Scraps of parchment and bullocks’ feet are highly valued as glue stock, since they are in fit condition for boiling without further preparation. They may yield up to 62 per cent. of their weight in glue.
Calf and sheep skins yield a superior glue; that from horse hides is usually dark and poor in quality, although with careful working a strong product can be obtained from the latter.
Of great value to the glue boiler are the so-called calves’ heads, which, after liming and drying, form a special article of commerce.
Skins of hogs, hares and rabbits yield a light-colored glue of little consistency. It is, therefore, best to use these last-named raw materials for the preparation of jelly, such as is used in sizing, in the manufacture of paper, etc.
The older the animals from which the skins have been derived, the more solid the glue will be. In many cases, especially where a certain quality of glue is to be produced, it may be recommended to separate the different kinds of skin refuse into lots, provided there is enough of each kind to boil it separately.
A considerable number of skins used for packing various articles, such as indigo from South Africa, have been so much damaged in transit as to render them useless for tanning, but they form good material for glue, frequently yielding 50 to 55 per cent.
In reference to judging glue stock some valuable notes are given in an article on glue, published by the American Provision Co., Chicago, Ills.:
“Dry, uncured or salted stock, such as raw hide or South American, if soaked for twelve hours in cold water, gains about 50 per cent. in weight, and still remains tough, and the water sweet. The moisture, dirt, and salt should not be over 10 per cent.
“Green salted stock, such as hide pieces, sinews, calf heads and pates, should have no excess of salt, nor be foul, discolored or heated; should be tough, with the hair not loose, and have a mild animal odor. Moisture and salt not over 40 per cent.
“Dry limed stock soaked twelve hours develops a characteristic odor, and should be firm, fibrous, and have no slimy pieces. The water should not be dark. Lime, sand and dirt, not over 5 per cent.