Glue consists of an impure form of gelatine (q.v.), and possesses generally the properties of that substance. It is manufactured chiefly from bones, which are ground coarsely and then digested with a dilute acid until soft, being afterwards freed from acid by thoroughly washing with water. They are then placed in large iron vessels known as digesters, in which they are subjected for some hours to the action of steam of from 2 to 3 atmospheres pressure - the resulting liquor, consisting of water, glue, fatty materials, etc., being then run off by pipes into tanks, from which the fat is skimmed and used for manufacture of soaps and greases. The liquor which remains is filtered through wire gauze, concentrated by boiling, and run into moulds to set, after which the cakes are dried at about 60° to 70° Fahr., and are then ready for storing for the market. If required to be of the best quality, the glue is bleached before concentrating, when it is obtained of a pale yellow colour. The bones left in the digesters are usually allowed to dry, and utilised for bone manures. The different varieties of liquid glues generally consist of gelatine, either pure or not, dissolved in acetic or some other acid, the solution forming a strong cement, which may be used for pottery, glass, etc.