THE GLUING OF WOOD 
49 
GLUING PLYWOOD OF DIFFERENT SPECIES 
The results shown in Figures 12 to 16 were obtained on joints 
made from lumber approximately three-quarters of an inch thick. 
Similar experiments on plywood joints were not made on all the 
species; however, a great deal of experimental plywood has been 
glued from some of the species, and it is believed that the same gen- 
eral principles, based on the inherent characteristics of the wood, 
hold for the plywood as for the lumber. 
The results obtained in gluing smoothly planed lumber may vary 
somewhat from those obtained in gluing rotary-cut or sliced veneer. 
When glued into plywood and tested, badly checked veneer shows a 
higher percentage of wood failure and a lower joint strength than 
firm, smooth veneer (<§). In a plywood test, therefore, the relative 
positions of the species in Figures 12 to 16 would probably vary 
somewhat. 
TENDENCY OF WOODS TO BE STAINED BY GLUE 
In gluing veneer into plywood the tendency of the different species 
to be stained by the glue is often a matter of importance. This 
applies particularly to the face veneer of plywood that is to receive 
a finish. The tendency of various woods to stain when glued with 
a strongly alkaline glue is indicated in Table 6. Most discoloration 
of the wood when glued is the result of a reaction between the free 
alkali of the glue and materials in the wood. The shade varies for 
different woods. For example, in oak the discoloration is brown ; in 
mahogany, dark red; and in redwood, almost black. The different 
kinds of glues vary in their tendency to stain. (See Table 1.) 
Blood-albumin glues do not ordinarily contain much free alkali; 
nevertheless the dark-colored blood albumin may penetrate porous 
woods to such an extent as to discolor thin veneers. Animal and 
liquid glues do not normally produce any objectionable discoloration. 
Table 6. — Tendency of various woods to be stained by a strongly alkaline glue 
Amount of staining 
Species of wood > 
Amount of st 
aining 
Species of wood i 
M 
m 
MS 
S a 
co o 
a 
a ° 
> 
CD 
M 
CS 
E3 
»i O 
> 
Ash 
X 
X 
X 
X 
X 
X 
Maple . . . . . .. 
x 
X 
X 
X 
Oak, red 
Birch 
X 
X 
X 
X 
X 
X 
X 
Redwood ____ ..... 
X 
X 
Spruce, Sitka 
X 
X 
X 
Gum, red . . . .. 
X 
Walnut, black 
X 
X 
i Heartwood was used in all species where it could be identified. 
31595°— 29 4 
