and closes up the
pores. As the material is moved down the kiln (as in the case of
"progressive kilns"), it absorbs a continually increasing amount of
heat, which tends to drive off the moisture still present in the
center of the piece, the pores on the outside having been closed up,
there is no exit for the vapor or steam that is being rapidly formed
in the center of the piece. It must find its way out in some manner,
and in doing so sets up strains, which result either in checking or
splitting. If the humidity had been kept higher, the outside of the
piece would not have dried so quickly, and the pores would have
remained open for the exit of the moisture from the interior of the
piece, and this trouble would have been avoided. (See also article
following.)

Shrinkage of Wood

Since in all our woods, cells with thick walls and cells with thin
walls are more or less intermixed, and especially as the spring-wood
and summer-wood nearly always differ from each other in this respect,
strains and tendencies to warp are always active when wood dries out,
because the summer-wood shrinks more than the spring-wood, and heavier
wood in general shrinks more than light wood of the same kind.

If a thin piece of wood after drying is placed upon a moist surface,
the cells on the under side of the piece take up moisture and swell
before the upper cells receive any moisture. This causes the under
side of the piece to become longer than the upper side, and as a
consequence warping occurs. Soon, however, the moisture penetrates to
all the cells and the piece straightens out. But while a thin board of
pine curves laterally it remains quite straight lengthwise, since in
this direction both shrinkage and swelling are small. If one side of a
green board is exposed to the sun, warping is produced by the removal
of water and consequent shrinkage of the side exposed; this may be
eliminated by the frequent turning of the topmost pieces of the piles
in order that they may be dried evenly.

As already stated, wood loses water faster from the ends than from the
longitudinal faces. Hence the ends shrink at a different rate from the
interior parts. The faster the drying at the surface, the greater is
the difference in the moisture of the different parts, and hence the
greater the strains and consequently also the greater amount of
checking. This becomes very evident when freshly cut wood is placed in
the sun, and st