CHAPTER II
THE MANUFACTURE OF GLUE
An understanding of the sources from which glue is derived and of the processes of manufacture will be found of practical importance to the glue user. It will give many valuable side-lights on the proper methods of preparation and handling.
Glue is an organic substance of adhesive properties obtained from the hides, skins, bones and sinews of cattle, sheep, deer, horses, and other animals. Tails, snouts, ears, and the pith of the horn are also used. Some glue is produced from the heads, bones and sinews of fish.
The tendons and intestines of many animals, the swimming bladder of many varieties of fish; rabbit skins, or “coney,” from which the fur has been removed; old waste leather, such as gloves, butchers’ offal, or “country bone;” “junk” bones, and much other apparently worthless matter, all contribute to the raw material of the glue-maker. In its broadest sense glue may be understood to include gelatine, but the use of the word is here confined to the substance known commercially as glue, and which in contrast with gelatine has greater adhesiveness, stiffness, and elasticity, and is also darker in color and more nearly opaque.
Neither gelatine nor glue exists already formed in nature; they are both the products of the action of heat and water on nitrogenous animal tissue. It is not definitely known just how this change takes place. Some writers regard glue as impure gelatine; others believe that there is a difference in nature between gelatine and glue. This question is without present importance for our purpose.
BOILING THE STOCK
Glue is produced by boiling the animal substances mentioned above, and drying the resulting liquor.
The following may be noted in connection with the use of skins. The outer covering, in which the wool, fur or hair is rooted, is of no importance to the glue-maker. The portion that produces the glue lies next to it, being composed of fibres which run in every direction and contain the fluid matter which aids in keeping the skin moist and pliable. The fat cells are directly beneath the glue-yielding portion, and as fat is undesirable, because it makes the glue greasy, the shreds of fat are saponified by being subjected to a lime bath. The lime bath is also useful in removing any hair still adhering; and is used also in preparing tissue, to remove bloody and fleshy particles.
This part of the process may consume from one to three days.
It may be noted in passing that the older the animal, the more solid the glue will be. On this account many manufacturers sort the skins before using.
Being animal stock, the raw material of glue is subject to decomposition, and the scraps of hide are therefore carefully preserved, especially during the summer season.
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.
When set, a little water is run over the surface, the jelly is detached from the cooler, cut into uniform slices of the thickness desired, and placed on galvanized or linen nets to dry.
Drying may be done in the open air if weather conditions are favorable, or in a drying-room. The latter method is preferable. Conditions can be regulated to insure uniform drying.
Piles of the nets, or “stacks,” are loaded on trucks and taken into the drying-room, where they are exposed to the effect of warm air currents induced by blower or pressure fans, or exhaust or suction fans.
The drying is a source of concern to the manufacturer. It is extremely important to keep the temperature at just the right point, to protect the glue from dust and dirt, and to avoid the possibility of bacterial growth in the glue-jelly, which is very susceptible to the development of harmful organisms.
The final form of the glue will be in sheets, strips or flakes, or ground. For commercial purposes it is put up in packages, bags and barrels.