42 BULLETIN • 1500, U. S. DEPARTMENT OE AGRICULTURE 
glue spreads as thin as 60 to 80 square feet of glue line per pound of 
dry glue may be used if other gluing factors are properly controlled. 
Differences of as much as 50 per cent in the area covered per pound 
of dry glue may be required between a thick or thin glue when used 
on rough, smooth, porous, or nonporous wood surfaces. 
GLUING END-GRAIN WOOD 
The recommendations thus far presented relate chiefly to the glu- 
ing of side-grain surfaces. Such joints can be made as strong as 
the wood itself. End-grain to end-grain joints, however (fig. 2, A), 
are much more difficult to glue and can not be glued strong enough to 
break the wood. 
For gluing end-grain surfaces with animal or casein glue both sur- 
faces should be sized with a mixture of 1 part of glue to 3 to 5 parts 
of water, depending on the grade of glue, and the sizing coat should 
be allowed to dry. Both surfaces should then be coated with a thick 
glue mixture (about 10 per cent less water than is used in side-grain 
gluing for any specified grade, and pressed at 200 pounds per square 
inch. 
LAYING FIGURED VENEER 
Highly figured burl, stump, and crotch veneers (pi. 2, A, and pi. 
11, C and D) generally necessitate the gluing of end-grain surfaces. 
It is considered good practice in gluing these veneers to size both 
sides of the veneer with warm animal glue mixed 1 part of glue with 
3 to 5 parts of water, depending upon the grade of glue, and allow 
the sizing coat to dry. 25 After the sizing has thoroughly air-dried, 
the veneer sheets should be redried between heated wooden cauls. 
While the face veneer is being sized and dried, the crossband veneer 
should be laid onto the core and then dried and surfaced. The dry 
cross-banded core should be spread next with a thick glue. Animal 
glue may be used on the crossband if allowed to chill somewhat 
before the face veneer is brought into contact because the warm 
cauls will soften the glue sufficiently to permit adhesion. Finally, 
the face veneer should be laid in place and pressed as quickly as 
possible. 
GLUING CHARACTERISTICS OF DIFFERENT WOODS 
While practically all species of wood commonly used in the United 
States can be glued successfully, some require more care in gluing 
than others in order to insure joints of the highest strength. 
Extensive gluing tests have been made at the Forest Products Lab- 
oratory on 40 common species, in which vegetable glue, casein glue, 
and animal glue were used. 2 ' 5 The wood, in pieces three-fourths of 
an inch thick. 5 inches wide, and 12 inches long, was glued into 
iy 2 by 5 by 12 inch blocks, then cut into specimens and tested as de- 
scribed in the Appendix, page 69. The gluing was done under con- 
ditions which cover the general range found in commercial prac- 
2Z Various secret sizing formulas are used in the woodworking trade, which, in addition 
to glue, usually contain other materials. 
26 Blood-albumen glues, owiug to their small use in this country, were not included 
iu these tests, and liquid glues, owing to the wide range iu grades, were also omitted. 
