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.