Alexander Watt

LOADING.—SIZING.—COLOURING.

Loading.—Sizing.—French Method of Preparing Engine Size.—Zinc Soaps in Sizing.—Colouring.—Animal or Tub-Sizing.—Preparation of Animal Size.—American Method of Sizing.—Machine-Sizing.—Double-sized Paper.—Mr. Wyatt's Remarks on Sizing.

Loading.—The very finest qualities of paper are usually made without the addition of any loading, as it is called, but for most other papers more or less loading material is added, according to the quality of paper to be produced. The loading material used for ordinary qualities is kaolin, or china clay, and for the better qualities sulphate of lime or pearl hardening, as it is termed in the trade. China clay, as it occurs in commerce, is in the form of soft lumps and powder, is nearly white, and when rubbed between the finger and thumb should present no hard particles of gritty matter. To prepare it for mixing with the pulp it is first worked up into a thin cream with water, which is usually done in a vessel furnished with an agitating arrangement by which the clay becomes intimately mixed with the water. The cream is then strained through a fine sieve to separate any impurities present, and is then allowed to flow into the beating-engine containing the stuff while in motion, by which it soon becomes mingled with the pulp. The proportion of china clay or other loading material which is to be introduced into the pulp depends upon the quality of the fibre and the requirements of the manufacturer, some makers using less of the material than others. From 3 per cent. to 10 or 15 per cent. appears to be about the extreme range for employing the material as a necessary ingredient, in the production of various classes of paper, above which figures the addition of loading material may be considered as an adulteration. Sometimes nearly twice the largest amount named is employed, no doubt to meet the exigences of keen competition—from foreign sources especially.

One effect of the loading, whether it be china clay or sulphate of lime, is to close the pores of the paper, whereby a smoother surface is obtained, while at the same time, if the material has been used in proportions suited to the quality of the fibre, and not in immoderate excess, a stronger paper is produced. A species of asbestos termed agalite has been introduced as a loading material, and since it has a fibrous texture, it blends with the fibres of the pulp, forming, as it were, a vegeto-mineral paper. It is stated that as much as 90 per cent. of the agalite used in the beating-engine enters into the manufactured paper, while not much more than half the china clay used is held by the pulp.

Sizing.—"Engine sizing," as it is termed, consists in adding certain ingredients to the pulp while in the beating-engine. The materials generally used are alum and resin soap, in proportions suitable to the paper to be produced. Resin soap is formed by boiling ordinary resin in a jacketed pan such as is used by soapmakers for preparing small quantities of fancy or other soaps, with a solution of soda crystals in the following proportions: Resin, 16 lbs.; soda crystals dissolved in water, 8 lbs.; and the boiling is kept up for about two hours, or until a soap is produced which is perfectly soluble in water. The method of preparing this soap as conducted at the soapworks has been described in the author's work on soap-making,[22] p. 64, from which the following abstract is taken: "Put into a pan capable of holding about 12 gallons, 2¼ gallons of fresh caustic soda ley at 30° B. Apply gentle heat, and when the ley begins to boil throw in, every few minutes, in small quantities at a time, finely powdered and sifted resin until 37 lbs. have been introduced. The mixture must be well stirred the whole time to prevent the resin from 'clogging' and adhering to the pan. It is important to moderate the heat, as the resin soap has a great tendency to expand and an excess of heat would cause it to boil over. The heat, however, must be kept to near the boiling point, otherwise the mass will become thick and of a very dark colour. When kept at near the boiling point it is always clear and its colour of a reddish yellow. If, during the boiling, the resin soap rises and threatens to overflow, the heat must be checked by throwing in a little cold water, only using sufficient to effect this object. It is absolutely necessary to stir the mass continually, otherwise the resin will agglomerate in masses and thus prevent the alkali from acting freely upon it. The boiling takes about two hours, when the soap is run into an iron frame and allowed to cool. It is very important that the resin used is freed from particles of wood, straw, etc., for which purpose it should be passed through a tolerably fine sieve."

