72 BULLETIN 150 0, IT. S. DEPARTMENT OF AGRICULTURE 
100 pounds, respectively. Army specifications (1926) were the same as for 
grade B Navy plywood. In 1927 the Navy adopted a new specification requiring 
but one grade of plywood with a dry strength of at least 285 pounds per square 
inch and a wet strength of at least 145 pounds per square inch. Some allow- 
ances are made for thickness of plywood and percentage of wood failure 
obtained in test. 44 Only the most water-resistant glues will make plywood 
which passes the grade A' wet-strength requirement. 
The plywood-shear test, like all joint tests, must be carefully made to be of 
value. A standard procedure must be adopted which insures correct use of 
the glue, a control of the factors in gluing, and uniform procedure in testing. 
When made in this way it will, within limits, give results which enable a com- 
parison of glues. Like the block-shear test, it is also valuable as a means of 
judging whether the glue is being used to the best advantage. It, therefore, has 
been used extensively at the Forest Products Laboratory for this purpose. 
Tests on Raw Materials 
Blood-albumin and some casein and vegetable glues are made by combining 
the separate raw materials at the time of their mixing with water. In these 
cases it is possible to make tests on the quality of the dried albumin, the 
casein, and the starch to be used. 
BLOOD ALBUMIN 
Dry blood albumin is seldom bought under a definite specification. A 90 per 
cent soluble albumin is satisfactory for blood-albumin glue and can be readily 
supplied in this form by the manufacturer. The albumin should be compara- 
tively free from entrained air to avoid a foamy or frothy glue. Color is not 
important, and the cheap dark albumin is as satisfactory as the light-colored 
albumin. Fat not in excess of 1 per cent, ash not more than 10 per cent, and 
a moisture content of about 8 per cent are known to be satisfactory limits for 
these factors. However, with even these requirements the albumin must be 
mixed into glue, and joints made and tested before there is positive assurance 
that it is of proper quality. 
CASEIN 
Commercial casein is not a uniform product ( ; /S), and, therefore, trouble 
may be caused in making glue unless the formulas are altered to suit such 
variations. In connection with its early studies of casein glues the Forest 
Products Laboratory made a series of analyses (10) on a large number of 
caseins manufactured by different methods. Each casein was mixed into glue 
in accordance with three different formulas. The analyses included tests for 
acidity, ash, fat, moisture, and nitrogen content. From this work it was con- 
cluded that any casein of reasonably good quality would make a satisfactory 
glue, provided changes are made in the proportions of the ingredients to suit 
the particular requirements of the casein. 
Of the characteristics of the casein studied, the ash content seems to be most 
closely related to the behavior of the casein in glue making {15). Caseins 
with high ash content require larger amounts of water for use in glue making 
than caseins with low amounts of ash. When caseins of different ash content 
are used in the same formula (laboratory formula No. 4B). the variation 
required in the ratio of weight of total water to casein was from slightly 
over 2 to a little more than 3.5 for caseins ranging from grain curd to rennet, 
respectively (15). The acidity, fat, moisture, and nitrogen content did not 
have as important an effect on the glue-making properties of the casein as 
the ash content. The analytical methods so far developed do not tell with 
certainty what modifications, other than those which give the correct water 
proportions, are necessary in a given glue formula to get the best results with a 
particular sample of casein. 
Caseins which meet the following requirements are considered satisfactory 
for glue-making purposes: 
Color. — White or very pale cream. 
Odor. — Sweet or only very faintly sour. 
41 For more detail, see U. S. Navy, Bureau of Aeronautics Specifications Xos. 40Pla and 
49Plb and TJ. S. Army Air Service Specification No. Sl'-S. 
