There are several kinds of filler in common use. The most common is China clay, of which the cleanest and finest grades are obtained principally in England. No equally good deposit has yet been successfully developed in this country. Clay is a product of the natural disintegration of feldspar. It is soft, plastic, and non-crystalline.
Agalite and talc, which are silicates of magnesia, are also used. They are cheaper and less desirable, both on account of color and their crystalline nature, which is more or less damaging to cutter knives and printing-plates. These fillers are used widely in the cheaper book-papers, and can often be detected by holding a sheet against the light, as the little, translucent crystalline particles then appear like pinholes.
Sulphate of lime, commercially known under such names as gypsum, pearl hardening, satinite, etc., is a white, crystalline substance. This is used to some extent in paper-making, but principally as a coating.
Barium sulphate, prepared chemically, and known as blanc fixe, is used largely for coating papers because of its brilliancy and purity of color.
Sizing Materials.—Starch was one of the earliest materials used for sizing paper, and is used considerably in addition to other materials, as it adds a hard, tinny character desired by the trade on certain grades. Silicate of soda is also used to impart similar characteristics.
Gelatine, or animal size, is obtained by boiling down suitable animal tissues. As a sizing agent, it is applied after the paper is made by passing the web of paper through a vat containing the hot liquid size.
Casein, which is sometimes used as sizing, is more important in its functions as an adhesive for the making of coated paper. It is prepared by treating skim milk with weak acid.
Rosin size, the most widely used size, is produced from rosin by cooking with soda ash, which produces a soft soap. The soap when mixed with water by agitation assumes a milky appearance. In this condition it is poured into the beater after all other ingredients have entered, and is precipitated by the addition of alum as a resinate of alumina.
Impurities in Paper.—Impurities, either chemical or physical, are sometimes found in paper, owing to lax methods or inferior materials.
Free acid occasionally occurs, and in some cases would be very deleterious. In papers that are to be bronzed, for example, this acid would tarnish the bronze. Needle papers, and paper for wrapping steelware, must be acid-free, otherwise they will cause rusting. The presence of free acid may only be determined by an analyst.