Glue is merely an impure Gelatine (q.v.). Almost every animal substance will yield it, hence all kinds of animal refuse find their way to the glue-makers' boilers. The refuse of tanneries, consisting of the clippings of hides, hoofs, ear and tail pieces of ox, calf, and sheep are preferred, because they can be dressed with lime, which removes the hair, and acts as an antiseptic. For this purpose they are placed in tanks with quicklime and water for two or three weeks. They are afterwards washed and dried, and are ready for use by the glue-maker, who usually gives them another heavier lime-dressing, and subsequently washes them; they are afterwards exposed to the action of the air for a time, to neutralise the caustic lime. When well drained, the pieces are placed in flat-bottomed copper-boilers, which have a perforated false bottom placed a little distance above the true one, to prevent the burning of the materials, and which have been supplied with rain or other soft water up to two-thirds the depth of the boiler, the pieces being piled up to some height above the top of the open boiler. The whole is kept at a gentle boiling heat until all the gelatinous part has dissolved out, and the mass of material has sunk down into the fluid. The boiling is sustained until, by repeated trials of small quantities, the operator knows the fluid is of the right consistency, when it is drawn off carefully into the congealing boxes.
The congealing boxes are of wood, and are nearly square, being slightly narrower at the bottom than the top; they are filled to the brim, and when their contents are sufficiently solidified the glue, with a little management, turns out in the form of a cube, which is cut into thin slices by a wire in the same manner as soap; and these larger slices are subdivided into smaller cakes by a wet knife. Frames, with nets stretched upon them, are provided for drying the cakes upon; and these frames, when covered with the cakes of glue, are adjusted one over another at a little distance apart, supported between four uprights, and, if in the open air, covered over with little wooden roofs, the whole being arranged so that the air can have free access to facilitate drying. This process is an anxious one for the manufacturer, as the changes of the weather have great and often completely destructive effects upon glue in this state. In Britain spring and autumn are the best drying seasons. Generally, after the open-air drying, the glue is taken to drying-rooms, heated slightly, where it hardens effectually; but it is not yet finished; the cakes at this stage have a dull, unsightly look, to remedy which they are dipped into cold water, or are wetted with a brush dipped in hot water, and redried, this wetting giving the cakes a bright varnished appearance.
While England does not excel in this manufacture, it is a recognised fact that Scottish glue—such as that made by Messrs Cox at Edinburgh—ranks in the front of the glues of all countries. A light-coloured glue is not necessarily good, nor dark-coloured glue necessarily bad. A bright clear claret colour is the natural colour of hide-glue, which is the best and most economical. Light-coloured glues (as distinguished from gelatine) are made either from bones or sheepskins. The glue yielded by these materials cannot compare with the strength of that yielded by hides. A great quantity is now made in France and Germany from bones. It is got as a by-product in the manufacture of animal charcoal. Although beautiful to look at, it is found when used to be far inferior to Scottish hide-glue. The latter is largely used by match-makers, piano-makers, and cabinet-makers, who export their goods to all parts of the world, and to whom, owing to the damp climates of many parts to which they export, a first-class glue is absolutely necessary. Besides its use in joinery, cabinet-making, book-binding, match-making, and similar operations, glue is used by paper-makers and in dressing silks; and for these last two purposes fine light-coloured kinds in thin cakes are made. Large quantities are employed by paper-hangers and others for sizing walls. It is also used for stiffening straw, cotton, horsehair, and other plaits for making bonnets and hats. See Dawidowsky, Glue, Gelatine, &c. (Eng. trans. 1884).
Marine Glue is not a glue, but a cementing composition used in shipbuilding, for paying seams in ships' decks after being caulked. In hot climates it is preferred to tar for this and other purposes, where the materials are exposed to the influence of wet. It consists of india-rubber cut very small, and digested at a gentle heat in a closed vessel with coal-tar naphtha until it is dissolved, when powdered shell-lac is added, and the digestion continued until it also is dissolved.