Respecting the preparation of resin soap, Davis says:—"The proportion of resin used to each pound of soda ash varies in different mills, 3, 4, or even 5 lbs. of resin being used to each pound of soda ash. The proportion of resin, soda ash, and water, can be best determined by practical experience, as no prescription could be devised which would be suitable to every case." M. d'Arcet, who modified the proportions recommended by M. Bracconot, recommends for the preparation of resin soap—

Theoretically speaking, only 2·45 parts of alum would be required to precipitate the resin; but the waters, which are almost always calcareous, neutralise part of the alum. Crystals of soda are much more expensive than soda ash, but on account of their greater purity they are sometimes preferred to the latter. At the present day the resin soap is preferably made by dissolving ordinary resin with a solution of carbonate of soda under boiling heat in a steam-jacketed boiler, the class of paper to be made governing the quantity of resin to be employed. The boiling usually requires from two to eight hours, according to the relative proportions of soda ash and resin used—the greater the proportion of soda used the less time is required for boiling—the process being completed when a sample of the soap formed is completely soluble in water.... About 3 lbs. of resin to 1 lb. of soda is the usual proportion. The resin soap is cooled after boiling by running it into iron tanks, where it is allowed to settle, the soap forming a dense syrup-like mass, and the colouring matters and other admixtures of the resin rising to the top are easily removed. It is important to run off the mother liquor (ley) containing the excess of alkali, for when the soap is used it consumes the alum to neutralise it."

When the impurities and ley have been removed the soap is dissolved in water, and if, from imperfect boiling, a portion of the resin is found not to have been saponified, a small quantity of a strong solution of soda crystals is added to the water used for dissolving the soap.

Where starch is used for stiffening purposes, the soap is mixed with a quantity of starch paste in the proportion of 1½ part of starch to 1 part of resin soap. Some manufacturers, Mr. Davis states, mix the starch paste with the kaolin in lieu of mixing it with the resin soap. In either case the materials should be thoroughly strained before being added to the pulp. From 3 to 4 lbs. of the mixture of resin soap and starch paste to each 100 lbs. of dry pulp are about the proportions in which the size is generally used, but the quantity added to the pulp in the beater depends upon whether the paper is to be soft-sized or hard-sized.

Sizing is chiefly applied to papers which are to be written upon with ordinary inks, and also, with a few exceptions, to printing papers, the object being to close the pores of the paper and render it non-absorbent, by which the spreading or running of the ink is effectually prevented. While the finest lines may be written upon a well-sized paper (as ordinary writing paper, for example) without spreading in the least degree, a similar stroke of the pen upon blotting paper, tissue, or unsized printing paper would spread in all directions, owing to the highly absorptive property of the cellulose.

The sizing of the pulp is conducted as follows:—After the loading material has been introduced and well mixed, the resin soap, previously dissolved in water, a little carbonate of soda being sometimes added, is mixed with a paste of starch prepared by dissolving starch in boiling water, and the mixture of soap and starch is then passed through a fine sieve to keep back any particles or lumps that may be present. The proportion of the materials used in sizing vary at the different mills, each manufacturer having formulÆ of his own; about 1 part of resin size to 3 of starch paste, and, say, from 9 to 12 lbs. of the mixture, may be used for 300 lbs. of pulp; and, if preferred, the respective ingredients may be put into the engine separately, a method adopted at some mills. Some manufacturers of the finest papers, instead of dissolving the starch in hot water, make it into a thin paste with cold water, in which condition it is introduced into the pulp, the object being to impart to the paper a particular feeling to the touch which is not obtainable by other means.

The mixture of resin size and starch paste, with or without the addition of water, is added to the pulp in the beater, in which the pulp is circulating, and the engine allowed to run until the materials are well incorporated in the pulp. At this stage a solution of alum (about 28 to 30 lbs. for 300 lbs. of pulp), or of sulphate of alumina,[23] is introduced, which causes the resin soap to become "separated," the sulphuric acid of the alum uniting with the alkali of the soap and setting the resin and alumina free in the form of minute particles; the resin in the subsequent drying on the calenders becomes fused, as it were, and thus cements the fibres and alumina together, at the same time rendering them non-absorbent and improved in whiteness by the precipitated alumina. Sometimes ordinary soap is added to the resin soap, which is said to impart a higher finish to the paper in the operation of calendering.

The so-called "concentrated alum," which contains a higher percentage of sulphate of alumina than the crystallised alum, is considered the most economical in use, being proportionately cheaper, and the variety known as "pearl alum" is specially recommended. "Aluminous cake" is another preparation which has found favour in many mills, but since it sometimes contains a large excess of free sulphuric acid it requires to be used with caution, since this acid, although it will brighten the colour of some aniline dyes, will discharge the colour from others, while at the same time it may injuriously affect the brass-wire cloths of the paper machine. The alum solution should be prepared in a lead-lined tank, fitted with a steam pipe for heating the contents when required.

