THE GLUING OF WOOD 27 
UNDUE DEFLECTION IN PRESSES 
Where heavy pressures are applied, such as in gluing veneer, there 
should be no undue deflection or bending in the presses or in the I 
beams. The jackscrew of the older types of veneer presses (pi. 7, A) 
are capable of withstanding 30,000 pounds pressure per screw. An 
average man on this type of screw using an 18-inch lever may apply a 
load of from 18,000 to 20.000 pounds. Where several such screws are 
mounted on one press they may, therefore, cause a la*ge deflection in 
the press. Loads applied on the first screw may be lessened as subse- 
quent screws are tightened and may, therefore, cause unequal pres- 
sure distribution over a part of the joint. Likewise, due to the bend- 
ing of the I beams unequal pressure may occur in bundles of veneered 
panels after their removal from the press. Deflection of presses or 
I beams may be detected by applying straightedges to the suspected 
member or by pressure-indicating devices placed under the bearing 
points. 
DURATION OF PRESSURE 
The length of time that glue joints should be left under pressure 
depends upon the rate at which the joint gains strength. Except in 
hot pressing, joints increase in strength primarily as a result of the 
drying of the glue. The most rapid setting occurs in a warm room 
when a thin spread of quick-setting glue is applied to thick layers of 
dry wood. Pressure applied momentarily and then released may give 
good results with joints made of thick members which fit accurately 
and which do not change their shape on account of the moisture added 
by the glue, provided that the joint is not disturbed until the glue 
has set. Such a condition seldom occurs in commercial operations, so 
that it is best to retain a pressure until the glue has at least partially 
set. In edge gluing in cores (fig. 1) a pressure applied for 30 min- 
utes to 1 hour generally gives satisfactory joints. In veneer work 
(fig. 3) the pressure should be retained on the joint from several 
hours to a clay. 
Rate of Setting op Animal, Casein, and Vegetable Glues on Lumber 
Forest Products Laboratory tests made to determine the rate at 
which glued joints gain strength during the pressing and condition- 
ing time show that animal and casein glues set at approximately 
the same rate when the room and the wood are at a temperature 
of about 70° F. Figure 5 shows how in thick stock the casein and 
the animal-glue joints increase in strength with age under the above 
conditions. The curve indicates that only a small percentage of 
the final strength of the thick joints exists at the end of one-half to 
one hour. The final strength of the joint when pressed for such a 
short period of time is contingent upon careful handling when it is 
removed from the press and upon a further conditioning period. 
Tests on similar joints which were pressed for periods of less than 
two hours and then thoroughly seasoned before being tested show a 
lower ultimate strength than those kept under pressure for longer 
periods. However, the lumber joints which were pressed for only 
one-half to one hour showed sufficient strength after conditioning 
for most kinds of construction. Nevertheless, where lumber joints 
