24 BULLETIN 1500, U. S. DEPARTMENT OF AGRICULTURE 
holes and onto lock-corner joints ; and revolving brushes for spread- 
ing it into dovetail joints. 
Animal, liquid, thin casein, and liquid-blood glues may be spread 
by hand with ordinary bristle brushes. However, metal-bristle 
brushes are more durable for the alkaline casein glues. Generally, 
mechanical means are required for spreading thick vegetable glue, 
thick casein glue, and the jelly form of blood glue. Wooden paddles 
or metal scrapers may, however, be used to spread thick glues on small 
areas. 
Speed in spreading is necessary where glues change rapidly in 
consistency after being spread, where there are several joints in one 
construction, or where, as in gluing plywood (figs. 3 and 17), the 
spread veneer sheets are piled together and pressed at one time. 
(PL 8.) High speed for mechanical spreaders is also desirable for 
quantity production but the speed must be controlled so as not to 
whip air into the glue. 
Uniform spreading by hand requires great skill, and even mechan- 
ical spreaders must be kept in proper adjustment for such spreading. 
In coating two sides of a piece of wood with glue in a double-roll 
spreader it is also necessary that the wood be of uniform thickness. 
It is safer to spread an excess amount of glue than to spread too 
little, for if too little is spread the joints may be weak. (P. 41.) 
More of a thin glue than of a thick glue is absorbed by the wood and 
is squeezed out under pressure; hence, a heavier spread is required 
for thin glues. Absorbent woods take up more glue than less 
absorbent woods and must therefore receive a thicker glue spread. 
Rough surfaces require more glue than smooth surfaces, and poorly 
fitted joints more than closely fitted joints. 
In most commercial gluing only one of the contact faces of a joint 
is spread (single spreading). However, when precautions are taken 
to insure strong joints, both faces are sometimes spread (double 
spreading). Tests at the Forest Products Laboratory using both 
methods have shown that in gluing flat surfaces there is no significant 
difference in the strength of joints made by these two methods under 
good gluing conditions. Under adverse gluing conditions, however, 
such as when the glue on the wood becomes thick before pressing, 
double spreading is more reliable. 
PRESSING AND CLAMPING 
In gluing wood, pressure is required to bring the surfaces of the 
joint into contact, to force the air and excess glue from the joint, to 
spread the glue into a thin film of approximately uniform thickness, 
to force the glue into the wood cells which are adjacent to the glue 
line, to hold the joint in correct position while the glue sets, and to 
prevent warping. Mechanical devices (14) used in applying pressure 
to glue joints, a few types of which are shown in Figure 4 and Plates 
5, 6, and 7, are therefore necessary in practically all wood gluing. 
AMOUNT OF PRESSURE 
It is possible to make well-glued joints with pressures which range 
from a few pounds, such as are exerted on rubbed joints, to 1,000 or 
more pounds per square inch (pp. 33 to 38). It may require as much 
