 67.

FIG. 68.

The apparatus for ac­com­plish­ing the evap­o­ra­tion varies with almost every mill. In some, it is very primitive and {181} crude, consisting perhaps of only a furnace for incinerating the residue, and over it a pan containing the liquor, the latter being heated and evaporated by the heat from the furnace. It is obvious that, with such an arrangement, a large quantity of heat must be wasted. To economise as much as possible of this waste heat, various plans have been suggested. That of Rœckner, of Newcastle, appears to be to a great extent efficacious. It consists practically of a series of shallow trays B (Fig. 67) placed in a brick chamber, alternated so as to allow the heated air from the furnace below to play upon the surface of each in succession, on its way to the chimney, with which the whole system is in connection. Above the chamber containing these trays, is a large tank C, containing a store of the liquor to be evaporated, placed there so as still further to economise the heat, and from which the liquor runs on to the trays. The furnace A is of the ordinary re­ver­ber­a­tory kind; below it, and connected with it by a kind of damper, is a large chamber J, where the calcined residue from the furnace is put to cool, thus preventing any nuisance from the smell of the burning mass. The chamber is provided with a pipe L, through which the vapours pass into the furnace. Several pipes E from the furnace pass through the {182} tank, to assist in warming the liquor. The residue, when cold, is drawn through doors from the chamber below the furnace. Rœckner has devised an apparatus (Fig. 68), consisting of a small chamber containing a series of pipes A, through which a stream of cold water constantly runs, in connection with the flue from his evaporator, for the purpose of condensing volatile bodies, and thus preventing, to a certain extent, contamination of the surrounding air.

A very economical form of evaporator is that invented by Porion, a French distiller, and named after him. It is shown in sectional elevation and plan in Figs. 69 and 70. It is largely used on the Continent, and also in England and Scotland. It consists of a large chamber k, the floor of which is slightly inclined from the chimney shaft, and through which the waste heat from the furnace a passes.

FIG. 69.

FIG. 70.

The liquor to be evaporated is run in at the end nearest the chimney from the tank placed above the chamber c. A number of cast-iron fanners i, dip into the liquor and revolve rapidly, usually at the rate of about 300 revolutions per {183} minute, producing and filling the chamber with a very fine spray, thus presenting a very large evaporating surface.

Between the furnace and the evaporator are placed the chambers c and f. In c a number of brick walls d are so placed that the flames from the furnace are intercepted and broken up. The object of this is to give time for all the products of combustion to be thoroughly burnt up, which would not be the case without the “smell-consumer,” as these chambers are called. This part is an addition to the original evaporator, and was devised by Messrs. Menzies and Davis. The liquor after having been concentrated in the chamber k runs into a trough placed alongside the doors h and flows into one or other of the furnace beds b where it is still further concentrated, and the residue ignited by the flames from the fires a. The draught can be regulated by the damper g, and also by one placed near the shaft j. The doors e, in the smell-consuming chamber, are for the purpose of cleaning out. The fanners i are worked by a small steam engine, not shown in the drawing. Under properly regulated conditions very excellent results can be obtained with this evaporator. The temperature of the gases near the chimney should not be higher than about 85°. By running the fanners at a very high speed the temperature of the gases may be even further reduced, thus showing the completeness of the evaporation.

This form of evaporator is open to the objection that the whole of the sulphur in the coal employed for the furnaces, finds its way into the recovered soda. It combines with the alkali to form sulphite of soda, part of which is decomposed in the furnace with formation of sodium sulphate, sulphide, and other sulphur compounds. The same objection, of course, applies, though perhaps in a less degree, to all systems of evaporation in which the flame is in contact with the liquor to be evaporated.

The Porion evaporator can be erected at very small cost, and costs but little for maintenance. It is capable of producing ³⁄₄ ton of recovered soda per ton of coal with liquors {184} of the usual strength. It has proved itself to be perhaps the most economical evaporator existing.

Some time ago there was erected in Lancashire an evaporator invented and patented by Mr. Alfred Chapman. It is shown in Figs. 71, 72, 73, and 74. The evaporation is effected at a low temperature in three vacuum pans E, and with the unusual result that the concentrated liquor gelatinises after leaving the third vacuum-pan, instead of taking the ordinary form of the concentrated products of other evaporators. It is said that this apparatus gives an excellent product, with great economy of labour and water, and with no drainage of foul liquor from the buildings. Observations extending over three months have proved that it evaporates 22 lb. of water from the liquor per lb. of coal used under the boiler. It is however very costly to erect.

FIG. 71.

FIG. 72.

FIG. 73.

FIG. 74.

The waste liquor is discharged into the tank A, whence it is pumped by the donkey-engine B, through the feed-heater C, {185} into the boiler D, which receives heat from the incinerating furnace H, and, in case of need, from an auxiliary furnace shown on the plan, under the feed-liquor-heater. The steam produced in D is taken to the first vacuum-pan at E, and having heated its contents, the products of evaporation pass over into the tubes of the second pan; this, in its turn, gives up its products of evaporation to the third, whence they go to the condenser of the vacuum-engine F. Thus the heat from the furnace H is used for incinerating the concentrated liquor {186} on its bed, for beating the feed-liquor in the feed-heater pipes, and for making steam out of the liquor itself in the boiler; this steam finally drives the donkey-pump and vacuum-engine, and causes the evaporation in the three vacuum-pans E. One advantage of this evaporator is the fact that the liquor is evaporated out of contact with the furnace gases.

Whatever be the method of evaporating or concentrating the liquor, the final treatment in the furnaces is much the same in every case. The furnaces shown in Figs. 69 and 70 may be taken to represent the ordinary form. The concentrated liquor is run on to either of the beds b, where the last portions of water are driven off by the heat from the fireplaces a, and the residual mass is ignited until all the organic matter contained in the liquors is carbonised and the soda is converted into carbonate of soda. This takes place in about 4 hours, according to the degree of concentration of the liquor as it is run into the furnaces. The running in of liquor should be done with great care, as explosions sometimes occur through the sudden liberation of steam on the liquor coming in contact with the hot beds. The charge should be allowed to remain in the furnace until it is thoroughly carbonised and all volatile matters have been driven off, otherwise a nuisance may be caused when the still burning mass is exposed to the air. Rœckner’s evaporator is provided with a special chamber into which the charge is drawn (J, Fig. 67).

The composition of the recovered soda varies with the nature of the liquors from which it has been obtained, and, as has been already pointed out, with the form of evaporator employed. It consists essentially of carbonate of soda, together with a certain amount of silicate of soda, if derived from liquors in which straw or esparto have been boiled, chloride of sodium, sulphate of soda, sulphite of soda, sulphide of sodium, and other sulphur compounds, the rest being made up of carbon and insoluble impurities. The amount of soda varies from 35 to 45 per cent. (Na₂O). The following analysis {187} will give some idea of the composition of Recovered Soda:—

The whole of the soda present as sulphur compounds is not lost, as a large proportion of it is present as sodium sulphite, most of which is converted into caustic soda by the causticising process.

A certain amount of soda is carried forward, partly mechanically and partly volatilised, to the flue leading to the chimney. This accumulates, and may be from time to time removed in the form of fine dust. It contains, besides carbonate of soda, much sulphate and chloride. In two different samples examined by the authors, the amounts of soda (Na₂O) present were 25·0 and 27·1 per cent.

This operation is known as “causticising,” and consists in heating a solution of the soda with lime. The decomposition which takes place is shown in the following equation:—

The recovered soda should be dissolved in separate vessels. Perhaps the best form of apparatus is a series of lixiviating tanks such as are used for dissolving the alkali in black ash. {188} By this means a nearly perfect exhaustion of the mass can be effected with a minimum of labour. Special tanks are sometimes made for the purpose, provided with mechanical stirrers.

It is essential in dissolving the recovered soda that a high temperature should be employed, as otherwise a portion of the soda present as silicate of soda will be lost, as it is only with difficulty soluble, and requires rather prolonged heating with water. Whatever the form of apparatus employed it should be so arranged that, after running off the strong liquor, the insoluble residue may be further treated with water. In the case of the vats mentioned above, this process is made continuous, pure water being run in at one end, and strong liquor flowing from the other. If other forms are used, the liquor after settling, may be run off by means of a pipe passing through the bottom or side of the vessel, and near the bottom, and consisting of two parts, one long, and one short. The short part is stationary, and is connected to the longer part by means of a movable knee joint, allowing it to be deflected. The liquor having settled sufficiently, the movable limb is lowered beneath the surface of the liquor which is then allowed to flow through. As the surface of the liquor falls, the pipe is gradually lowered. In this way the clear solution can be run off without disturbing the residue at the bottom. The open end of the pipe is usually covered with coarse wire gauze, to keep back insoluble impurities. With properly calcined recovered soda, the solution should be bright and almost colourless. If at all brown in colour, and if it has an empyreumatic odour, it indicates imperfect calcination. The residue in the dissolving tanks consists chiefly of carbonaceous matter, together with some soda, insoluble matter, &c.

The liquor is now ready to be causticised. This should be done in a separate vessel, although it is the practice in many mills to perform this operation in the same vessel in which the solution of the soda has been conducted. A good form of causticiser can be made from an old egg-shaped boiler, by cutting it in two along its length. {189}

It should be provided with two or more vertical steam pipes, connected at the bottom of the boiler with a horizontal pipe perforated with numerous holes. The vertical steam pipes should be furnished with injectors, whereby air is drawn in, and forced with the steam through the holes in the horizontal pipe. The stream of air serves the double purpose of thoroughly agitating the liquor and of oxidising any sodium sulphide in the recovered soda. The liquor before causticising should be reduced in strength to about 20–25 degrees Twaddle, which may be done with the washings of the residue from the recovered soda, or from the washings obtained subsequently from the lime-mud. This strength should never be exceeded, otherwise imperfect conversion into caustic soda is the result. This is due to the fact that concentrated solutions of caustic soda react upon calcium carbonate, forming sodium carbonate, and calcium hydrate, the reaction being the reverse of that indicated in the above equation. If the liquors are very strong in carbonate of soda, and comparatively free from sulphate, they should not be causticised at much over 20° Twaddle, if they contain much sulphate, and therefore less carbonate, the higher strength can with safety be adopted.

