ANIMAL GLUES: THEIR MANUFACTURE, TESTING, AND PREPARATION 
Varieties and Classes of Animal Glue 
The term "animal glue" refers "broadly to all glues made from 
the skins, "bones, etc., of cattle, goats, sheep, deer, horses, pigs, 
rabbits, or other mammals, unless otherwise specified; however, the 
term usually refers to cattle glue only. The market value attached to 
the glues from the seven sources mentioned is a"bout in the order named. 
Cattle glues are divided into three main classes: hide glue, 
sinew glue, and bone glue. Hide glue is the strongest and most reliable 
of the three. Bone glue, as a rule, is quite inferior. Sinew glue lies 
"between hide and bone glue as regards strength and value. Although these 
are the three main classes of cattle glue, it is manufactured frroai a wide 
variety of raw materials. Hides, sinews, tendons, horn pith, bones, head 
pieces, ears, trimmings, fleshings, tannery waste, etc., are utilized for 
glues of different kinds. The raw material has an important influence on 
the character of the resulting glue. 
Each class of glue is sold in cake, flake, ground, pearl, 
shredded, and other forms, hut the form of the glue is no reliable indi- 
cation of quality. The chief difference between the various forms is in 
the quickness and convenience with which they can "be put in solution. 
The finely divided forms absorb water more rapidly and can be dissolved 
more easily than the cake and flake forms. 
The higher-grade glues, in flake form, are usually light in 
color and nearly transparent. Inferior glues tend to be dark in color 
and opaque. Color and transparency, however, are not safe indications as 
to the quality, for low-grade glues are sometimes bleached. On the other 
hand, foreign substances such as zinc white, chalk, etc., are frequently 
added to transparent glues to produce what are technically known as opaque 
glues. The added materials, while they apparently do no harm, do not 
increase the adhesive qualities. Aside from the fact that they give an 
inconspicuous glue line in a joint, the "opaque" or whitened glues have 
no apparent advantage over other glues of the same grade. 
Glues are graded according to a variety of tests which will 
be described later. In the past no two manufacturers have tested their 
glues in exactly the same way, nor have they followed the same system of 
grades. The oldest known system of classification is that of Peter Cooper, 
a manufacturer whose product was early established in the American trade. 
His grades, arranged in order of value, are: A Extra, I Extra, 1, 
lx moulding, lx, 1-1/4, 1-3/8, 1-1/5, 1-5/8, 1-3/4, 1-7/8, and 2. Later 
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