THE GLUING OF WOOD 
39 
in the woodworking plant or by the production requirements and 
often can not be altered without considerable inconvenience and 
expense. 
In the case of animal glue the effect of assembly time is closely 
related to the temperature of the room and the wood. Low tempera- 
tures of the room and the wood are best with short assembly periods 
and high temperatures of the room and the wood are best with long 
assembly periods. This is illustrated graphically in Figure 10. The 
data used are averages for joints of 12 hardwood species of high 
density. The ^-min- ^ 
ute and 1-minute "^-c 2400 
assemblies with the 
room and wood at 70° 
F., the 3 and 5 at 80°, 
the 8, 12, 18, and 25 
at 90°, and the 12,18, 
and 25 with the room 
at 90° and the pre- 
heated wood at 120° 
all gave joints of high 
quality although the 
joints on the pre- 
heated wood are not 
quite the equal of the 
others. The short-as- 
sembly periods at the 
high temperatures 
gave inferior joints. 
Long assemblies at 
the low temperature 
would have also pro- 
duced unsatisfactory 
joints with the pres- 
sures used. 
The assembly time 
for the casein and the 
vegetable glues has 
less effect on the 
strength of the joints 
than for the animal glue, either in blocks (parallel-grain construc- 
tion) or in plywood. The consistency of the casein and the vege- 
table glues changes more slowly; therefore, moderate pressures (100 
to 200 pounds per square inch) give satisfactory joints under assem- 
bly periods such as from one-half to 20 minutes. However, maxi- 
mum joint strengths are normally not obtained in less than 5 to 7 
minutes. For the casein glue, an assembly period of from 20 to 25 
minutes is frequently not as good as an intermediate assembly period 
such as from 5 to 15 minutes. 
If the assembly period must be long, it is best to use a thin glue 
heavily spread upon wood which is not excessively dry. For short 
assemblies thick glue spread thin on verv dry wood is recommended. 
If the other controlled factors do not bring the glue to the preferred 
consistency in the time available, a pressure must be used to suit the 
consistency of the glue. 
°* ^ 2000 
Q,^» 1600 
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Pressure —Lbs, 
400 SOO 
per <Sq, /n. 
Figure 9. — Effect of gluing pressure on the joint strength 
of hardwoods glued with casein glue. Based on 19 
hardwood species. Each average value shown represents 
at least 756 specimens 