The causticising vessel should be provided with a stout iron cage or basket, into which the lime can be put. This should be securely fastened to the vessel, and should dip into the liquid.

The liquor having been properly diluted, is now heated by means of the steam pipes, and the lime put into its cage. It should be put in in lumps. As the liquor reaches the boiling point, the reaction will proceed rapidly, and the lime will gradually disappear; fresh lumps should be added if necessary. If the liquor is sufficiently heated the causticising will be complete in from two to three hours. The liquor should be tested from time to time; this is usually done by a workman. He withdraws a sample of the liquor, and after allowing the calcium carbonate to subside, pours off a portion of the clear liquid into a glass vessel. He {190} then adds an excess of either sulphuric or hydrochloric acid. If any effervescence takes place, due to the evolution of carbonic acid gas, he knows that the operation of causticising is incomplete; the heating must therefore be continued. It is difficult, without an undue expenditure of time and steam, to convert the whole of the soda into caustic: it should however be so perfect, that on testing only a very slight effervescence occurs. It is quite easy to convert as much as 95 per cent. of the soda, or even more. The actual amount converted can only be ascertained by a careful analysis of the liquor.

The amount of lime used is generally somewhat in excess of the theoretical quantity; 106 parts of sodium carbonate (Na₂CO₃) require 56 parts of lime (CaO): it is necessary, however, to add about 60 parts. A very good plan is to conduct two or even three causticisings in the same vessel without cleaning out or removing the calcium carbonate, using in the first operation a large excess of lime. The causticising being completed, the calcium carbonate and excess of lime are allowed to settle down, and the clear liquor run off by an arrangement such as that already described in the dissolving process. Fresh solution is then run in and the whole mass heated for some time, until the excess of lime is converted into carbonate. Fresh lime is then added if necessary until the conversion of the carbonate of soda is complete. The liquor is then allowed to settle, and is run off as before: this operation may again be repeated.

The residual calcium carbonate, or “lime-mud” as it is called in alkali works, is then washed once or twice by running in water, boiling up, allowing to settle, and running off the clear liquor. If these liquors are too weak for use in boiling fibres, they may be used for diluting fresh recovered soda liquor before causticising, or for dissolving the soda.

Some arrangement should be provided for removing as much as possible of the liquor from the lime-mud before throwing it away or otherwise disposing of it. This is best done by throwing it on a filter made of layers of stones, {191} ashes and sand, and covered at the top with perforated iron plates. The filter is connected with a vacuum pump. In this way very perfect draining is ac­com­plished, and the mud forms a hard mass on the surface of the filter, from which it can be easily removed with spades. In this form it contains only 50–60 per cent. of water. If properly washed it should contain in this state only about 2 per cent. of alkali (Na₂O). By careful manipulation, even this amount can be reduced.

The importance of thoroughly washing the mud can hardly be too much insisted upon. Where proper means are not employed for draining, the washing should be made more perfect. The lime-mud consists chiefly of carbonate of lime, together with silicate, free lime, &c. The following analysis is of a mud obtained by causticising recovered soda derived from the liquors in which esparto and straw had been boiled:—

As already pointed out the liquors contain a certain amount of soda, as sodium sulphide and other sulphur compounds. The presence of the former, if in large quantities, is objectionable, as it is liable to discolour fibres boiled in liquors containing it. It is therefore best to remove it. This can be conveniently done by blowing air into the liquors during the process of causticising: this has the effect of oxidising it to sulphate of soda, in which form it is harmless.

The air can of course be blown into the liquor by means of a pump; the most economical way is to connect with the steam pipe an injector constructed on the principle of the injectors used for feeding boilers and for other purposes. By this means a strong current of air is drawn in, and {192} being forced with the stream to the bottom of the liquor, passes through it in a number of fine streams.

If the amount of sulphide present be very high it may be necessary to prolong the oxidising operation beyond the time necessary for complete causticising.

In many paper-mills the causticising is conducted in circular vessels furnished with mechanical agitators. These are more expensive than the simple form described above, and they possess no special advantages. The use of causticisers in which neither mechanical agitation nor agitation by means of air is provided for is exceedingly wasteful of labour, time, steam, and soda. The lime-mud settles at the bottom as a hard mass, very difficult to manipulate.

CHAPTER XIII. PAPER TESTING.

There are two points of view from which a paper may be tested: first, of physical or mechanical properties; secondly, of material composition. We shall consider the subject according to this division.

(I.) Quantitative measurements of such properties as resistances to breaking and tearing strains are seldom made by English paper-makers. In Germany, on the other hand, the matter has been very thoroughly investigated in connection with the work of the Königl. Techn. Versuchsanstalt, Berlin, and through the agency and influence of Prof. Sell, and C. Hofmann, a department has been organised exclusively for the work of paper testing. The results of the tests are becoming widely recognised by practical men and the trade in that country, as affording a true index of the quality of a paper. It is therefore of importance to give an outline of the methods employed.

The determination of the strain or weight which a paper is capable of supporting is a very obvious measure of the strength of the paper. Observations of the limiting strain or breaking weight are sometimes made by paper-makers, but the apparatus and method employed are usually crude. The simplest means consist in clamping the paper—a strip of standard length and breadth, arbitrarily chosen—at one end, the clamp being firmly held in a fixed support, and to the other attaching by means of a similar clamp, an ordinary scale pan, the whole arrangement hanging vertically. Into the pan, weights are added in due succession until fracture of the strip is determined. It is scarcely necessary to point {194} out that the errors of experiment with such a method are very great: indeed it has been found that even with the refined apparatus about to be described the errors are not inconsiderable. However, by exhaustive investigation, according to the well-known “law of errors,” these have been quantified, and a careful operator can therefore obtain results which are trustworthy. The apparatus in question is the Hartig-Reusch machine.14 It is shown in sectional elevation and plan in Figs. 75 and 76.

The principle will be readily grasped by inspection of the diagrammatic representation of its essential parts—Fig. 77. The strip of paper is held horizontally by the clamps a and b, a being held by the fixed support A, b by the movable carriage B. B is connected by means of a swivel with the spiral spring F, and this again is similarly connected with the screw, which is made to rotate by the wheel D. By turning D, therefore, the spiral may be extended, and a corresponding strain communicated through B and b to the paper. The paper undergoes a certain elongation under the strain, and the carriage B moves from right to left in consequence. The rotation of the screw is continued, and the extension of the spiral proceeds until the paper is fractured. At this point it is required to determine, (1) the elongation of the spiral which is the measure of the breaking strain, and (2) the distance through which the carriage has moved, i.e. the elongation of the paper. Both these effects are communicated to the pencil G, the latter directly, since the pencil-holder is in rigid connection with B, the former through the rod I, from which, by a special arrangement, the horizontal is converted into a vertical motion of the pencil. This, therefore, traces a curve, of which the ordinates represent the strains, and the abscissæ the elongations of the paper produced by the strain.

The scale shown in Fig. 76 indicates the exact position of the clamp A. {195}

FIG. 77.

FIG. 76.

The values for the spiral spring—i.e. extension for a given load—having been determined by previous observations in a special apparatus, the curve obtained is at once a measure and a permanent record of these cardinal factors, breaking strain and elasticity. As with all other such instruments, the recording apparatus introduces certain errors, which, however, by careful investigation and modification in accordance with the results, have been reduced to a minimum. Nevertheless, the director, Dr. Martens, has recently adopted a simpler instrument, altogether similar in principle, but based upon a direct reading of the two movements, in which of course these errors do not appear. For the student, however, the recording instrument is the more instructive, and we have given it preference for de­scrip­tion here, more especially as no difference in essential parts is involved.

Those who wish to pursue the matter into the most interesting details of the investigations made upon the subject, are referred to the papers published by the Institute for 1885.15

In testing the strength of papers by this or similar machines, it is important to observe the hygrometric state of the atmosphere at the time the trials are made, as this has been found to exert a considerable influence on the results, a paper being weaker the moister the atmosphere.

The results of the tests are expressed in the following terms:—The elongation is given directly in percentage of the original length. This is uniformly taken at 180 mm., a length arrived at after laborious investigation, as minimising the errors of experiment; in other words, as giving mean value with the minimum of variation. For the breaking strain an ingenious expression has been arrived at, viz. the length of the paper which suspended vertically, with one end hanging freely, the other fixed, would determine fracture at the fixed end. As the breaking strain would vary with the thickness, the numbers obtained in {197} units of force or weight for strips of constant breadth, would need correction in order to admit of strict comparison with one another. By substituting an expression in terms of the paper itself—since a paper of greater thickness, and requiring therefore a proportionately greater force to fracture it, weighs more per unit of area, and in the same proportion—all the numbers for breaking strains are strictly comparative one with the other. In the same way also the question of width may be disregarded.