The proportions of the materials used in sizing differ considerably in different mills, but the following may be taken as an average for common writing and printing papers:—

To the sizing solution is generally added from 30 to 50 parts of kaolin. When a colour is present on which alum would have a prejudicial effect this is usually replaced by about one-third of its weight of sulphate of zinc. Many mineral substances have from time to time been added to paper stock, principally to increase its weight, and in 1858 Sholl took out a patent for adding carbonate of lime, a substance which, however, had long been fraudulently used in order to increase the weight, but he found it to have the property of fixing the ink in the pores of the paper, thus rendering it immovable. The only useful addition is kaolin, or some similar aluminous compound, as it attaches itself to the fibre, and, while giving the required opacity and a good surface, takes both printing and writing ink well, and has the advantage, from a manufacturer's point of view, of increasing the weight. It has been proposed that small quantities of glycerine be added to the pulp, in order to give the paper greater flexibility, and especially to give copying-paper the quality of taking up colour readily.[24]

French Method of Preparing Engine Size.—Thirteen pails of water are boiled in a copper-jacketed pan capable of holding about 150 gallons; 90 lbs. of soda crystals are then introduced and allowed to dissolve, when 200 lbs. of finely-powdered resin are gradually introduced, with constant stirring, and the boiling is sustained for about two hours after the last portion of resin has been added. A further addition of water is now made by putting in five pails of cold water, and the water is then boiled for an hour and a half longer. The resin soap is then transferred to stock-chests, in which it is allowed to remain for ten days or longer, fresh batches being prepared in rotation, to meet the requirements of the mill.

To determine whether an excess of resin soap or of alum has been added to the pulp, red and blue litmus papers should be employed, the former turning blue if an excess of resin soap be present, and the latter red when alum or sulphate of alumina is in excess. For uncoloured papers the aluminous material should be added until the pulp becomes faintly acid, which will be indicated by the blue litmus paper turning slightly red when immersed in the pulp.

Besides resin soap, various substances have been proposed as sizing materials, including wax dissolved in a strong solution of caustic soda and precipitated with alum, but the cost would be an objection to the use of this material except for the highest classes of paper. It is stated that 12 lbs. of gum tragacanth to each 500 lbs. of resin has been used in preparing some kinds of engine-sized papers, and is said to impart to them an appearance equal to that of tub-sized papers.

Zinc Soaps in Sizing.—According to a paragraph in the Papermakers' Monthly Journal, a somewhat novel method of sizing is employed in Germany, which consists in the precipitation in the stock of zinc soaps. Cottonseed oil soap or Castille soap is worked up in the engine with the stuff, and after it has become well mixed with the pulp a solution of sulphate of zinc is added, which results in the formation of a white and heavy zinc soap, which is insoluble, and adheres well to the fibres. The weight and whiteness of the zinc soap are the main points in favour of this method, which is said to yield good results.

Colouring.—The pulp, after passing through the various processes described, although apparently white, invariably presents a yellow tinge when converted into paper. To obviate this it is usual to "kill" the yellow tint by adding to the pulp small quantities of blue and pink colouring matters. The blue colours generally used are ultramarine, smalts, and various aniline blues, and the pinks are usually prepared from cochineal, either in a liquid form or as "lakes" (compounds of cochineal and alumina) or aniline dyes, the former being preferable, as it is not injuriously affected by the alum used in sizing. The ultramarine should be of good quality, otherwise it will become decomposed, and its colouring property destroyed by the action of the alum, but more especially so if the alum contains an excess of free acid. Smalts blue, which is a kind of coloured glass, is not affected by acids. In preparing the colouring matters for mixing with the pulp they must first be mixed with water, and the liquid should then be strained, to keep back any solid particles that may be present in the material. Aniline blues should be dissolved in hot water, or alcohol, and then diluted. Samples of the pulp are examined from time to time until the desired effect is produced, which the practised eye of the beater-man can readily determine.

Animal or Tub-sizing.—Another process of sizing, termed "animal-sizing," "tub-sizing," or "surface-sizing," is also adopted in the manufacture of certain classes of paper, and is either accomplished by hand or on the machine. The former method having been elsewhere described (p. 132) we will now describe the operation of sizing on the machine, to which the term tub-sizing is also applied. The size employed, which is prepared from what are called "glue pieces," or clippings of "limed" and unhaired skins of animals, requires to be as colourless as possible, in order that the colour of the paper may not be injuriously affected by it.

