THE GLUING OF WOOD 37 
Consistency of Casein, Vegetable, and Blood-Albumin Glues 
Most casein and vegetable glues are thick enough to press imme- 
diately after being spread, but if an unusually thin glue is used it 
should be allowed to thicken before pressing. Blood glues applied 
in jelly form are thick enough to press immediately, but when ap- 
plied in liquid form they should be allowed to thicken slightly before 
being pressed. Blood glue may be spread on thin sheets of paper and 
dried. In this form it may then be inserted between wood layers 
and hot pressed. 24 Such blood glues retain moisture enough to allow 
the glue to unite the wood layers under the combined influence of the 
heat and pressure. 
TYPES OF JOINTS RESULTING FROM DIFFERENT GLUING CONDITIONS 
Plate 10 illustrates a well-made joint and three types of weak 
joints. The weak joints were produced as a result of not having 
a proper combination of glue consistency and pressure. The four 
sets of 10 specimens tested represent four joints, each glued under a 
different condition. In the first joint (pi. 10, A) failure occurred 
entirely in the wood because the joint was stronger in shear than 
the wood itself. In the other three joints of quite different appear- 
ance, failure occurred entirely in the glue line. Plate 10, B shows the 
" starved " type of joint, which is clean in appearance and shows 
no distinct film or layer of glue. The chilled type of joint (pi. 10, C) 
is due to the glue jellying (congealing without drying) and to the 
use of insufficient pressure to cause the glue to give complete contact. 
It occurs chiefly with animal glues. The "dried" joint (pi. 10, D) 
may occur with any glue that has lost so much water that it will not 
adhere to wood even under a very heavy pressure. In both the 
chilled and dried joints, contact occurs over only a part of the joint 
areas. 
The relation of pressure and temperature to joint strength is 
illustrated in Figure 8, which shows the results of tests made on 
birch blocks glued with a good grade of animal glue at 70° and at 
90° F. and at various pressures. The joints illustrated were coated 
with glue and placed together for 3, 12, and 25 minutes, respectively, 
before being pressed. The results of the three assembly periods, how- 
ever, are averaged for each pressure used. At 70° the glue after 
being spread chilled in 1 to 2 minutes, and at the end of 3 minutes 
it formed a firm jelly. Such a firm jelly used with low pressures 
can not be relied upon for obtaining the best results. The curve 
at 70° shows a constant increase in average joint strength with 
increase of gluing pressures up to 400 pounds pressure. 
The test results are very different, however, with the room and 
wood at 90° F. At this temperature the glue after being spread 
changed slowly in consistency, and at the end of 25 minutes it had 
not formed a firm jelly. The high pressures used at 90° F. squeezed 
out too much of the glue on the 3-minute assembly period, and 
therefore gave low average results. Good average results, how- 
ever, were obtained at 50 and 100 pounds pressure per square inch, 
24 A similar process is covered by the following : McClain, J. R. wood veneer. (U. s. 
patent no. 1,299,747.) U. S. Patent Office, Oft. Gaz. 261 ; 286, illus. 1919. 
