48 BULLETIN 1500, U. S. DEPAETMENT OF AGRICULTURE ■ 
the casein and vegetable glues is thought to have counteracted the 
interference of the infiltrated materials in the wood with the glue 
adhesion. The heart wood of birch glued very satisfactorily when 
the animal glues were used but glued less satisfactorily when the 
casein and vegetable glues were used. In the tests described on 
page 42 the heartwood of Osage-orange glued satisfactorily with 
animal glue but unsatisfactorily with casein glue. (Figs. 13 and 14.) 
TREATING THE WOOD BEFORE GLUING 
In certain species treating the wood with a chemical before gluing 
improves the joint strength. Of several treating materials tested at 
the Forest Products Laboratory a 10 per cent solution of caustic soda 
generally proved to be the most effective. 30 In these tests the wood 
surfaces to be joined were brushed with the caustic soda solution; 
after about 10 minutes they were wiped with a cloth to remove any 
excess solution or dissolved material and allowed to dry before being 
glued. The same grade of glue was used, and the density of the wood 
tested was substantially the same. These tests show a decided im- 
provement in the joint strength of the hard maple, yellow birch, 
white oak, red oak, red gum (both heartwood and sapwood), black 
cherry, and basswood over tests on untreated woods of the same spe- 
cies. Osage-orange treated with the caustic solution and then glued 
with a casein glue gave satisfactory joints. Furthermore, treating 
with caustic so$a the surfaces to be joined prevents starved joints. 
This is indicated by the test data obtained from hard maple glued 
with an animal glue, which is given in Table 5. 
Table 5. 
-Results of caustic soda treatment on joints of hard maple glued with 
an animal glue 1 
Condition of specimen 
Shear 
strength 
of joint 
Wood 
failure 
Lbs. per 
sq. in. 
2,440 
3,5&4 
Perct. 
13 
82 
i Each test value shown represents 50 specimens. 
Although chemical treatments of the wood surfaces before gluing 
result in better joints with certain species that require great care in 
gluing, such treatments are time consuming and therefore add to the 
expense of gluing. Ordinarily, good glued joints can be secured by 
properly regulating the gluing practice (p. 32) so that the chemical 
treatment of the wood is unnecessary. 
30 With hickory, black cherry, and red gum (heartwood) glued with casein glue, a treat- 
ment of milk of lime (10 parts of hydrated lime to 90 parts of water) gave slightly better 
results than were obtained with the caustic soda treatment. 
