results with normal conditions of pressure and assembly time. The spread 
may, of course, be varied, but a decrease in the strength of the joint will 
probably be noticed if the amount of glue is cut so as to get more than 55 
or 60 square feet of glue line per pound of dry glue. On the other hand, 
there is some danger in increasing the amount of glue spread too much be- 
cause of the amount of water added during the gluing operation. Vegetable 
glue, after being spread on the wood, ordinarily remains in a condition 
satisfactory for pressing for periods up to about 25 minutes. A pressure 
of about 200 pounds per square inch gives good results when the assembly 
time varies over a rather wide range. In commercial operations, however, 
a pressure of from 100 to 150 pounds per square inch is commonly used with 
success. If the assembly time is short, say from 1 to 5 minutes, a pressure 
of about 100 pounds per square inch may give slightly stronger joints than 
one of 200 pounds per square inch. Since vegetable glues generally contain 
large proportions of water, it is usually necessary to recondition glued 
stock after removal from pressure before further machining or finishing. 
Glued stock for interior use should have a moisture content of 6 to 8 per- 
cent before final machining. 
Durability of Vegetable-glue Joints 
Vegetable glues are generally low in water resistance and are very 
similar to animal glues in this respect. Conventional shear-test specimens 
of yellow birch plywood (three-ply, 3/l6-inch) bonded with a typical 
vegetable glue were exposed under each of several repeating cycles. The 
cycles consisted of exposure, at a temperature of 80° F. for each phase, 
for 14 days at a high relative humidity or of soaking in water for 2 days, 
followed by exposure for 12 or 1^ days at 30 percent relative humidity. 
Such cycles caused swelling and shrinking of the wood and subsequent mechan- 
ical stresses on the joints. At some of the higher relative-humidity 
conditions softening of the glue also occurred. The exposure cycles are 
given in table 1. 
Specimens subjected to test No. 6 (table l) separated completely, 
either in the first cycle or in the drying that immediately followed. 
Specimens exposed to test No. 5 showed at the end of the first complete 
cycle an average joint strength of less than kO percent of the original. 
All specimens failed before the end of the fourth cycle, or in somewhat 
less than 16 weeks. A relative humidity of 97 percent is sufficiently high 
to permit development of molds and to bring the moisture content of the 
wood to about 28 percent during the wet half of the cycle. 
Specimens exposed to test No. k failed less rapidly than those ex- 
posed to test No. 5. They had decreased in strength, however, by some 50 
percent by the end of the first test cycle, and all specimens had failed 
by the end of 36 weeks. Moisture content of the wood during the wet half 
of this cycle probably reached about 22 percent. 
In test No. 3, more than 20 weeks of exposure were required before 
the average test values had fallen to 50 percent of the original. By the 
Report No. R30 -6- 