A further mechanical test, forming a part of the scheme of investigation, is the resistance of the paper to rubbing. This test is an altogether empirical one, as the following brief de­scrip­tion will show:—A piece of the paper, about 6 inches square, crumpled by successive folding in two directions at right angles, is grasped by the thumb and forefinger of each hand, at a distance of 3–4 inches apart. It is then rubbed upon itself across the thumbs a given number of times (seven is the number chosen) and held up to the light. If no holes are visible, the rubbing is repeated. The number of times necessary to repeat the rubbing until holes appear is the measure of the resistance. A sufficient uniformity in the results of this test has been observed to make it the basis of a classification of papers, in regard to their resistance to such disintegration; they are divided into the following eight groups, beginning with the lowest:—

The classification of papers on the results of these tests cannot be more lucidly given than in the following scheme, under which the results are officially recorded:—

This classification is based on the results of some hundreds of observations. It is interesting to note the differences observed in the numbers for a and b according to the direction for the test in which the paper is cut, i.e. in the direction in which it was run on the paper machine, or at right angles (see Chap. XI. p. 171). The mean ratio for the breaking lengths (strains) may be taken as 1 : 1·6, i.e. the paper is about 40 per cent.16 weaker across the web; the elongation under strain on the other hand is about double.

It is also of interest to note the influence of the glazing process (p. 167) upon the quality of the paper as determined by these tests.

First, we must notice the effect of the treatment upon the substance of the paper itself. The mean reduction of thickness is 23 per cent. On the other hand, the reduction of weight, calculated per unit of surface (square metre), is 6·7 per cent., whence we may infer an increase of surface, flattening out, in the process. These quantities, but more particularly the latter, will doubtless vary with the various methods of glazing and with the materials of which the paper is composed.

The breaking length (strain) shows a mean increase of about 8 per cent.; the elongation under strain, on the other hand, a diminution of 6 per cent.

For an interesting discussion of the question of the relative strengths of machine and hand-made paper see ‘Paper,’ by Richard Parkinson.

FIG. 78.

The thickness of a paper may be determined by means of an ordinary micrometer, such as is shown in Fig. 78. The paper is placed in the jaws of the instrument, and the screw {199} advanced until it touches the paper. The thickness is then read off on the scale. Other forms of apparatus are sold for the same purpose. In making a determination of the thickness of a paper it is necessary to take the mean of a series of observations at different points of the sheet, as the thickness may vary somewhat.

CHAPTER XIV. GENERAL CHEMICAL ANALYSIS FOR PAPER-MAKERS.

A knowledge of the methods usually employed for the qualitative and quantitative analysis of the various chemical substances met with in paper manufacture is of considerable importance. The scope of this work will not allow us to enter fully into the necessary details of manipulation; for these the reader is referred to standard works on analysis; we will merely indicate the methods.

In testing recovered soda, it is necessary to boil the finely powdered ash for some time with water; unless this be done, the amount of alkali will probably be under-estimated, as part of the soda is present as a difficultly soluble silicate.

It is sometimes desirable to determine the amount both of sodium hydrate and carbonate in a sample of alkali. For this purpose an excess of a solution of barium chloride is added to a solution containing a known quantity of the substance under examination, and the mixture made up to a definite volume. It is then filtered, and an aliquot portion of the filtrate titrated with standard alkali. The filtrate contains only the alkali that was originally present as hydrate, the carbonate of soda having formed with the barium chloride, insoluble barium carbonate, and sodium chloride.

The method of estimating the amount of available chlorine in bleaching powder by determining the quantity of ferrous sulphate it is capable of oxidising is fallacious, as by this means calcium chlorate is included in the result.

Bleaching powder should contain about 35 per cent. of available chlorine, but the percentage frequently falls below this amount, especially in warm weather.

Great care should be exercised in the sampling of bleaching powder, as indeed of all chemicals, in order to insure an average result. Small portions should be taken from different parts of the bulk; the whole should then be carefully mixed, and, if necessary, reduced to powder. Portions of the mixture should be taken, pounded, and again thoroughly mixed.

If this process be repeated once or twice, a perfectly uniform sample will be obtained.

• (1) The alumina is estimated as such by precipitation with ammonia. Ferric oxide is also thrown down with alumina, and must therefore be deducted from the amount of the mixed oxides obtained.

• (2) The percentage of iron oxide present is determined in the following way. A considerable quantity of the sample under examination is dissolved in water, and to the boiling solution a large excess of caustic potash solution is added. The caustic potash precipitates both the alumina and ferric hydrates, the former, however, redissolves. The solution is filtered, and the precipitate of ferric hydrate washed, dissolved in dilute hydrochloric acid, and reprecipitated with excess of caustic potash. It is again thrown on to a filter, washed, redissolved in hydrochloric acid, and again reprecipitated with ammonia. In this way it is obtained free from alumina.17

  17 For a method of estimating minute quantities of iron in alums see ‘Journ. Soc. Chem. Ind.,’ April 1887, p. 276.

• (3) Free Acid.—This can be estimated indirectly, i.e. by estimating the alumina and ferric oxide, calculating the amount of sulphuric acid necessary to combine with them, and deducting this from the total amount of sulphuric acid obtained. In the case of alum it is of course necessary to make due allowance for the sulphate of potash or ammonia present. The sulphuric acid is estimated as barium sulphate in the usual way.

  It should be borne in mind that sulphates of alumina are sometimes met with containing uncombined alumina. A convenient method of estimating free acid consists in digesting a weighed quantity of the finely powdered sample in strong alcohol. The alcohol dissolves away the free acid, which can be estimated in the solution by means of a standard solution of alkali.

  Free acid can be detected by means of a solution of {208} Congo-red, which is turned blue with free acid, but not with pure sulphate of alumina.

The same solution serves also for the estimation of sulphurous acid in sodium and other sulphites.

When perfectly cold, the “stiffness” of the pastes should be compared. This may be done by noting the relative resistance to weights placed upon their surface. Information on this subject is contained in a paper by W. Thomson in the ‘Journ. Soc. Chem. Ind.,’ March 1886.

The identification of starches by means of their microscopical appearances is tolerably simple, as each particular kind possesses characteristic differences.

Raw materials intended for the preparation of gelatine may be similarly examined. They yield the whole of their gelatine on heating with water. The effect of prolonged heating on solution of gelatine should be borne in mind. The amount of residue left after treatment with water should be carefully determined.

The chemical examination is both difficult and somewhat misleading. A microscopical examination will often throw much light on their composition.

For comparing the tinctorial power of different samples, weighed quantities should be ground up with starch or some other white powder. In this way some idea of the intensity of colour can be readily arrived at.

CHAPTER XV.SITE FOR PAPER-MILL, WATER-SUPPLY, WATER PURIFICATION, ETC.

In choosing a spot on which to build a paper-mill, the manufacturer has to take into consideration several very important circumstances. Chief of these is the necessity for having a large supply of water at command. Not only is a large quantity needful, but it should be free from impurities, such as suspended matter and iron. The former, it is true, can be removed by settling and filtration; the latter cannot, and is liable to injuriously affect the colour of the paper. Again, as a question of economy in working, it is advantageous to have convenient water-power; therefore for this, as well as for the former reason, paper-mills are usually situated on the bank of a stream. In choosing such a site, paper-makers are probably also influenced by the fact that it affords a ready means for the removal of impurities. In properly conducted mills, where suitable apparatus is employed for evaporating the liquors in which the raw material has been boiled, the stream should not be polluted to any very great extent. Generally speaking, the greater the pollution, the more are valuable materials being lost to the manufacturer. It is obvious that the site for a mill should also be chosen with reference to its proximity to means of transit for the raw and manufactured materials.

The disposition of the different divisions of a paper-mill depends of course upon the nature of the paper it is intended to make and to a large extent upon local circumstances. Fig. 79 will give some idea of the general arrangement of a mill for manufacturing esparto paper. {211}

FIG. 79.

The impurities of water consist of two classes, insoluble and soluble. The former can be readily removed by processes of settling and filtration. For this purpose most paper-mills are provided with large ponds capable of holding several days’ supply. These are sometimes supplemented by filtering beds. Insoluble impurities can also be removed by passing the water through filter-presses.

Fig. 80 is an illustration of Rœckner’s patent clarifier, which may also be applied to this purpose.

The cylinders C dip below the surface of the water to be purified, which is contained in the reservoir A. They are open at their lower ends, but are closed at the top by the domes D. These are connected with the pump I by means of the pipes H. On starting the pump the water rises slowly in the cylinders. As soon as it reaches the level of the top, the action of the pump is stopped and the cocks K opened, when the water commences to flow down the pipes G, which together {212} with the cylinders form a kind of siphon. If the reservoir A be kept full, the siphons can be made to act continuously. The flow of water being slow, insoluble impurities have time to subside.

The impurities may from time to time be removed by means of the small pump L connected with the bottom of the reservoir.

The clarifier can also be used for the purification of the effluent water from paper-mills.

FIG. 80.

Insoluble impurities can be removed from water by means of the Stanhope purifier, shown in Fig. 81.

The soluble impurities of water consist mainly of carbonate and sulphate of calcium and iron, and occasionally organic matter. It is a moot point among paper-makers {213} whether or not the presence of the two former in a water is objectionable. For boiling and bleaching purposes it undoubtedly is, as when mixed with caustic soda or bleaching liquor they form a precipitate of carbonate of calcium, which lines the insides of boilers, breakers, and potchers as a hard scale, which is always liable to become detached, and to find its way into the finished paper. Moreover, carbonate of calcium is precipitated in the fibre, and carries with it a certain quantity of colouring matter, the subsequent removal of which is difficult.

Carbonate of calcium, though practically insoluble in pure water, is soluble in water containing carbonic acid. When this carbonic acid is neutralised by lime or soda, the carbonate is precipitated. The sulphate of calcium is unacted upon by lime, but by the action of caustic soda is converted into free lime and sodium sulphate. The lime then neutralises the free carbonic acid in the water, and forms carbonate of calcium, which is of course precipitated.

