GLUE, is a tenacious cement, principally used by ca binet-makers, joiners, book-binders, case-makers, and hat ters.
The substances from which glue is made, are the shreds or parings of hides; the ears before they are immersed in the tanner's vats ; the cuttings and raspings of horn, from the comb-maker, the button-maker, and the horn lanthern maker ; and the hoots and horns of oxen, calves, and sheep, from the butcher ; the pelts of the hare, rabbit, beaver, &c. from the hat makers, beaver-cutters, and furriers ; and the parings of vellum and parchment from the white leather dresser, glover, &c.
These substances are indiscriminately mixed together, and are purified from all grease and dirt by digestion in lime water, the greatest care being taken to remove eve ry piece that is in the slightest degree putrescent. The materials are next steeped and washed in clean water, with frequent stirring, and are afterwards laid in heaps, and the water pressed out. They are then boiled in a large brass kettle with clear water, the fat and dirt being con stantly skimmed off as they rise, and when the whole is dissolved, a little melted alum or finely powdered lime is added. After the skimming has been continued for some time, the whole is strained through baskets, and suffer ed to settle, in order thai the remaining impurities may subside, and the fat rise to the top. The impurities and fat being removed, it is then returned into a clean kettle, and suffers a second evaporation and skimming. When it acquires a clear darkish brown colour, and a sufficient consistence, which is known by the appearances during ebullition, it is lifted out by &scoop, into frames or moulds, about six feet long, one foot broad, and two deep, where it is allowed to cool gradually. It is then cut by a spade into square cakes, and each of these is afterwards divided into three pieces, by an instrument like a bow, having a brass wire for its string. The pieces thus cut are dried in the open air, on a kind of net-work, (generally old her ring nets,) fastened in moveable sheds of four feet square, each containing six or eight rows of net-work. When the glue is dry, each piece is rubbed gently with a wet cloth, to give it that glazed appearance which the London glue always possesses. The different pieces are then packed carefully up in separate rows in barrels or hogs heads, and are ready for sale.
The best glue swells considerably, without melting, by three or four days immersion in cold water, and recovers its dimensions and properties by drying. When glue looks
thick and black, or has got frost in the drying, it should be melted over again with a sufficient quantity of fresh glue. Good glue is distinguished by its having a strong black colour, and by being free of cloudy and black spots, when held between the eye and the light.
In France, glue-is made from whole skins, which, when fresh, are steeped 24 hours in large tubs ; but a longer time when they are dry. They are then placed upon hand barrows, formed of strong basket work, to allow the wa ter to drain off,,,and are afterwards well washed in a run ning stream, where they are shaken in baskets with a long toothed rake, till the water runs through them quite clear. The materials are now steeped in weak lime water, the liquor being z efreshed every 13 cl.iys with a bucket of fresh lime water, and the skins being occasionally turned. By this process, the grease is removed, and the skins con verted nearly into the state of parchment. The parts of the materials that still retain the hair, require a stronger lime water. The skins are again steeped and drained in hand barrows, and sometimes the water is squeezed out of them by a press. The skins are no•w thrown into the kettle, which has sometimes stones at its bottom, and sometimes'a wooden grate, to prevent the skins from stick ing to it. The boiling is carried on very cautiously and gradually, and the evaporation is known to have been suf ficiently great by dropping a little upon a plate, and find ing it of the proper tenacity. The glue, when hot, is then filtered through a bed of long straw, into a tub, the ope ration being performed in a warm place, in order to pre vent the glue from congealing. In this tub the glue is left to refine for two or three hours, and when still fluid it is run off by stop cocks, at different heights, into wet wooden boxes, where it congeals. The glue which flows from the different stop cocks, has different degrees of fine ness, the uppermost being the purest. After lying 24 hours in the boxes, the cakes are taken out, divided, dried, and packed up for sale.
Glue has also been successfully prepared from the bones of animals. Parmentier found that 6 pounds of but tonmaker's raspings yielded a pound of glue not inferior to the English glue. Glue from ivory was less transpa rent. See Annales de Chinde, vol. xii. p. 292.