56 BULLETIN 15 0, U. S. DEPARTMENT OE AGEICULTUEE 
and face veneers. Prevailing commercial practice varies as to the 
maximum size of knots and blemishes permitted, but in general it is 
from one-fourth to one-half of an inch in diameter. Doty and 
decayed wood have a different shrinkage from sound wood and under 
moisture changes this shrinkage difference may become noticeable 
on highly finished surfaces. 
The best core woods for high-grade panels are of low density, of 
low shrinkage, of slight contrast between the spring wood and the 
summer wood, and of the species which are easily glued (o2). Edge- 
grained cores are better than flat-grained cores because of their low 
shrinkage in width, and. in softwoods with pronounced summer 
wood they are better for the additional reason that the hard bands 
of summer wood are less likely to show through thin veneer in edge- 
grain lumber than in the flat-grained lumber. In most species a 
core made of all quarter-sawed or all flat-sawed material remains 
more uniform in thickne-s with moisture-content changes than one 
made by combining these two types of material. This advantage is 
not of great practical significance, however, where the laminations 
are narrow and the glue joints are strong. In addition to its 
properties, the cost price, the percentage of clear cuttings, and the 
quantity available generally determine the extent to which a wood 
is used for cores. Yellow poplar and chestnut are deservedly popular 
core woods. Basswood and the gum woods are also extensively used 
for cores. 
The use of the chestnut, which is a very desirable core wood, de- 
serves special mention. This wood is now being forced on the market 
because of the ravages of a chestnut blight which is slowly exter- 
minating the species. Because of lack of a good market, much of the 
dead chestnut timber is being left in the woods to rot despite the 
fact that the wood from dead trees, so long as it is not decayed, is in 
every way as good as that from live trees. Manufacturers who use 
chestnut wood are therefore salvaging a wood that might otherwise 
be wasted. 
T\ Trite pines, western yellow pine, and Douglas fir are woods that 
are used very largely for cores in doors. Such cores are commonly 
built from small pieces of wood which are incident to the manufac- 
ture of sash and other millwork and are faced with thick veneers. 
However, for cores in high-grade veneered panels the coniferous 
woods which have pronounced summer wood must be used with dis- 
cretion to prevent their growth rings from showing through the 
finished face veneers. 
PARALLEL GRAIN CONSTRUCTION 
Parallel-grain or laminated construction, a? distinguished from 
cross-banded construction, refers to two or more layers of wood 
fastened together with the grain of all layers approximately parallel. 
The size, shape, number, and thickness of the laminations or plies 
may vary greatly. (Fig. 17.) Parallel-grain construction may be 
used as a base for veneer, as in cores for doors, table tops, and other 
furniture panels, or it may be used unveneered. as in automobile 
sills, steering-wheel rims, porch columns, airplane propellers, and 
wooden pulleys. 
