The tanneries supply most of the hide stock, but only waste pieces reach the glue manufacturer, as leather is more valuable than glue, and the larger portion is therefore reserved for the tanner’s use. Various names are used to describe the parts of hide that the tanner discards for the glue-maker’s use—the heavy trimmings are “pieces;” the hide pared off the hair or grain side, “skivings;” the parts scraped from the flesh side are “fleshings.”
At the packing houses the heads, feet, ribs, and other bony structures go direct to the glue-room. If bone is sweet and fresh it is known as “green,” or “packer” bone. The waste of button and knife factories is also used.
Bones are usually ground, and they are treated with a sulphuric acid bath to attack and separate the lime and gelatine of which the bone is composed. Bones, after being treated in this way, become pliable and soft, and the sulphuric acid is then removed by centrifugal force.
The acid must all be removed, as the glue will granulate if any remains.
Other parts of the stock are always carefully washed before boiling.
After the stock has been prepared, it is placed in a boiler with false bottom provided with an opening through which the liquid may be run off. The boiling of the stock is an operation that must be carefully conducted, as the application of a greater degree of heat, or for a longer time than is necessary, damages the glue.
The boiler is heated by direct firing. As the boiling proceeds, test quantities of the liquid are run off for examination, and when a sample is found on cooling to form a stiff jelly, it is ready to draw off. The first boiling usually occupies about eight hours. When the liquid has been run off from this boiling, more water is added and the boiling is continued. This operation is repeated until the stock has yielded all of its gelatinous matter. As many as six or eight boilings may be made.
The liquid first run off—the “first boiling”—is always best, as the effect of repeated or prolonged application of heat is to weaken the glue tissue. The later boilings are also as a rule darker in color than the earlier ones.
DRYING
The glue-solution from the boiling process is run into wooden troughs or “coolers,” about 6 feet long, 2 feet broad, and a foot deep, in which the solution sets in a firm jelly.