Preparation of Animal Size.—This operation is generally conducted at the mill, the materials from which the size is produced being the cuttings or parings of animal skins and hides, or pelts, which have undergone the processes of "liming" and unhairing preparatory to being tanned. The cuttings, or pates, commonly called "glue pieces," are first soaked in a mixture of lime and water, placed in large tubs for several days, after which they are put into a wooden cylinder, or drum, five or six feet in diameter, and about ten feet in length, which revolves upon a horizontal shaft, which, being hollow, admits the passage of water to the interior of the drum. The drum is perforated, and revolves in a large tank, while a continuous stream of water is allowed to pass through it, and the dirty water escapes through the perforations in the drum. When the cuttings are sufficiently cleansed in this way, they are transferred to an iron copper, furnished with a false bottom and steam-pipe, or a jacketed pan. The cuttings are next covered with water; steam is then turned on, and the liquid brought to a temperature below boiling point, or say, about 180° to 190° F., it being very important that the liquid should not actually boil. This operation is carefully kept up for twelve to sixteen hours, according to the nature of the cuttings, by which time all the material excepting any membranous or fatty matters that may be present, will have become dissolved and a solution of gelatine obtained. The liquor is then allowed to settle for a short time to allow fatty matters to rise to the surface and membranous substances to deposit, and the fatty matters must afterwards be carefully removed by skimming. The liquor should next be strained to separate any floating particles of a membranous character. Sometimes the gelatine solution is clarified by adding a small quantity of powdered lime, which is thoroughly mixed by stirring, after which it is allowed to rest. When it is found that the impurities and lime deposit too slowly, a little weak sulphuric acid is added, which, forming an insoluble sulphate of lime, the solid matters quickly subside, leaving the liquor quite clear. The solution is next filtered through felt, and is afterwards treated with a solution of alum, which at first causes the liquid to thicken and become nearly solid, but it becomes fluid again, however, on the addition of more alum solution. When this condition is finally attained, the liquid is ready for use in the process of sizing. The addition of the alum (which should not contain any free acid) to the gelatine greatly improves its sizing property, besides preserving it from decomposition. The treatment of the glue pieces for the purpose of obtaining gelatine solutions is fully described in the author's work on "Leather Manufacture," p. 401.[25]

American Method of Sizing.—Another method of preparing size, and which is adopted in America, is the following:—In large paper mills the size is generally prepared in a room devoted to the purpose, and is commonly situated near the machine. The finest grades of light hide and skin clippings are used for No. 1 letter papers, but less costly stock is employed for the lower grades of animal-sized papers. To preserve the glue pieces the tanners and tawers macerate the clippings in milk of lime and afterwards dry them. As the clippings require to be freed from the lime, the first treatment they receive at the paper-mill is to put them in large wooden tubs partly filled with water, in which they are allowed to soak for several days. They are afterwards more perfectly cleansed by means of a drum-washer, such as we have before described. Fresh hide and skin clippings, that is, those which have not been limed and dried at the tanneries, and which are occasionally purchased by the paper manufacturers, require to be used as soon as possible after they arrive at the mill as they readily decompose, and are placed in tubs partly filled with water, in which 2 per cent. by weight of caustic lime has been dissolved. The pieces, if from calfskins, are allowed to remain in the lime bath for ten to fifteen days, clippings of sheepskins fifteen to twenty days, and trimmings from heavy hides, as ox, etc., twenty-five to thirty days, the milk of lime being renewed once or twice a week, and the material well stirred from time to time. The glue-stock, as it is sometimes termed, is afterwards thoroughly washed in the drum-washer, and when this operation is complete the material is spread out in the yard to drain, and when sufficiently dried is ready for boiling, or may be stored until required for use.

To prepare size from the material treated as described, it is placed in a boiler of cast or wrought-iron or copper, furnished with a perforated false bottom, and capable of holding from 100 to 400 lbs. of the raw material, according to the requirements of the mill. Several such boilers may be placed close to each other. At the bottom of the boiler is a stop-cock for drawing off the gelatine solution when required. When the requisite charge of glue-stock has been introduced into the boiler, water is poured over it and steam turned on, which passes through a pipe fixed beneath the false bottom, and care is taken that the temperature of the contents of the boiler should not exceed 200° F., which heat is kept up for ten to eighteen hours, according to the nature of the materials treated. The gelatine solution is drawn off from the boiler as it is formed, into wooden tubs, and at the same time carefully strained to remove membranous matters and suchlike impurities. Several boilings are made from the same batch of glue-stock, and all the solutions are afterwards mixed together in the receiving tubs, and a solution of alum is added in such proportions as to be recognised by tasting the liquor. One object in adding the alum being to prevent the gelatine from decomposing, more of this substance should be added in warm than in cold weather.

