68 BULLETIN 1500, U. S. DEPARTMENT OF AGRICULTURE 
wide variation with the humidity of the atmosphere and of the uncertainty of a 
constant variation between grades, the moisture-content test is rarely of any 
direct value in judging the quality of a glue. However, a very low moisture 
content in animal glue is known to be associated with brittleness and crazing. 40 
A knowledge of the moisture content is indirectly of value in preparing solu- 
tions for viscosity and jelly-strength tests. Under careful testing methods, mois- 
ture-content determinations should be made for this purpose. 
PROTEIN CONTENT 
Chemical analyses of the protein content of animal glues have been sug- 
gested, but they involve highly refined methods and equipment, and their value 
is very questionable. Even though they should in the future prove to be signifi- 
cant, they probably would not be practical for most woodworkers. 
MECHANICAL TESTS 
Many attempts have been made to measure the strength of glues in joints. 
For this purpose metal, porcelain, glass, and other materials (22, 34) have been 
tried, but wood is the material usually employed. A discussion of wood- joint 
tests will be found later under " Tests for blood-albumin, casein, vegetable- 
protein, and starch glues." For animal glues, wood-joint tests are secondary in 
importance to the jelly, viscosity, and other tests. However, joint tests have 
been used to some extent as supplementary tests in the belief that they supply 
information that is not obtained from the other tests, such as information on 
the ability of glue to adhere to the wood substance. They have especially been 
applied by users of glue for woodworking who have, in general, mistakenly 
placed greater confidence in the joint tests than in the other forms of tests. The 
extreme difficulty of making the joint tests accurately and of using the glue to 
the best advantage, together with the variable properties of wood, make such 
tests questionable for use in judging even the adhesive quality of an animal 
glue. Poor gluing may result in weak joints with high-quality glues, while very 
careful gluing may result in strong joints with low-quality glues. Therefore, 
mechanical tests afford only a means of determining the strength of glued joints 
and may mislead a purchaser into accepting from a number of glues one of 
lower quality. 
TESTS FOR LIQUID GLUES 
Tests for judging the quality of liquid glues are not so well known as are 
those for animal glues. Some of the properties of liquid glues which have been 
described 41 as important are the following: Viscosity, jelly point, moisture con- 
tent, rate of setting, hygroscopicity, chloride content, acidity and alkalinity, 
and keeping qualities. 
The viscosity, jelly point, moisture content, acidity or alkalinity, and keeping 
qualitites are determined in a manner similar to those described for testing 
animal glues. A determination of chlorine content in the form of chlorides re- 
quires the services of a chemist. The rate of drying and the hygroscopicity 
may be determined by exposing a film of the glue to certain fixed atmospheric 
conditions (51). 
Wood-joint tests are of more significance and value for liquid glues than for 
animal glues. The extremely wide variation among the different brands and 
grades of liquid glue make it possible to eliminate by wood-joint tests those 
which are the least desirable for wood joints. Furthermore, liquid glue is sold 
in prepared form, which reduces the chances of error in making wood-joint tests. 
However, very careful control of gluing conditions and of the technic of making 
the wood-joint test is required to obtain an accurate comparison of liquid glues. 
40 Bogue (6) describes this condition as follows : " The whole mass will crumble to small 
cubical and rectangular fragments ranging usually from a thirty-second to an eighth of an 
inch on a side. Such a glue is spoken of as crazed, and since it is the farthest removed 
from the elastic and pliable forms it is naturally given the lowest rating by the ' inspection 
test.' " 
41 Tressler (6, 51) has described tests for liquid slues and the properties important for 
joining work. He finds that " fish glue of the ordinary viscosity contains from 50 to 55 
per cent of glue and weighs from 9% to 10 pounds to the gallon " ; that the best fish 
glues have a jelly point of about 45.5° F., and " should not contain more than 0.02 per 
cent of chlorine as chlorides " ; and that the proper reaction is " neutral to litmus and 
very slightlv acid to phenolohthalein." To determine keeping qualities, he suggests storage 
at 99° for a month, and, for a determination of the rate of drying and hygroscopicity, an 
exposure of a film to constant atmospheric conditions. 
