THE GLUING OF WOOD 5 
ANIMAL GLUES 
The desirable properties of animal glue for woodworking are 
high strength in joints, free-flowing consistency, quick setting, stain- 
lessness, and adaptability to different use conditions. In practice no 
other glue has been found to be as satisfactory as animal glue for 
hand spreading on irregularly shaped joints, although a cheaper 
glue would be ve/y desirable. The cost of animal glue and the pre- 
cautions necessary in applying it are the chief factors which limit 
its use. For some purposes the fact that it is not naturally highly 
water resistant is also a drawback. A formula for Increasing the 
water resistance of animal glue is given in the Appendix. 
Animal glue may be sold in different forms, of which the cake, 
flake, and ground forms are the most common. Shredded and pearl 
(^7) 3 glues are two forms recently developed. There is no distin- 
guishable difference in strength or other property between these 
forms. Ground, pearl, and shredded glue can be mixed and melted 
more quickly and conveniently than the others, but some consumers 
avoid the ground glue because they consider it easier to adulterate. 
While adulteration is possible, ground glue which conforms to ac- 
ceptable specifications or standards may be used with confidence. 
Ground glue was used by the Government during the World War 
for gluing airplane propellers because it was easier to sample and 
test and more convenient to mix than the other forms then available. 
Most of the animal glue used in woodworking is made by cooking 
in water the hides or bones of cattle, but some is made from fleshings, 
sinews, cartilage, horn piths, and other animal parts, and the liquor 
thus obtained is drawn off, concentrated by evaporation, jellied, 
and further dried (1, 6, 41). The same raw material with fresh 
water added is reheated to yield liquors for other batches of glue. 
The first batch of glue gives the highest grade. There is also a wide 
range in the grade of glue obtained from each class of raw material. 
All manufacturers of animal glue grade their product on account 
of variations in quality. The tests upon which grades are based are 
discussed in the Appendix. 
The oldest system of grades in America, and one of the best known, 
is that established about 1844 by Peter Cooper (7), in which glues 
were classified in order from the highest to the lowest grades as AA 
extra, A extra, I extra, No. I, IX moulding, IX, 1%, 1%, iy 2 , 1%, 
1%, 1% an d 2. The Peter Cooper system was not generally accepted 
as a standard. A major objection was that it depended upon a set 
of samples the measurable physical properties of which were not 
clearly defined. Recently the National Association of .Glue Manu- 
facturers, comprising a considerable number of the animal-glue 
makers of the United States, worked out and adopted uniform, clearly 
defined, and accurate testing methods by means of which test results 
(17) can be duplicated in any suitably equipped laboratory. On the 
basis of tests of the jelly strength and viscosity of glues the asso- 
ciation has established a system of grades. The new grades are num- 
bered 1 to 21 in ascending order of quality. Most of the glues used 
in gluing wood are included in the middle third of this range (30). 
The new grading system promises to bring about a very desirable 
3 Reference is made by italic number in parentheses to " Literature cited," p. 75. 