When the solutions are cool they are ready for use, and the gelatine is removed from the receiving tubs and dissolved in a separate tub as required for use, the dissolving tub being provided with a steam-pipe. The proportion of water—which should only be lukewarm—used in dissolving the gelatine varies from a quarter to half the bulk of the latter, the nature of the fibre and thickness of the paper regulating the proportion of water to gelatine, the strength of the size liquors being greater for thin papers and weak fibres than for thick papers and strong fibres.

The operation of sizing is considered one of the most difficult and uncertain with which the paper-maker has to deal, since the material (gelatine) is greatly influenced by the conditions of the atmosphere, both as regards its temperature and humidity, while the temperature of the liquid size itself has also an important influence on the success of the operation. The condition of the paper, again, also affects the result, for if it be highly porous it will probably be weak, and consequently there may be considerable waste during the process of sizing from the necessary handling it is subjected to; moreover, should the paper have been blued with ultramarine, a strongly offensive odour is often imparted to it; this, however, may be obviated by employing fresh size and drying the paper as completely as possible. There are two systems of animal-sizing employed at the mill, namely, hand-sizing and machine-sizing, which is also called tub-sizing, the former being applied to papers of the finest quality. Papers that have been made by the machine, after being cut into sheets, are hand-sized, as described in the next chapter.

Machine-Sizing.—The lower-priced papers, to be machine-sized, are first partly dried over a few cylinders, after which the paper passes through a tank containing liquid size, from whence it passes between two rollers, which squeeze out the superfluous size; it is then wound on to a reel on which it remains some time to enable the size to thoroughly permeate the paper, after which it is wound on to another reel, and from thence it passes over a series of wooden drums or cylinders, each of which is furnished with a revolving fan; by this means the paper becomes dried slowly, whereby a more perfect sizing of the material is effected.

Double-Sized Paper.—This term is applied to paper which, after being sized in the engine in the usual way, is afterwards "surface sized," as it is called, with animal size in the manner described.

Respecting the drying of paper after it has been tub-sized there seems to be some difference of opinion as to whether it is best to hang it in a loft to dry or to dry it over the cylinders of a drying machine. Upon this point the New York Paper Trade Journal makes the following remarks:—"When the paper is passed through the size-tub, it is again wet; the fibres expand, and their hold on each other is relaxed. Now it must make a difference to the subsequent strength and quality of this paper whether it be hung up in a loft to dry or run over a drying machine. If it is hung in the loft no strain is put upon it and the fibres are at liberty to shrink, or slowly contract, in all directions; whereas if it is run over a drying machine, consisting of from 50 to 100 reels, the longitudinal strain prevents the fibres from shrinking and reassuming their normal position in that direction. Attempts have been made to obviate this defect by regulating the speed of each section of the machine in such a manner as to allow for the shrinking, but this only remedies the evil by preventing the paper from breaking as it travels over the machine. Everything else being equal, it would seem that loft-dried paper must be superior to that dried over the drying machine. Our home manufacturers endorse this view, inasmuch as they continue to prefer the system of loft-drying to the less expensive machine methods."

Mr. Wyatt's Remarks on Sizing.—Mr. James W. Wyatt, in a paper on the "Art of Paper-making,"[26] makes the following observations on engine-sizing and animal-sizing which will be read with interest:—"Engine-sizing renders the paper fully as non-absorbent as animal size. The latter penetrates the sheets only slightly and forms a coating or skin on each surface, whereas the engine size surrounds each fibre and impregnates the whole mass. Surface-sizing, however, produces a stronger, firmer sheet, and is smoother for the pen to travel over; the manufacturer also gets the benefit in the price of the paper of the additional weight of the size, amounting to 7 per cent. on the average. On the other hand, as the animal size is mostly a skin on the surface, if the coating be broken anywhere by the use of a knife in scratching, the paper will only imperfectly resist ink in that place, a great disadvantage for account and office-books and ledgers. Engine-sized paper is much cheaper to produce than animal sized, and is therefore used principally for the lower qualities of writings and for almost all kinds of printings where firmness and smoothness is not so much a desideratum. Most tub-sized papers have a certain portion of engine size mixed with the pulp. This not only ensures the thorough sizing of the sheet, but also is a measure of economy in reducing the absorbing power of the paper for the animal size. Papers for ledgers and office-work are best given an extra proportion of engine size to ensure their ink-resisting properties, and they are also sized by hand in animal size and loft dried." The following rough estimate of the comparative cost in materials and wages of engine-sizing and animal-sizing paper may be of interest: