38 
BULLETIN 1500, U. S. DEPARTMENT OF AGRICULTURE 
but the minimum values were low. The most dependable results 
were obtained at 150 pounds pressure, where both the average and 
the minimum values were high. More uniform strength values could 
be obtained throughout the range of pressures and temperatures em- 
ployed by the use of the correct assembly time for each temperature 
and pressure. 
With the casein and the vegetable glues the consistencies are nor- 
mally between those represented by thinly mixed warm animal glues 
and those represented by firmly jellied animal glues. The pressures 
that give the best joints with these glues are intermediate between 
those shown as optimum on the two curves of Figure 8. This is illus- 
trated by the pressure-joint strength curve of Figure 9, which is 
based on joint-strength tests of 19 different hardwoods glued with a 
relatively thick casein glue. Here again the results for 1, 5, 12, and 
25 minutes, closed assembly periods, are averaged. The best results, 
30OO 
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Room and Wood at 90° F 
Room and Wood at 70° F 
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Pressure —Pounds per Square Inch 
Figure 8.— Relation of gluing pressure to joint strength. The tests were made on 
yellow birch blocks glued with an animal glue. The shaded portion of the aver- 
age points indicates the percentage of wood failure developed in testing the 
joints. Each average value shown represents at least 32 specimens 
as indicated by the average joint-strength values and the average 
percentages of wood failure, were obtained with pressures of 200 
and 400 pounds per square inch. The slight inferiority of the 100 
pounds pressure would possibly disappear with a thinner mixture of 
glue and more easily glued woods. 
ASSEMBLY TIME 
The time which elapses between the spreading of the glue on the 
wood and the application of pressure is called the " assembly time." 
In gluing plywood and veneered panels the common practice is to 
build up a stack of panels 2 feet or more in height and to place them 
under pressure at one time. (Pis. 5, 6, and 8.) In this method of 
assembly, 20 minutes or more may elapse from the time the first 
panel of a stack is spread with glue before it is under pressure, 
whereas the last panel spread with glue may be pressed within 2 
minutes. In gluing thick pieces of wood the assembly time is usually 
short and in edge gluing it is generally less than 1 minute. The 
assembly times are largely influenced by the kind of apparatus used 
