28 
BTJLLETIX 1500, U. S. DEPARTINIEXT OP AGRICULTURE 
of uniformly high strength are required a pressing period of four 
hours or more is safer practice. 
Tests have also been made on lumber joints glued with vegetable 
glue. The average rate of increase in the strength during the press- 
ing and conditioning time for such joints was appreciably slower 
than that obtained for the animal and casein joints. On all of the 
vegetable-glue joints, however, there was a wide variation in the 
strength of individual specimens tested, so that in some cases the 
vegetable-glue joints compared favorably in strength with the joints 
made with animal and casein glues of the same age. 
The strength of lumber joints made with animal glue in a warmer 
room and with heated wood showed a more rapid increase than that 
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Joint Acje in Hours 
Figure 5. — Increase in strength of animal and casein glue joints during the press- 
ing and conditioning time. Tests were made on hard maple of approximately 
three-fourths of an inch thick and 
were both at approximately 70° F. 
rlued into blx-ks. Gluing room and the wood 
indicated in Figure 5. This was apparently the result of the thinner 
glue line and the faster drying under the warmer conditions. At 
70° F. the animal glue thickened quickly, and at the time of press- 
ing was similar in consistency to the casein glue. Heating the wood 
and gluing in a room of high temperature hasten the drying and set- 
ting of the glue, but there is a serious danger of producing joints 
of lower final strength. This is discussed later under " Control of 
gluing conditions," page 32. 
Rate of Setting of Animal. Casein, and Vegetable Glues on Veneer 
The initial set of the glue in gluing veneer joints is as quick as in 
the lumber joints, but the rate of approaching the full strength of 
the. joint while in the press is slower. Thin layers of veneer do not 
absorb rapidly or completely the moisture from the glue and there- 
fore require a I01112 pressing period. Hence, it is customary to leave 
veneered panels and plywood under pressure for periods of 18 to 2i 