These reactions may be represented by the following equations:—

If sodium carbonate be used instead of caustic soda, the decomposition will take place thus:—

It will be seen from the above equation that in the boiling processes the lime salts are removed from the water at the expense of an equivalent quantity of caustic soda. The amount thus decomposed is not sufficiently large to make it advisable {214} on that account to purify the water. It is indeed so small that processes of purification, based upon the use of lime and caustic soda, are now largely used, the cost for chemicals rarely exceeding 1d. per 1000 gallons. But for the reasons we have stated above, and also from the fact that it serves to remove dissolved iron and organic matter from a water, such a purification process is in certain cases advisable.

The processes now in use consist, as we have indicated, in the addition to the water of lime and caustic soda or carbonate of soda, the quantities being regulated according to the hardness of the water, and the relative proportions of carbonate and sulphate of calcium.

The method of removing carbonate of calcium from water by the addition of lime is due to the late Dr. Clark, of Aberdeen, and the process is still called after him. It is in use in its original form in many places. The plant necessary consists simply of a tank for mixing the water and the lime, and of large settling tanks in which the carbonate of calcium subsides.

Various modifications of this process have been proposed, chiefly in the direction of improved plant.

In the Porter-Clark process the carbonate of calcium is removed by passing the water after the addition of lime (in the form of lime-water) through a filter-press.

FIG. 81.

In Fig. 81 is shown the Stanhope purifier, which is largely used for the purpose of softening water. The following details will render its action clear: A is a store tank containing caustic soda solution. B is a tank into which the water to be treated flows, and which is maintained at a constant level by means of a valve and float. C and C are two tanks which are used alternately, and in which lime water is prepared. A definite quantity of caustic soda solution is added from B; this mixture forms the reagent by which the softening of the water is ac­com­plished. D is a small tank for the purpose of maintaining a constant head of reagent. The reagent and the water are mixed together in the pipe F, the quantities being regulated by the cocks {216} shown at H. The pipe F leads into the vessel E, which is fitted with a number of -shaped shelves, placed at an angle of 45°, and riveted alternately to opposite sides of the vessel. This arrangement causes the water to take a serpentine course. The position of the shelves is indicated by the dotted lines.

As soon as the mixture of water and reagent flows down the pipe F, and enters the vessel E, the calcium carbonate commences to precipitate and settles upon the -shaped shelves. At the top of the tank a layer of wood shavings, inclosed in wire-netting, is placed as shown by the dotted lines. This acts as a filter, and intercepts any particles that may not have subsided. The clear purified water passes away by the pipe I.

The precipitate that collects on the -shaped plates is from time to time drawn off by the cocks shown at G; this may be done without interfering with the working of the apparatus. While there can be no doubt that the use of soft water is advantageous in the boiling, washing, and bleaching processes, and also of course for supplying steam boilers, it is probably an advantage to use a hard water for diluting the pulp before running it into the machine.

In the case of papers which are loaded with any of the forms of calcium sulphate (pearl-hardening, crystal-hardening, &c.) the use of very soft water is objectionable from the fact that a certain quantity of calcium sulphate is dissolved. This would not take place to the same extent with water which is already charged with sulphate and carbonate of calcium.

The removal of soluble iron from a water is effected by the softening processes described.

CHAPTER XVI. ACTION OF CUPRAMMONIUM ON CELLULOSE. PREPARATION OF WILLESDEN PAPER.

The action of a solution of copper oxide in ammonia upon cellulose has been already referred to (see p. 5). Celluloses, and also lignocelluloses, when treated with such a solution, gradually gelatinise and finally dissolve. On evaporating the solution to dryness, a gummy amorphous mass is obtained, containing the cellulose intermixed with copper oxide.

If the cellulose be in excess, e.g. when the solution is evaporated on the surface of paper, calico, &c., merely dipped in the solution, the copper oxide is often not formed at all, but a green varnish-like mass of cellulose combined with copper oxide, which coats the surface of each filament, welding and cementing them together. This cement-like cupro-cellulose, as it may be termed, being insoluble in water, communicates water-resisting properties to the material so treated: moreover, the presence of copper renders the fabric less prone than before to be attacked by insects and mould, so that animal and vegetable life of a parasitic nature and fungoid growths are rarely, if ever, to be observed, even when the material is kept under conditions where boring worms, ants, rot, and mould, would be likely to attack it.

As has been already indicated (p. 5), the solution of cu­pram­mon­ium hy­drox­ide is preferable to one containing cu­pram­mon­ium salts; not only is the action on cellulose more energetic, but various other advantages are obtained.

Preparation of the Copper Solution.—The cu­pram­mon­ium solution is prepared according to the patent of Dr. C. R. Alder Wright (No. 737, 1883). {218}

A series of cast-iron towers, two to three feet in diameter and ten to twelve feet high, is so arranged that a current of air can be blown by a powerful engine successively through the whole series. The towers are then nearly filled with fragments of metallic copper (crumbled up sheet, cuttings, &c.), and solution of ammonia: the air-current being turned on, oxidation of the copper and solution of the oxide so produced are rapidly effected.

The ammonia solution employed is previously impregnated with a considerable quantity of copper by passing a stream of water over scrap copper in similar towers, a current of ammonia gas mixed with a suitable proportion of air being at the same time forced in. The liquor passing from one tower is used over again to supply a second instead of water, and so on through the series: finally the liquor is brought up to full copper strength in the series of towers described above.

The spent air issuing from the towers carries with it a notable quantity of ammonia; this is intercepted by means of an “exhaust” scrubber containing copper, and well supplied with water, whereby a comparatively weak solution of ammonia and copper is obtained, which can be used instead of water in the first series of towers.

In order to produce the maximum effect on the cellulose, the solution should contain from 100 to 150 lbs. of ammonia, and from 20 to 25 lbs. of copper per 100 gallons.

By decomposing a cu­pram­mon­ium solution by means of metallic zinc, a corresponding solution of zinc-ammonium hydroxide can be obtained. This solution is also capable of gelatinising cellulose, but not to the same extent as the copper solution. It may, however, be advantageously employed in certain cases in conjunction with a copper solution. Such a solution can be conveniently prepared by substituting brass for copper in the dissolving-towers. It is worthy of note that although, as we have seen, zinc has the power of replacing copper in a cu­pram­mon­ium solution, iron is without any action, although it readily replaces copper in a solution {219} of copper sulphate. This fact is of very great industrial importance, as it enables the manufacturer to employ vessels and machinery of iron.

The “Willesden” goods may be divided into two classes. Goods of the first class, such as rope, cordage, netting, &c., are prepared by simply dipping the made-up materials in a bath of cu­pram­mon­ium solution, using certain precautions as to the mode of immersion and its duration, and the strength of the solution. On subsequently drying the dipped fabrics, they are obtained coated and impregnated with cupro-cellulose, which thus not only forms a kind of varnish-like surface dressing, but further adds strength to the fibres by more or less intimately cementing them together.

Goods of the second class constitute a much more important group to which at present the Willesden Company more especially devotes its attention. These fabrics are essentially of three kinds, viz. Willesden Canvas, Willesden Scrim, and Willesden Paper. The two former of these classes are prepared in much the same way as the goods just described, saving that the fabric to be treated is usually unwound from one roller and rewound upon another, after passing through the bath and over a succession of drying cylinders.

Willesden Paper.—This may be divided into two departments, viz. (1) Willesden unwelded; (2) Willesden welded, the first class being prepared from a single web of paper by passing it through the bath, rolling and drying. Certain coarse varieties furnish a waterproof material excellently adapted for lining packages, &c. Finer qualities furnish envelopes and stationery, possessing the valuable property of not being affected by water. Letters written on such paper would remain perfectly legible, even after prolonged immersion. It may be interesting to point out here that the cu­pram­mon­ium solution offers a very simple means of fastening envelopes in such a way as to be proof against any attempts at tampering. The method consists in using a concentrated solution as the fastening material; the envelope {220} is then closed and ironed with a warm flat-iron, whereby the gelatinised cellulose is converted into an insoluble cupro-cellulose, and the cover is fastened so securely that the only possible mode of opening is by tearing the paper.

Willesden Welded Goods are prepared by simultaneously dipping two or more sheets of paper and pressing them into one compact homogeneous sheet whilst the surfaces are still in a gelatinous state. In this way a continuous length of fabric of extraordinary durability is produced that is scarcely affected by water, even when heated in it for some weeks at a pressure of 60 lbs. per square inch.

As, on drying fabrics treated with cu­pram­mon­ium solution, the whole of the ammonia in the solution absorbed by them is volatilised, it is necessary, in order to make the process economical, to collect and re-use this ammonia. This is effected by conducting the drying process in closed chambers, from which the ammonia gas is conducted by suitable appliances and recovered.

In addition to the advantages already mentioned, it may be stated that Willesden paper is much less inflammable than ordinary paper.

Among the many uses to which Willesden goods can be applied, the following may be cited:—Roofs and sheds; huts and tents; partitions; tanks and pipes; damp-proof foundations; underlining slates; sails, awnings, &c. &c.

CHAPTER XVII. STATISTICS.

Raw Materials.

