THE GLUIKG OF WOOD 55 
Opposite plies to be symmetrical and balanced must have about the 
same moisture content when glued. Variations in moisture content 
of corresponding plies at the time of gluing bring about shrinkage 
differences that result in warping. Large changes in the moisture 
content of the wood after gluing should be avoided as much as pos- 
sible because they induce internal stresses of large magnitude and 
cause warping, checking, and weak joints. 
The use of cross-grained veneer is one of the most common causes 
of warping and cupping in cross-banded construction. Deviations 
in the grain of only 5° to 10° from straight cause considerable dis- 
tortion. While it is impossible to eliminate all cross-grained veneer, 
that which shows an excessive amount of cross grain should be re- 
jected in plywood manufacture or used in such a manner or for such 
purposes that its effect is not harmful. 
Figured veneer, cut from burls, crotches, stumps, and similar 
growth irregularities is not straight grained but is used because of 
its desirable appearance. It shrinks with both the width and length 
of the sheet, whereas a plain veneer shrinks chiefly in one direction. 
This difference in shrinkage between these two types of veneer causes 
warping when they are used together in thin panels. "With such 
veneer it is not practical to have a strictly balanced construction, 
and the effects of such unsymmetrical arrangement must be com- 
pensated for in some other way. Ordinarly, by laying figured veneer 
of this kind over a thick and properly cross-banded core the con- 
struction is made resistant enough against warping to prevent the 
unbalanced forces of two thin faces from excessively distorting or 
warping. In gluing thick 5-ply veneered panels (fig. 3, D) this con- 
struction makes it possible to use a figured veneer on the panel face 
and a straight- grained veneer on the panel back. 
It is especially important that the crossbands should be of good 
quality, for they particularly affect the shape and permanence of 
the form of the panel. They should be straight grained and smoothly 
cut and should be of sound wood which is even in grain and texture. 
Imperfections in the crossbands, such as marked differences in the 
texture of the wood or irregularities in the surface, are easily seen 
in the panel through thin surface veneers. This is especially true 
in highly polished panels which have a conspicuous grain in the 
crossbands. The warping of thick 5-ply panels is commonly the 
result of using cross-grained veneer in one or both of the crossbands, 
which shrinks and swells abnormally lengthwise. A very small 
change in the length of one crossband allows the panel to become 
distorted. Where the grain runs diagonally across both sheets of 
veneer, the warping is very pronounced unless the crossbands are 
laid with the grain parallel to each other. 
REQUIREMENTS FOR CORES 
Where the crossbands and face veneer are relatively thin, the cores 
for high-grade panels must be practically free of knots, limb mark- 
ings (local areas of cross grain occurring in the region of knots), 
and doty and decayed wood. Unless removed, such defects may be 
visible on the faces of panels after they have received a finish. The 
size of defects that may be allowed in cores without showing upon 
the finished faces depends largely on the thickness of the crossband 
