[CHAPTER XI.]
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—
| Powdered resin | 4·80 | parts. |
| Soda crystals at 80° (French, alkalimeter) | 2·22 | " |
| Water | 100 | " |
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.