The statistics concerning the trade in raw materials for paper-making are not published in a detailed form, all the various kinds of fibre, &c., being grouped under two heads, viz. rags (linen and cotton), and esparto (which embraces other fibres, and probably wood pulps). The figures for recent years are shown in the accompanying tables:—

But a portion of these imports was exported again, the figures being as follows:—

The relative proportions of raw material furnished by the various countries in 1884 (the latest available return) are shown in the subjoined tables:—

A considerable export of raw material for paper-making also takes place from British shores, the figures, embracing rags and other materials, for 1884 being as below:—

• (a) Exports of home produced rags and other paper-making materials in 1884:

  To	Tons.	£
  United States	59,222	550,924
  Holland	649	2,703
  British America	390	4,255
  Germany	262	1,673
  Other countries	401	3,348
  	60,924	562,903

• (b) Exports of foreign produced ditto in 1884:

  (i.) Linen and cotton rags.
  To	Tons.	£
  United States	14,191	167,801
  Other countries	514	5,519
  	14,705	173,320
  (ii.) Esparto and other fibres.
  To	Tons.	£
  All countries	140	701
  (iii.) Pulps and other materials.
  To	Tons.	£
  United States	11,290	97,620
  Other countries	363	3,023
  	11,653	100,643

Our imports of “rags and other paper-making materials” from Norway increased from 23,483 tons, value 138,098l., in 1880, to 48,199 tons, 279,679l., in 1884, mainly owing to the development of the wood-pulp industry. Later figures will probably show a similar increment. The same articles from Denmark grew from 359 tons, 3406l., in 1880, to 1403 tons, 16,352l., from the same cause. German statistics have even a stronger upward tendency: 11,587 tons, 196,051l., in 1880, against 196,051 tons, 331,591l., in 1884. Our receipts of esparto from Algeria show an advance, but not a marked one, being 60,612 tons, 421,343l., in 1880, and 88,357 tons, 515,232l., in 1884. But shipments of the same fibre from Spain show a decline from 51,413 tons, 454,713l., in 1880, to 40,159 tons, 314,927l., in 1884; and from Morocco they have fallen away from 2879 tons, 18,231l., in 1880, to 260 tons, 1290l., in 1884; while the figures relating to Tunis and Tripoli also show a disposition to recede, though not at such a rapid rate.

The wood-pulp industry of Norway for the year 1886 shows a very large increase upon the figures of a few years back, albeit prices have ruled very low. This latter circumstance is attributed, not so much to over-production, as to excessive competition among the sellers of this article. The quantity exported during the year 1886 is about 120,000 tons; in the year 1885 it was 107,651 tons; 1884, 88,220 tons; 1883, 70,464 tons; 1882, 58,884 tons; 1881, 42,194 tons; 1880, 26,055 tons. Several of the old works have extended their production during the past year, and several new establishments are in the course of erection, so the production this year may probably be put at 150,000 tons wood pulp with 50 per cent. water. The greatest part of the Norwegian wood pulp is exported to England, France, and Belgium; in Russia, the increase in the duty has stopped business, and the same can almost be said of Germany. America, too, has drawn part of her supply from Norway, but this trade is not expected to continue. {225}

• CHAPTER XVIII. BIBLIOGRAPHY.

  • J. Murray.
    Edinburgh: 1829.
    Practical Remarks on Modern Paper.
    
  • L. S. Le Normand.
    Paris: 1834.
    Manuel du Fabricant des Papiers.
    
  • G. Planche.
    Paris: 1853.
    L’Industrie de la Papeterie.
    
  • L. Müller.
    Berlin: 1855.
    Die Fabrikation des Papiers.
    
  • Proteaux.
    Philadelphia: 1866.
    Manufacture of Paper and Boards.
    
  • C. Hofmann.
    Philadelphia: 1873.
    Manufacture of Paper.
    
  • Hugo Müller.
    Leipzig: 1873.
    Pflanzenfaser.
    
  • T. Routledge.
    London and New York: 1875.
    Bamboo considered as a Paper-making Material.
    
  • Vétillart.
    Paris: 1876.
    Études sur les Fibres Végétales.
    
  • Arnot.
    London: 1877.
    Technology of the Paper Trade (Cantor Lecture, Society of Arts).
    
  • J. Dunbar.
    London: 1881. {230}
    The Practical Paper-maker.
    
  • Edinburgh: 1884.
    Forestry and Forest Products (Edinburgh Forestry Exhibition).
    
  • C. F. Cross and E. J. Bevan.
    London: 1885.
    Cellulose.*
    

    * Gives a list of works on Cellulose and its derivatives.

  • R. Parkinson.
    Preston: 1886.
    A Treatise on Paper.
    
  • C. T. Davis.
    Philadelphia: 1887.
    Manufacture of Paper.
    
  • Tomlinson.
    Manufacture of Paper.
  • London: since 1872.
    Paper-makers’ Monthly Journal.
    
  • New York: since 1872.
    Paper Trade Journal.
    
  • Berlin: since 1876.
    Papier Zeitung.
    

CHAPTER XIX. ADDENDA.

The Yaryan Process of Evaporation.

This process, the invention of Mr. H. J. Yaryan, of Toledo, Ohio, U.S.A., has been considerably developed in America in its several applications, and as we have authentic information of its successful working for the recovery of soda in some of the largest paper-mills in the States, we think it deserving of mention. The principle of the process is that of “multiple effects,” upon which, indeed, all the modern methods of economic evaporation are based. The principle may be briefly explained as follows:—A liquid is converted into vapour under ordinary conditions of boiling, by overcoming the pressure of the atmosphere upon its surface. The quantity of heat required to vaporise, as also the temperature of the ebullition, will be less as the pressure to be overcome is less. Further, the vapour continuously driven off carries with it a quantity of heat, which is its heat of condition or latent heat. This heat it imparts to any colder body (e. g. a further quantity of the same liquid) with which it comes in contact, direct or indirect; if the quantity of the latter be relatively small, it will raise its temperature approximately to that of the ebullition of the first liquid. If now the pressure (atmospheric) on the surface of the latter be slightly reduced, by any means, it will boil. The vapour from this can be made to boil a third quantity of the liquid, under a further diminished pressure.

The successive effects in economic evaporation consist, therefore, in utilising the latent heat of a vapour given off from a liquid under a certain pressure (e. g. that of the atmosphere) to vaporise a further quantity of the liquid under a pressure maintained by mechanical means below that of the first. In the ordinary methods the vapour does its work in the successive effects by passage through systems of tubes, the liquid to be heated being in contact externally; in the Yaryan system, on the other hand, the arrangement {232} is reversed. The liquid to be evaporated traverses the system of tubes which are heated externally by the vapours. At the end of each effect, the liquid is caused to impinge, in a special chamber, upon a disc: in this way a complete separation of liquid and vapour is effected, each then passing on to the next effect, the former through the tube-system, the latter to the chamber inclosing these. The flow of liquid is maintained by a force-pump, and the diminished pressure by a vacuum-pump suitably disposed. This system differs from that described on page 180, in that the evaporation is continuous, the dilute liquors entering the apparatus and the highly concentrated liquors leaving it in an unbroken stream. The rate of flow is such that the evaporation of the caustic liquors from wood boiling from 8–10° to 80° Twaddell, in a quadruple effect, requires only a few minutes. At the latter concentration it is ready for the incineration process, which by means of a rotary furnace, such as that of Mr. J. W. Hammond, of the firm of S. D. Warren and Co., is also effected continuously. It is found, moreover, that the excess of heat available from this process is sufficient for the evaporation.

INDEX.

