By this incomplete treatment with hydrochloric acid, a certain quantity of the phosphates remains in the cartilage and is inclosed in the glue prepared from it, the finished product acquiring thereby a dirty whitish color, which is by many considered an evidence of its quality. This mechanical admixture of phosphates, however, does not affect the adhesive power of the glue, neither increasing or decreasing it. Such white and opaque glue is manufactured to answer the demand in certain quarters of the trade, and, as above mentioned, heavy white substances are often intentionally incorporated with skin glue, as well as bone glue, to give it the appearance of Russian glue. These heavy powders add to the weight of the product, though when incorporated with it in small quantities do not injure its adhesive power, but large quantities render the product weaker.
Patent glue. This term is applied to an indefinite number of preparations, but particularly refers to a very pure variety of bone glue of a deep dark-brown color not showing net marks. It is very glossy, and swells up much in water. To satisfy the demand for thick cakes, they must be cut from very concentrated jelly to insure their drying.
Gilder’s glue is found in commerce in very thin, pale yellow cakes tied up in packages weighing about 2 lbs. each. It is a variety of skin glue bleached with chloride of lime, and dissolves with difficulty in water. The first runnings from the boiler are used for its manufacture.
A very superior article of gilder’s glue is obtained by cutting rabbit skins into fine shreds and boiling in water, then turning the mixture into a basket through which the liquid passes, leaving the refuse behind. About 100 grammes (3.52 ozs.) of sulphate of zinc and 20 grammes (0.705 oz.) of alum are then separately dissolved in pure boiling water and poured into the first-mentioned liquid, and the whole well stirred together while hot. The mixture is then passed through a sieve into a rectangular box, in which the jelly remains twenty-four hours in winter, or forty-eight in summer. The solid mass is taken from the box, cut into slices of proper thickness, and dried upon nets.
Size glue and parchment glue are manufactured in the same manner. Both are skin-glues, and can be readily produced by following the directions given for the manufacture of skin-glue.
Paris glue is used for sizing. It is brown, opaque, and almost always soft. Being very hygroscopic, and imparting a suitable flexibility to the felt, it is better adapted for hatter’s use than any other variety. For its manufacture only the generative organs, or the thick tendons of the legs of cattle and horses, are used, or other waste and fleshy parts, and substances mixed with small bones, which, if thoroughly cleansed, might yield a good quality of glue, but are intentionally transformed by too long-continued boiling, whereby the gelatinous solution is largely deprived of its adhesive power, and yields a hygroscopic product.
Liquid Glues. These are chiefly combinations of glue with some ingredients added to destroy the gelatinizing property and yet not impair its adhesiveness. They remain for a long time clear and syrupy, and are used for a variety of purposes. Below are given a few receipts for such glues.
1. Dissolve 38 parts of glue in small pieces in 100 parts of acetic acid. Solution is promoted by exposing the vessel to the sun or placing it in hot water.
2. Dissolve 50 parts of light-colored glue in 50 parts by weight of hot water, in which 14 parts of fused magnesium chloride have been dissolved. The solution on cooling does not gelatinize, but remains syrupy, the density varying according to the quantity of water used. In the preparation of printing inks it can be used as a substitute for gum.
3. Dilute 10 parts of strong phosphoric acid with an equal weight of water, and then gradually add 4 parts of ammonium carbonate in the dry state. When the effervescence has subsided, add a further 5 parts of water and warm on the water-bath or steam-chest to 158° F. Now add 20 to 40 parts of glue, according to the consistency required, and stir until all is dissolved. Cool.