An understanding of the sources from which glue is derived and of the processes of manufacture will be found of practical importance to the glue user. It will give many valuable side-lights on the proper methods of preparation and handling.
Glue is an organic substance of adhesive properties obtained from the hides, skins, bones and sinews of cattle, sheep, deer, horses, and other animals. Tails, snouts, ears, and the pith of the horn are also used. Some glue is produced from the heads, bones and sinews of fish.
The tendons and intestines of many animals, the swimming bladder of many varieties of fish; rabbit skins, or “coney,” from which the fur has been removed; old waste leather, such as gloves, butchers’ offal, or “country bone;” “junk” bones, and much other apparently worthless matter, all contribute to the raw material of the glue-maker. In its broadest sense glue may be understood to include gelatine, but the use of the word is here confined to the substance known commercially as glue, and which in contrast with gelatine has greater adhesiveness, stiffness, and elasticity, and is also darker in color and more nearly opaque.
Neither gelatine nor glue exists already formed in nature; they are both the products of the action of heat and water on nitrogenous animal tissue. It is not definitely known just how this change takes place. Some writers regard glue as impure gelatine; others believe that there is a difference in nature between gelatine and glue. This question is without present importance for our purpose.
BOILING THE STOCK
Glue is produced by boiling the animal substances mentioned above, and drying the resulting liquor.
The following may be noted in connection with the use of skins. The outer covering, in which the wool, fur or hair is rooted, is of no importance to the glue-maker. The portion that produces the glue lies next to it, being composed of fibres which run in every direction and contain the fluid matter which aids in keeping the skin moist and pliable. The fat cells are directly beneath the glue-yielding portion, and as fat is undesirable, because it makes the glue greasy, the shreds of fat are saponified by being subjected to a lime bath. The lime bath is also useful in removing any hair still adhering; and is used also in preparing tissue, to remove bloody and fleshy particles.
This part of the process may consume from one to three days.
It may be noted in passing that the older the animal, the more solid the glue will be. On this account many manufacturers sort the skins before using.
Being animal stock, the raw material of glue is subject to decomposition, and the scraps of hide are therefore carefully preserved, especially during the summer season.