Abaca, see Manilla hemp

Abies pulp, 60

Acetic acid in bleaching, 114

Acid, action on paper, 163

― in alum, estimating, 207

― ― ―, evils of, 163

― processes for wood, 69

― radicles and cellulose, 8

Acids added in bleaching pulps, 112

― and cellulose, 12

Adansonia fibre, 53

― ―, boiling, 103

― ―, cutting, 103

― ―, willowing, 103

Adipo-cellulose, 26

Adulterating paper, 130

Agalite loading, 133

Agave americana fibre, 33, 44

Alkali boiled with rags, 82

― process for isolating cellulose, early methods, 65

Alkaline processes for isolating cellulose, 62

Alkalis and cellulose, 14

― and ligno-cellulose, 21

Aloe fibre, 33, 39, 44, 46

― ―, dimensions, 39

― perfoliata fibre, 39

Alum, action on cellulose, 163

―,― on gelatine, 163

―, analysing, 206

― cake, analysing, 206

―, estimating acid in, 207

―, freedom from acid, 163

― in sizing, 139

―, proportion required, 141

― solutions, table of strengths, 143

Alumina resinate, 137

Alumina sulphate, analysing, 206

― ― in sizing, 140

Aluminous cake, 140

Ammoniacal solution of cupric oxide, 5

Amyloid, 12

Analysis of plant substances, 42

Analysing alum, 206

― ― cake, 206

― alumina sulphate, 206

― antichlor, 208

― bleaching powder, 205

― caustic soda, 205

― dyes, 209

― gelatine, 208

― loading material, 203, 209

― paper, 199

― pigments, 209

― recovered soda, 205

― size, 203

― soaps, 208

― soda ash, 205

― sodium sulphite, 208

― ― thiosulphate, 208

― starches, 208

― wood pulp, 201

Ananassa sativa fibre, 44

Aniline blues, 141

― colours, solutions, 37

― for diagnosing plant substances, 43

― pinks, 142

― sulphate solution, 37

Animal cellulose, 7

Annandale’s damping method, 159

Antichlor, analysing, 208

―, lime and sulphur, 129

―, proportion required to neutralise bleaching powder, 128

―, removing bleaching liquor by, 127

Antichlor, sodium hyposulphite, 127, 129

―, ― sulphite, 125

―, ― thiosulphate, 127, 129

―, testing, 130

Apparatus for microscopic examination of fibres, 35

Ash of cotton pulp, 136

― of esparto pulp, 136

― of linen pulp, 136

― of paper, estimating, 134

― of paper, natural, 136

― of straw pulp, 136

― of wood pulp, 136

Aspen for mechanical wood pulp, 108

Available chlorine in bleaching powder, 110

Bachet and Machard’s process forwood, 69

Baobab fibre, 53

― ― composition, 54

Barium hydrate and cellulose, 7

Bark fibres, 2

Barley straw, 98

Barre’s acid process for wood, 69

Bast fibres, 34

― ―, dimensions, 39

― filaments, 34

―, nature of, 2

― tissues, 53

Beater bed-plate, 120

― knives, 120

― ―, cutting in situ, 121

―, lead-lined, 120

Beaters, 117

―, direct-driven, 126

―, engine for driving, 126

―, water for, 125

Beating esparto, 117

―, proportion of water to pulp, 118

― pulp, 117

― ―, duration of, 119

― wood pulp, 117

Bed-plate of beater, 120

Beech, composition, 61

Bertram’s beater, 122

― damping cylinder, 159

― direct-driven beaters, 126

Bertram’s rag boiler, 82

― smoothers, 158

― strainers, 148

Bibliography, 229

Birch, composition, 61

―, pulp yield, 77

Bisulphite, action on cellulose compared with soda, 76

―, colour of pulp produced by, 77

― process, advantages, 76

― processes for wood, 71

Bleached pulp, freeing from bleach, 114

― ―, improving colour, 114

Bleaching, acetic acid in, 114

― action should be confined to non-cellulose, 113

― by chlorine, 114

―, electrolytic, 115

― pulp in potchers, 111

― pulps, 110

― rags, 115

― refractory pulps, 114

―, time required by, 113

―, with sodium hypochlorite, 113

Bleaching-liquor, clear, 111

―, removing by antichlor, 127

―, removing from half-stuff, 127

―, storing, 111

―, strength, 111

―, washing out from half-stuff, 127

Bleaching-powder, accelerating action of, 112

―, analysing, 205

― and cellulose, 10

―, available chlorine in, 110

―, keeping, 111

―, neutralising by lime and sulphur, 129

―, ― by sodium hyposulphite, 127, 129

―, ― by sodium sulphate, 128

―, ― by sodium thiosulphate, 127, 129

―, preparing, 110

―, proportion of antichlor required to neutralise, 128

―, quantities needed, 112

Blitz’s sulphide process, 67

Blondel’s acid process for wood, 69

Blotting paper, starching, 139

Blue colours, 141

Boehmeria, 50

― nivea fibre, 39

― puya fibre, 44

Boilers for esparto, 90

― for resolving wood, 74

― for straw, 99

―, iron, 76

―, lead-lining, 75

Boiling adansonia fibre, 103

― esparto, 89, 91

― jute, 103

― manilla hemp, 103

― rags, 82

― straw, 100

Boiling-liquors, recovering soda from, 179

Bombax heptaphyllum fibre, 39

Books on paper-making, 229

Bottoms, caustic, 177

Broke paper, grinding mill, 105

― ―, using up, 104

Bromine and jute, 18

Broussonetia kæmpferia fibre, 54

― papyrifera fibre, 39, 54

Burning recovered soda, 186

Calcining recovered soda, 186

Calendering, 167

Calenders, 158

―, deflection, 171

Calcium carbonate mud, 190

― hypochlorite, removing from pulp, 114

― sulphate, estimating, 203

― ― loading, 131

Cannabis sativa fibre, 39, 49

Carbonic acid bleaching, 115

Carbonising recovered soda, 186

Caustic soda, analysing, 205

― ―, brands, 178

― ―, commercial forms, 177

― ― drums, emptying, 179

― ―, prices, 179

― ― solutions, table of strengths, 109

Causticisers, 188, 192

Causticising, 187

― by ferric oxide, 232

―, lime used, 190

―, oxygenating during, 191

Cellular tissue in straw, 103

Cellulose, 4

―, absorption of acids and basic oxides, 7

―, action of alum on, 163

―, ― of cu­pram­mon­ium on, 5, 217

―, ― of lime on, 84

―, ― of soda and bisulphite compared, 76

―, ― of sulphur on, 130

―, ― on light, 6

― and acids, 12

― and alkalis, 14

― and barium hydrate, 7

― and bleaching powder, 10

― and chlorine, 10

― and humus, 15

― and hydrochloric acid, 13

― and lead solutions, 8

― and metallic salts, 8

― and nitric acid, 13

― and oxygen, 10

― and sulphuric acid, 12

― and water at high temperature, 14

― and zinc chloride, 13

―, animal, 7

―, behaviour under Schweitzer’s reagent, 5

―, bleaching should not affect, 113

―, burning, 23

―, characters, 4

―, combinations, 7

―, ― with acid radicles, 8

― compared with jute, 17

―, composition, 4

―, compounds, 7, 16

―, decomposition, 12

―, ― by ferments, 15

―, ― by heat, 23

―, destructive action of soda on, 64

― di-nitrate, 10

―, early form of alkali process for isolating, 65

― formed by vinegar plant, 16

―, formula, 4

― gelatinised, 5

―, general method of isolation, 4

― hexa-nitrate, 9

―, identification, 6, 18

Cellulose in contact with iron, 13

― in fibre, estimating, 42

― in straw, 99

―, isolating by sulphurous acid, 64

―, ― from plant substances, 62

―, ― from wood, Schultze’s method, 21

―, lignification, 17

―, mineral constituents, 5

―, modifications of, in the plant, 16

―, nitration of, 8

―, oxidation, 10

―, penta-nitrate, 9

―, percentage yielded by jute, 19

―, proportion in esparto, 96

―, ― of, in well-boiled pulp, 113

―, reactions, 6

―, soluble in Schweitzer’s reagent, 5

―, ― sulphides for isolating, 67

―, solubility, 5

―, synthesis, 15

―, tetra-nitrate, 10

―, triacetyl, 8

―, tri-nitrate, 10

―, yield from esparto, 95

―, ― from straw, 98

Celluloses, compound, hydrolytic resolution, 63

―, ―, treating for paper, 63

Cellulosic fermentation, 16

Chapman’s evaporator, 184

Chemical analysis of plant substances, 42

― examination of paper, 201

Chestnut, pulp yield, 77

China clay, estimating, 203

― ― loading, 131

― grass, 39, 46, 50

― ―, character, 50

― ―, composition, 51

― ― fibres, dimensions, 39

― ―, forms in which employed, 51

― ―, general chemical characteristics, 51

― ―, micro-chemical reaction, 51

― ―, microscopic features, 51

Chlorinating straw pulp, 102

Chlorinating wood tissue, 19

Chlorine and cellulose, 10

― and jute, 18

― as a bleaching agent, 114

―, available, in bleaching powder, 110

― water solution, 37

Clarifying water, 125

Classification of paper as to strength, 197

Cleaning straw, 100

Closing plates of strainers, 150

Cochineal lakes, 142

Collodion pyroxyline, 10

Colophony, 137

Colour, improving, 114

― of pulp produced by bisulphite, 77

Colouring paper, 141

Commercial statistics, 221

Composition of paper, testing, 199

Compound celluloses, hydrolytic resolution, 63

― ―, treating for paper, 63

Conifer pulp, 60

Constitution of plant fibres, 1

Cooke and Hibbert’s beater, 123

Corchorus capsularis fibre, 39, 44, 52

― olitorius fibre, 39

Cork, 26

Cotton, 46

―, classification, 46

―, composition of raw fibre, 47

― fibres, dimensions, 39

―, forms in which employed, 48

―, general chemical characteristic, 47

― in paper, detecting, 199, 201

―, micro-chemical reaction, 47

―, microscopic features, 46

―, nature of, 1

― pulp, ash, 136

― ―, beaten, 118

― ―, dimensions of fibre, 119

― rags, statistics, 221, 222

― ―, treatment for paper, 79

― sections, 47

Couch rolls, 156

Cream caustic, 177

Cross’s boiler, 76

Crotalaria juncea fibre, 44, 50

Crystal carbonate, 138

Cuprammonium, 217

Cuprammonium solution, preparing, 217

Cuticular tissue, 26

Cutting, 171

― adansonia fibre, 103

