the National Glue Manufacturers' Association (8)— adopted standard methods 
of testing and a uniform system of grades. 
Animal glue can "be used anywhere in the woodworking industry 
except where a strongly water-resistant adhesive is required. High-grade 
hide glues are used mostly in joints of thick stock; lower grade glues 
for veneer work. 
Manufacture of Animal Glue 
The process of making animal glue is, "briefly, as follows (2): 
The stock, after certain preliminary treatments to remove foreign matter 
and to get it into suitable condition for rapid conversion, is heated in 
water. The collagen, a complex insoluble protein "body present in the raw 
materials, is thus hydrolyzed into the soluble, jelly-forming substances 
constituting glue. The hydrolysis continues "beyond the formation of these 
products and gives rise to compounds having little or no jelly-forming 
power and therefore of slight value as adhesives. For this reason the 
liquor is drawn off after a certain period of heating and a fresh supply 
of water added for a second run. A number of successive "cooks" may be 
made, "but the glue obtained from the first is generally of the highest 
grade. There are many and varied details in the steps above outlined, 
depending upon the kind of stock used and the plant in which the glue is 
made. Any detail of manufacture may be expected to have its effect upon 
the character of the resultant glue. 
The solution of glue from the "boiling kettles is next concen- 
trated by "boiling off the water in vacuum dryers until the percentage of 
glue in the solution is high enough to make a firm jelly on cooling. If 
too high a temperature is reached during the concentrating process, the 
quality of the glue may "be lowered. 
When sufficiently concentrated, the glue solution is cooled "by 
refrigeration, either after "being run into pans or as it is carried upon a 
traveling "belt. As it cools it forms a jelly firm enough to handle. From 
the pans, the jelly is removed, sliced with wires or a knife, and placed 
upon screens to dry. If a belt is used, the jelly is formed in a contin- 
uous sheet; this is cut into sections and placed on screens as it travels 
along. The screens are then placed in a drying chamher and left until the 
glue is dry. The glue may easily "be injured during the drying process if 
the temperature conditions are not properly controlled. The form of the 
glue when dry depends upon the shape in which it was placed upon the 
screens. If carefully sliced to the proper thickness, cakes of regular 
shape will be formed. Sheet glue from the belt "breaks into thin, 
irregular pieces as it comes from the drying screens. It is then commonly 
-Figures in parentheses refer to references at the end of the discussion. 
Rk32 _2- 
