THE GLUING OF WOOD 69 
TESTS FOR BLOOD- ALBUMIN, CASEIN, VEGETABLE-PROTEIN, AND STARCH GLUES 
There are at present no generally accepted physical and chemical tests for 
blood-albumin, casein, vegetable-protein, or starch glues. The technic of testing 
these glues is less refined and perfected than for animal glues. The joint-mak- 
ing strength of all of these adhesives and the water resistance of blood-albumin, 
casein, and vegetable-protein glues are the properties most often tested. Un- 
fortunately, joint tests are the only methods yet available for determining the 
suitableness of these glues. 
WOOD-JOINT TESTS 
A number of methods and specimens have been used for measuring the strength 
of glues by testing glued- wood joints. The joints for wood tests are made in 
various ways, for example, some end to end (fig. 18, I, J. and O), others side to 
side with the grain approximately parallel (fig. 18, A, B, C, D, E, G, H, K, L, 
M, and P), and still others side to side with the grain at right angles. (Fig. 18, 
F and N. ) Joints are tested generally in tension or shear ; however, the exact 
type of test varies with reference to the character of the work to be expected 
of the glued joint and with reference to the testing apparatus available. The 
" Spandau " test used in Germany and the British aeronautical-inspection test 
used in Great Britain are perhaps the best-known European tests. The Spandau 
test (43) is made by gluing the end grain of the two pieces together and break- 
ing the specimen chiefly by bending. (Fig. 18, O.) The British aeronautical- 
inspection test (42) is a tension test on a joint made between a portion of two 
relatively thin pieces of wood with the grain parallel. (Fig. 18, H.) The 
British and the German tests are both regarded as rather unsatisfactory for 
testing the quality of a glue (24, 43). 
Forest Products Laboratory Joint Tests 
Two wood- joint tests have been used extensively at the Forest Products 
Laboratory; namely, the block-shear test and the plywood-shear test. These 
were adopted after experimentation with numerous methods and specimens. 
The block-shear test is made on lumber stock, usually about 1 inch thick, and 
the plywood-shear test on veneer glued into 3-ply panels. (Fig. 18, A and F.) 
In both tests side-grain faces are glued together ; in the block-shear test the 
grain is approximately parallel, and in the plywood-shear test it is at right 
angles in adjacent plies. In the block-shear test the joint is subjected to a 
compressive shearing force (pi. 12), and in the plywood-shear test to a tensil 
shearing force. (PI. 13.) These tests are quick, simple, and moderately 
accurate. 
Two pieces of selected wood, usually hard maple, although other high- 
strength woods may be used, each about 1 by 2V 2 by 12 inches in size, are glued 
together face to face. The exact dimensions of the pieces may be varied some- 
what without seriously affecting the test. 
Care is taken to make sure that the surfaces to be joined are smooth and 
true and that the pieces are uniform in thickness. Extreme precautions are 
also taken to properly control the many other factors in gluing (p. 32). After 
the glued joints have aged sufficiently — for example, a week or longer — they 
are cut into shear specimens. (PI. 12, A.) These specimens are placed in a 
testing machine which is equipped with a special shearing tool (pi. 12, B) 
and tested to destruction. The pressure required to produce failure is noted 
and the percentage of area of the joint where the wood surface is torn out by 
the glue is estimated. Both of these values are then used in considering the 
quality of the joint. 
Failure of such a wood joint may occur entirely in the glue line, entirely in 
the wood, or partly in each. Unless the wood is of good quality, such as 
particularly high in density, dry, and free of defects, the failure usually occurs 
wholly or partly in the wood with the use of glues of medium quality or better. 
Inferior glues show little or no tearing of the wood. 
42 The test specimens and the shearing tool here described and used with glued joints 
are distinctly different from the standard specimen and tool used in previously published 
reports on solid wood. 