― beater knives, 121

― jute, 103

― manilla hemp, 103

― rags, 81

― straw, 100

Dahl’s sulphide process, 67

Damping-cylinders, 159

Dandy roll, 155

Dark paper from jute, 103

Deckle, 144, 152

Densities, Twaddle, and sp. gr., 109

Detecting adulteration, 203

Diagnosis of plant substances, 42

Di­cot­y­le­don­ous stems, fibres of, 33, 46

Direct-driven beaters, 126

Discoloration of paper from mechanical wood pulp, 108

Doctor, 157

Draining bleached pulp, 114

― boiled rags, 84

― straw pulp, 100

Drum washer for boiled rags, 85

Dry pulp, preparing, 119

Drying cylinders, 157

Dusting rags, 82

Dyes, 141

―, analysing, 209

Economy of sulphide processes, 68

Edgerunner mill, 119

Edgworthia papyrifera fibre, 54

Ekman’s bisulphite process, 72

― process, preparing wood for, 73

Electrolytic bleaching, 115

Emptying soda drums, 179

Engine for driving beaters, 126

― sizing, 137

Esparto, 46, 55

―, amount of bleaching powder needed by, 112

―, beating, 117

―, boiled, washing, 95

― boiler, Roeckner’s, 93

― ―, Sinclair’s, 94

―, boiling, 89, 91

―, character, 55

―, cleaning machine, 90

―, composition, 57

― fibres, dimensions, 39

―, general chemical characteristics, 56

― in paper, detecting, 199, 201

― liquor, disposal of, 98

―, micro-chemical reaction, 56

―, microscopic features, 56

― paper mill, arrangement, 210

―, picking, 90

―, presse-pâte system, 96

―, pressure for boiling, 93

―, proportion of cellulose in, 96

― pulp, ash, 136

― ―, dimensions of fibre, 119

―, quantity of soda for, 92

― section, 56

―, soda required by, 89

―, statistics, 221, 222

―, substances removed from, by caustic soda, 97

―, time of boiling, 92

―, treating for paper, 63

―, yield of cellulose, 95

Estimating acid in alum, 207

― added mineral matter in paper, 136

― mineral matter in paper, 134

Evaporating soda liquors, 180, 231

Evaporators, 180, 231

―, Chapman’s, 184

―, Roeckner’s, 181

―, Porion’s, 182

―, smell consumer, 183

―, Yaryan, 231

Felting, 31

― caused in potchers, 113

Fermentation, cellulosic, 16

Ferments decomposing cellulose, 15

Ferric oxide causticising process, 232

Fibre bundles, 34

―, interweaving to form paper, 30

―, theoretical value in paper, 30

Fibres, analysis, complete, 43

― and cells, 1

―, apparatus for microscopic examination, 35

― classified according to iodine reaction, 46

―, dimensions, 38

―,― in pulps, 119

―, estimating cellulose in, 42

―, ― purity, 39

―, isolating for examination, 38

―, microscopic features, 32

―, nature of, 1

― of di­cot­y­le­don­ous stems, 33

― of mono­cot­y­le­don­ous stems, 33

―, physical structure, 30

―, raw, analysing, 42

―, reagents for examining, 36

―, sections for examination, 38

―, separation, 34

―, table of characters, 44

―, ― of length, 39

―, Webster on analysis of, 42

Fibrous tissue, 34

Fibro-vascular bundles, 2

― ― ―, dimensions, 39

Filaments, 34

―, dimensions of cells and fibres, 39

―, table of lengths, 39

Filter bags for water, 126

― paper, starching, 139

Filtering lime mud, 190

Finish, 159

Finishing-house, 175

Fir pulp, 61

Flax, 39, 44

―, characters, 48

―, composition of raw fibre, 48

― fibres, dimensions, 39

―, forms in which employed, 49

―, general chemical characteristic, 48

―, micro-chemical reaction, 48

―, microscopic features, 48

― sections, 48

Forbes’ beater, 122

Fourdrinier machine, 145

Francke’s bisulphite process, 72

Friction-glazing, 169

Fry, Ransome, and Wilkie’s wood-splitting machine, 74

Furfural, 21

Gaskell and Deacon’s crystal carbonate, 138

― sodium sulphite, 128

Gelatine, action of alum on, 163

―, analysing, 208

―, estimating amount of, 204

― for sizing, 161, 162

― size, making, 162

― with soap, 163

Glaser’s straw pulp chlorinating process, 102

Glazing, 167

―, influence on strength, 198

Gossypium fibres, 39, 46

Gould’s beater, 122

Greenbank soda, 178

Grinding straw pulp, 101

― wood to pulp, 105

Guillotine cutter, 174

Gun-cotton, 9

Half-stuff defined, 88

―, freeing from bleaching liquor, 127

―, removing bleaching liquor from, by antichlor, 127

―, washing out bleaching liquor from, 127

Hand-made paper, 144

Hartig-Reusch paper-testing machine, 194

Hemp, 39, 46, 49

―, characters, 49

―, composition, 50

― fibres, dimensions, 39

―, forms in which employed, 50

―, general chemical characteristic, 50

―, micro-chemical reaction, 50

―, microscopic features, 49

―, section, 50

Hermite’s electrolytic bleaching, 115

Hibiscus strictus fibre, 44, 46

Humus and cellulose, 15

Hydracellulose, 12

Hydrochloric acid and cellulose, 13

― ― ― ligno-cellulose, 22

― ― in bleaching pulps, 112

Hydrolytic resolution of compound celluloses, 63

Igniting recovered soda, 186

Iodine and jute, 17

― ― lignified tissue, 17

― ― starch paper, making, 130

― reaction, details of manipulation, 40

― solution, 36

― ―, testing, 36

Iron boilers, 76

― oxide causticising process, 232

Jolly’s balance, 135

Jordan beater, 121

Jute, 33, 39, 44, 46, 52

― and bromine, 18

― ― chlorine, 18

― ― iodine, 17

―, boiling, 103

―, character, 52

― compared with cellulose, 17

―, cutting, 103

―, dark paper from, 103

― fibres, dimensions, 39

―, forms in which employed, 53

―, general chemical characteristic, 53

―, micro-chemical reactions, 52

―, microscopic features, 52

―, objection to acid in bleaching, 112

―, reactions, 17

―, treating for paper, 64

―, willowing, 103

Kaolin, estimating, 203

―, loading, 131

Kingsland beater, 121

Knives for cutters, 171

― of beater, 120

― ― ―, cutting in situ, 121

Knotter, 151

Laid paper, 156

Lead lining to beater, 120

― solutions and cellulose, 8

Leaden boilers, 75

Leaf fibres, 2

Lignification estimated by aniline, 43

― of cellulose, 17

Lignified tissue and chlorine, 18

― ― ― iodine, 17

Ligno-cellulose, 20

―, aldehydic character, 25

―, and dilute alkalis, 21

― and dilute mineral acids, 20

― and hot water, 21

― and hydrochloric acid, 22

― and mineral acids, 20

― and nitro-sulphuric acid, 22

― and sulphuric acid, 22

―, animal digestion of, 23

―, decay of, 23

―, decomposition by heat, 23

―, distribution, 23

―, treating for paper, 64

Lime, action on cellulose, 84

― and sulphur for neutralising bleaching powder, 129

― boiled with rags, 83

― for causticising, 190

―, milk of, 177

― mud, 190

― ―, composition, 191

― ―, filtering, 190

― salts in water, 212

― sulphate loading, 131

― unsuited for esparto, 89

Lime-tree, composition, 61

Linden bast fibres, 39, 54

Linen, 46

― in paper, detecting, 199, 201

― pulp, ash, 136

― ―, beaten, 117

― ―, dimensions of fibre, 119

― rags, statistics, 221, 222

― ―, treatment for paper, 79

Linum usitatissimum fibre, 39, 44, 46

Literature, 229

Loading, estimating, 134

― materials, analysing, 209

― paper, 130

― with agalite, 133

― with china clay, 131

― with lime sulphate, 131

― with magnesium silicate, 134

― with pearl-hardening, 131

Loft-dried paper, 145

Lunge’s bleaching method, 114

Lygeum spartum fibre, 55

Machine, 145

Magnesium silicate loading, 134

Manilla hemp, 44, 46, 59

― ―, boiling, 103

― ―, composition, 60

― ―, cutting, 103

― ―, micro-chemical reaction, 60

― ―, microscopic features, 59

― ―, sections, 60

― ―, treating for paper, 64

― ―, willowing, 103

Masson, Scott, and Bertram’s beater, 123

― ― ― esparto-cleaning machine, 90

― ― ― strainer, 149

Mechanical wood pulp, 105

― ― ―, quantitative estimation, 202

Menzies and Davis’ smell consumer, 183

Metallic salts and cellulose, 8

Microscope equipment, 35

Microscopic features of fibres, 32

Microscopical examination of paper, 201

Milk of lime, 177

Mill, arrangement, 210

―, site for, 210

Millboard machine, 166

Mineral acids and cellulose, 20

― ― and ligno-cellulose, 20

― matter, added, estimating, 136

― ― in paper, estimating, 134

Mixing pulps, 119

Mono­cot­y­le­don­ous stems, fibres of, 33, 46, 54

Mounting solution, 36

Musa paradisiaca fibre, 44

― textilis, 59

Names in paper, 145, 156

Nettle, 52

Neutral mounting solution, 36

New Zealand flax, 39, 44, 46, 59

― ― ―, dimensions, 39

― ― ― general chemical characteristics, 59

― ― ― micro-chemical reaction, 59

New Zealand flax, microscopic features, 59

Nilgherry nettle, 44

Nitric acid and cellulose, 8, 13

Nitro-sulphuric acid and ligno-cellulose, 22

Oak, pulp yield, 77

Oat straw, 98

Oxidising sodium sulphide, 191

Oxycellulose, 13

Oxygen and cellulose, 10

Oxygenating liquors while causticising, 191

Paper ash, estimating, 134

―, broke, pulp from, 104

―, colouring, 141

― from mechanical wood pulp, faults of, 108

―, hand-made, 144

―, loading, 130

―, loft-dried, 145

― machine, 145

― makers’ soap, 163

― making and textiles, 3

―, measuring strength, 193

―, natural ash, 136

―, printed, pulp from, 105

―, sources of raw material, 1

―, thickness, 153

―, toned, 142

― trade, statistics, 225

― waste, pulp from, 104

―, width, 152

Paper-mulberry fibres, 39, 46, 54

Partington’s bisulphite process, 72

Patterns on paper, 145, 156

Pearl hardening, 131

― ―, estimating, 203

Pectic acid, 28

