THE CAUSES OF WARP IN LUMBER SEASONING
By Edwin Knight, Research Engineer
Western Pine Association
Lumber degrade due to warp has long been a matter of much concern
among lumbermen. Warp is complex. It seems as if each one of us chosen to
view it in a different way, and unless we investigate carefully all of the
factors involved, we are likely to know too small a part of the problem when
trying to apply corrective measures.
Definition of Warp
Let us define the forms of warp in order to all use the same terms.
Warp in lumber can be defined as any deviation from flatness or straight-
ness. One form is cupping, the deviation from flatness transversely across
the face of a board. Bow is the same type of deviation but in.a lengthwise
direction from end to end of the piece. Crook is an edgewise deviation and
twist is a variation of cup in which the board takes on a spiral propeller
like form from end to end. One' of the worst types of warp from a standpoint
of degrade is kink, the sharp, short bend in the piece.
Causes of Wart
Many of the causes of warp are inherent in the wood and are not con-
trollable. Others are induced by external forces which can and should De
controlled. Still others seem to be a combination of causes and effect of
which a partial control is possible if we know the basic causes and learn
how to apply this knowledge. The fact that it is possible to exert some
measure of control over warp is good reason to make every effort to do so,
and to stop looking for short cuts to quality. There are so few of them in
the lumber business.
Most softwood lumber is sawn by turning the log to produce flat sawn
lumber in the boards. Differential shrinkage becomes a major problem in
flat sawn lumber. The fact that tangential shrinkage exceeds radial
shrinkage accounts for the cupping that occurs. Because more of the grain
is tangentially disposed on the sap side of a board than on the heart side
the board always cups toward the outside, or the sap side. Moreover, since
the shrinkage differential