26 BULLETIN/ 15 0, U. S. DEPARTMENT OF AGRICULTURE 
as 50 pounds pressure per square inch before the entire surfaces of 
warped pieces of thick wood are close enough together to permit the 
making of satisfactory joints. Some woods, after being wet from the 
water in the glue, crush at about 200 pounds pressure per square inch. 
In most commercial gluing operations pressures of 100 to 200 pounds 
per square inch will give satisfactory joints and will not injure the 
wood. 
In hand-screw clamps and presses the amount of pressure applied 
is usually unknown to the operator. Fortunately, the range of pres- 
sures throughout which good joints may be obtained (p. 34) is so 
broad that trained operators of screw clamps and presses are usually 
successful even though they have no accurate knowledge of the amount 
of pressure they 'are applying. Satisfactory power presses (pi. T) 
are equipped with pressure-indicating devices which enable the opera- 
tor to adjust the amount of pressure (p. 74) to the size of the joint, 
the kind of wood, or other gluing conditions. 
Efficient use of a power press demands that it shall be continuously 
available for service. For example, piles of panels are stacked at the 
glue spreader, then transferred to the press where, as the load is 
applied, they are quickly bound with I beams and turnbuckles, and 
then removed from the press. (PI. 5.) By this method a stack of 
panels may be pressed every few minutes with one press. 
EVEN DISTRIBUTION OF PRESSURE 
Pressure should be applied uniformly over the surface of the wood 
to be glued. This presupposes that the surfaces to be joined are true 
and fit accurately and that the pieces are of uniform thickness. In 
order to secure a sufficiently uniform pressure the clamps and the 
screws must be properly spaced with the bearing surfaces parallel. 
For the same reason thick cauls should be used under the bearing 
surfaces when gluing thin pieces such as veneer, and occasional in- 
spections should be made of power presses to insure that the platens 
are true and parallel. The staple and wedge devices shown in Figure 
4, A and B, do not distribute pressure evenly and therefore are not 
suitable for use on high-grade glued joints. 
The cauls generally used on the top and bottom of a stack of 
glued veneer (pi. 8) should be smooth and of uniform thickness, and, 
where heavy loads are applied, should be 3 to 4 inches thick. Thinner 
cauls of wood, metal, or other material may be used at various places 
throughout the stack to compensate for any irregularities in the 
stock. In commercial gluing cauls are frequently used with every 5 
to 10 panels. TThere valuable veneered stock with two good faces 
is being glued, it is well to use thin cauls alternately with the panels 
throughout the stack. If the panels have only one good face, the 
cauls may be at intervals of two panels laid face to the caul. (PL 
8, B.) Curved cauls are used where the veneer is pressed into curved 
panels, such as in chair seats. Such cauls are cut out or formed to 
the desired shape, and the veneer panels are pressed between them. 
(PI. 5.) Cauls should be coated with wax. grease, or other suitable 
substances to keep the glue from adhering to them and also to make 
them easy to clean. In some operations paper is placed between 
high-grade veneered panels to prevent the panels from sticking to- 
gether and to absorb some of the moisture which is added by the glue. 