Pecto-cellulose, 28

― ―, treating for paper, 63

Phormium tenax fibre, 39, 44, 59

Physical structure of fibres, 30

Picking esparto, 90

Pictet’s sulphurous acid process, 70

Pigments, 141

―, analysing, 209

Pine, composition, 61

―, pulp yield, 77

Pine, white, for mechanical wood pulp, 108

― wood fibre, 61

Pineapple fibre, 44, 46

Pink colours, 141

Pinus fibre, 60

Plant fibres, constitution, 1

―, structural elements, 1

― tissues mainly cellulose, 4

Pochin’s aluminous cake, 140

Poplar, composition, 61

― fibre, 60

Porion’s evaporator, 182

Porter-Clark process for water, 214

Potchers, felting caused in, 113

― for bleaching pulp in, 111

― for rags, 80

Presse-pâte system for esparto, 96

― ― ― for straw, 101

Pressure for boiling straw, 100

― in relation to soda, 64

Prussian blue, 142

Pulp, beating, 117

―, bleaching, 110

―, ― refractory, 114

―, colour of, produced by bisulphite, 77

―, dimensions of fibre in, 119

―, dry, preparing, 119

―, duration of beating, 119

―, freeing from bleach, 114

―, improving colour, 114

―, mixing, 119

―, removing calcium hypochlorite from, 114, 127

― saver, 114, 155

― yield from straw, 99

― ― ― woods, 77

Purification of water, 211

Rags, bleaching, 115

―, boiled, draining, 84

―, ―, washing, 85

―, boiling, 82

―, ― with lime, 83

―, cutting, 81

―, dusting, 82

―, potchers, 89

―, proportion of alkali, 83

―, quantity of water for boiling, 84

―, sorting, 80

―, statistics, 221, 222

―, time of boiling, 84

―, treatment for paper, 79

―, ― in washer, 89

―, willowing, 81

Ramie fibres, dimensions, 39, 50

Raw material for paper, 1

Reagents for examining fibres, 36

Recovered soda, analysing, 205

― ―, carbonising, 186

― ― causticising, 187

― ―, composition, 186

Refractory pulps, bleaching, 114

Repped paper, 171

Resinate of alumina, 137

Retree, 175

Rhea fibre, 50

Roeckner’s esparto boiler, 93

― evaporator, 181

― strainer, 150

― water clarifier, 211

Rosin, 137

― soap, 137

Rotary boilers unsuited to esparto, 90

Rubbing test for paper, 197

Rye straw, 98

Salle, 175

Salmon’s guillotine, 174

Save-all, 154

Schultze’s method of isolating cellulose from wood, 21

Schurmann’s calender rolls, 171

Schweizer’s reagent, 5

Sections of fibres for examination, 38

Seed hairs, 46

― ―, dimensions, 39

Self-cleaning strainer, 149

Settling ponds, 125

Seventy per cent. white caustic, 178

Sheave, causes of, 90

Silk fibre, 31

Sinclair’s esparto boiler, 94

Single cylinder machines, 164

― sheet cutter, 173

Siphon washer for boiled rags, 87

Site for mill, 210

Sixty per cent. white caustic, 177

Size, estimating amount of, 204

―, ― nature of, 203

―, making with gelatine, 162

Sizes of sheets, 175

Sizing, 137

Slicer, 151

Smalts, 141

Smell consumer, 183

Smoothers, 158

Smoothness, 159

Soaking wood, 107

Soap, analysing, 208

― and gelatine, 163

― for paper-makers, 163

Soda, action on cellulose compared with bisulphite, 76

― ash, analysing, 205

― bicarbonate for accelerating action of bleaching powder, 112

―, brands, 178

― carbonate for sizing, 138

―, commercial kinds, 177

―, destructive action on cellulose, 64

― drums, emptying, 179

― for esparto, 89

― for isolating cellulose, 63

― in relation to pressure, 64

― liquor from esparto, treatment of, 98

― ―, evaporating, 180, 231

― ―, removing sodium-sulphur compounds, 191

―, prices, 179

―, proportion for straw, 99

―, recovered, analysing, 205

― ―, carbonising, 186

― ―, causticising, 187

― ―, composition, 186

― recovery, 179

― solutions, table of strengths, 109

―, substances removed from esparto by, 97

Sodium hypochlorite in bleaching, 113

― hyposulphite, analysing, 208

― ― for neutralising bleaching powder, 127, 129

― sulphide, oxidising, 191

― sulphite, analysing, 208

― ― to neutralise bleaching powder, 128

― -sulphur compounds in soda liquors, 191

― thiosulphate, analysing, 208

― thiosulphate for neutralising bleaching powder, 127, 129

Soft water, objections to, 216

Soluble sulphides for isolating cellulose, 67

Solutions, alum, table of strengths, 143

―, aniline colours, 37

―, aniline sulphate, 37

―, chlorine water, 37

―, iodine, 36

―, neutral mounting, 36

Sorting, 175

― rags, 80

Specific gravity, Twaddle converted into, 109

Spherical rag boiler, 82

Stanhope purifier, 212, 214

Starch and iodide paper, making, 130

―, estimating, 203

― paste for sizing, 138

Starches, analysing, 208

Starching blotting paper, 139

― filter paper, 139

Statistics, 221

Steam and ligno-celluloses, 21

Steaming wood, 107

Stipa tenacissima fibre, 39, 55

Strainers, 147

―, closing plates, 150

―, knotter, 151

Straw, 46, 57

―, amount of bleaching powder needed by, 112

―, boiled, washing, 100

―, boilers for, 99

―, boiling, 100

―, cellulose yield from, 98

―, cleaning, 100

―, composition, 59, 99

―, cutting, 100

―, dimensions of cells, 58

―, general chemical characteristics, 58

― in paper, detecting, 200, 201

―, microscopic features, 57

― paper, source of weakness, 103

―, pressure in boiling, 100

―, proportion of soda for, 99

― pulp, ash, 136

― ―, chlorinating, 102

― ―, dimensions of fibre, 119

― pulp, draining, 100

― ―, dry, preparing, 119

― ―, grinding, 101

― ―, yield, 99

―, soda treatment, 98

―, treating for paper, 63

― washer, 101

Strengths of alum solutions, table, 143

― of paper, classification, 197

― ―, influence of glazing, 198

― ―, measuring, 193

Sugar-cane fibre, 59

Sulphide processes, economy of, 68

― ―, malodours of, 68

Sulphite processes, objection to, 75

Sulphur, action on cellulose, 130

― and lime for neutralising bleaching powder, 129

Sulphuric acid and cellulose, 12

― ― and ligno-cellulose, 22

― ― in bleaching pulps, 112

Sulphurous acid for isolating cellulose, 64

― ― processes for wood, 70

― ― processes, objection to, 75

Sunda, see Sunn hemp.

Sunn hemp, 44, 46, 50

― ―, characters, 50

― ―, composition, 50

― ―, general chemical characteristics, 50

― ―, micro-chemical reactions, 50

― ―, microscopic features, 50

Synthesis of cellulose, 15

Table of processes for resolving wood, 78

― of strengths of alum solutions, 143

― ― ― of caustic soda solutions, 109

Teazing needles, 38

Telegram paper, 104

Test paper, making, 130

Testing antichlor, 130

― iodine solution, 36

― paper, 193

― ― by rubbing, 197

Textile trade waste, treating for paper, 62

Thickness of paper, gauging, 153

― ― ―, measuring, 198

Thompson’s rag-bleaching process, 115

Thune’s mechanical wood pulp, 107

Tilghmann’s sulphurous acid process, 70

Tilia grandifolia fibre, 39, 54

Time of boiling rags, 84

Toned paper, 142

Trade statistics, 221

Triacetyl cellulose, 8

Tub sizing, 137

― ― on the machine, 160

Twaddle degrees, converting into specific gravity, 109

Ultramarine, 141

Umpherstoil’s beater, 123

Urtica dioica fibre, 52

― heterophylla fibre, 44

Vacuum strainer, 148

Victory cutter, 174

Vinegar plant forming cellulose, 16

Vomiting boiler, 91

Washing boiled esparto, 95

― ― rags, 85

― ― straw, 100

― out bleaching liquor from half-stuff, 127

Waste paper, pulp from, 104

Water and ligno-cellulose, 21

― at high temperature and cellulose, 15

―, clarifier, 211

―, clarifying, 125

―, filter bags, 126

― for beaters, 125

―, lime salts, 212

― marks, 145, 156

―, Porter-Clark process, 214

― proportion to pulp in beating, 118

―, purification, 211

―, quantity for boiling rags, 84

― resolving wood by, 65

―, settling ponds, 125

―, soft, objections to, 216

―, soluble impurities, 212

Water supply, 210

Web glazing, 167

Webster on analysis of fibres, 42

Weighing paper ash, 134

Weight per ream, 176

Weighting paper, 130

Wet end of machine, 154

― felt, 157

Wheat straw, 98

White fir fibre, 61

― pine for mechanical wood pulp, 108

Width of paper, gauging, 152

Willesden canvas, 219

― paper, 219

― scrim, 219

― welded goods, 220

Willowing adansonia fibre, 103

― jute, 103

― Manilla hemp, 103

― rags, 81

Winding, 160

Wire-cloth, 153

― in rag washer, 87

―, shifting, 156

Wood, acid processes, 69

―, amount of bleaching powder needed by, 112

―, bisulphite processes, 71

―, boiling with sulphites, 74

―, chemical, in paper, detecting, 200

―, cutting up for pulp, 105

―, grinding, 105

―, mechanical, in paper, detecting, 200, 201

―, objection to acid in bleaching, 112

― processes, table of, 78

― pulps, 60

― ―, ash, 136

― ― beating, 117

― ―, dimensions of fibre, 119

― ―, dry, preparing, 119

― ― from aspen, 108

― ― from white pine, 108

― ―, mechanical, 105

― ―, ―, quantitative estimation, 202

― ―, statistics, 224

―, resolving by water, 65

―, Schultze’s method of isolating cellulose from, 21

―, soaking, 107

― splitting machine, 74

―, steaming, 107

― suitable for Ekman’s process, 73

―, sulphide treatment, 67

―, sulphurous acid processes, 70

―, treating for paper, 64

Woods, pulp yield of, 77

Wool fibre, character, 31

Wove paper, 156

Wrapping paper, 104

Yankee machine, 164

Yaryan evaporator, 232

Young and Pettigrew’s acid process, 69

Yucca gloriosa fibre, 44, 46

Zinc chloride and cellulose, 13