58 BULLETIN" 1500, IT. S. DEPARTMENT OP AGRICULTURE 
While the properties of parallel-grain products are generally 
similar to those of solid wood, the gluing of them together makes it 
possible to manufacture wide or thick articles out of smaller and 
cheaper material and generally with less waste of wood than if 
solid wood alone were used. For example, glued table tops can be 
made in widths which it is impossible to obtain from most species in 
solid wood. Such articles, if well constructed, are less likely to 
warp and check than if made of a single wide board. Curved mem- 
bers (fig. 17, F, G, and H) can be made either by pressing or wrap- 
ping thin laminations around curved forms or by gluing the indi- 
vidual segments together. (Fig. 17, I and J.) On account of the 
direction of the grain of the wood in these products, their strength 
is far superior to solid wood cut to the same size and shape. 
Differences in shrinking or swelling are the fundamental causes 
of internal stresses and laminations should be of such character that 
they shrink or swell the same amounts in the same directions. The 
laminations should therefore be straight grained, of the same shrink- 
age properties (Table 9), and at the same moisture content when 
glued. Lengthwise shrinkage in a straight-grained piece of normal 
wood is only about one-third of 1 per cent, while the shrinkage side- 
wise may be ten or twenty times as much. In a cross-grained lam- 
ination the shrinkage across the grain changes the length of the 
piece more than the normal amount and consequently sets up stresses 
in long pieces which may cause serious warping. 
If two or more pieces having different shrinkage properties are 
glued together, even though they are straight grained, a moisture 
change will cause them to shrink or swell different amounts and thus 
set up stresses. If internal stresses are to be avoided, flat-grained 
wood should not be glued to edge-grained wood, nor should differ- 
ent species be glued together unless they have similar shrinkage 
properties. 
Combining pieces at different moisture content results in stresses 
when they later come to a common moisture content. The pieces 
should therefore be conditioned to about the same moisture content 
before being glued. 
The relative density of the different pieces of the same species in 
parallel-grain construction is not very important, as test specimens 
show little weakening or warping from this cause (26). 
In making glued parallel-grain articles it is important to avoid 
as much as possible the development of internal stresses when the 
article is exposed to conditions which change its moisture content. 
In an article like an airplane propeller, where extreme refinement 
of shape and balance is necessary, the greatest precautions against 
internal stresses must be observed. However, in gluing ordinary 
articles, especially in edge-to-edge gluing, moderate attention to 
shrinking and swelling will be sufficient. 
The stresses which develop in parallel-grain articles due to differ- 
ences in the properties of the various members will gradually disap- 
pear if the glued article is kept for a long time at a constant moisture 
content. This is because the stressed members, stretch, check, or 
warp until all of the stresses are relieved. Stresses due to moisture 
differences in the wood at the time of gluing will not reappear after 
having once been relieved. Stresses due to cross grain or to dif- 
