Photo by U. S. Forest Service._]

The tendency of logs to split emphasizes the importance of
converting them into planks or timbers while in a green
condition. Otherwise the presence of large checks may render
much lumber worthless which might have been cut out in good
condition. The loss would not be so great if logs were perfectly
straight-grained, but this is seldom the case, most trees
growing more or less spirally or irregularly. Large pieces crack
more than smaller ones, quartered lumber less than that sawed
through and through, thin pieces, especially veneers, less than
thicker boards.

In order to prevent cracks at the ends of boards, small straps
of wood may be nailed on them or they may be painted. This
method is usually considered too expensive, except in the case
of valuable material. Squares used for shuttles, furniture,
gun-stocks, and tool handles should always be protected at the
ends. One of the best means is to dip them into melted
paraffine, which seals the ends and prevents loss of moisture
there. Another method is to glue paper on the ends. In some
cases abroad paper is glued on to all the surfaces of valuable
exotic balks. Other substances sometimes employed for the
purpose of sealing the wood are grease, carbolineum, wax, clay,
petroleum, linseed oil, tar, and soluble glass. In place of
solid beams, built-up material is often preferable, as the
disastrous results of season checks are thereby largely overcome
or minimized.

TEMPERATURE

The effect of temperature on wood depends very largely upon the
moisture content of the wood and the surrounding medium. If
absolutely dry wood is heated in absolutely dry air the wood
expands. The extent of this expansion is denoted by a
coefficient corresponding to the increase in length or other
dimensions for each degree rise in temperature divided by the
original length or other dimension of the specimen. The
coefficient of linear expansion of oak has been found to be
.00000492; radial expansion, .0000544, or about eleven times the
longitudinal. Spruce expands less than oak, the ratio of radial
to longitudinal expansion being about six to one. Metals and
glass expand equally in all directions, since they are
homogeneous substances, while wood is a complicated structure.
The coefficient of expansion of iron is .0000285, or nearly six
times the coefficient of linear expansion of oak and seven times
that of spruce[50].

[Footnote 50: See Schlich's