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.