The use of S-shaped thin steel clamps to prevent large checks and splits is now a common practice in this country with crossties and poles as it has been for a long time in European countries.  These devices are driven into the butts of the timbers so as to cross incipient checks and prevent their widening.  In place of the regular S-hook another of crimped iron has been devised. (See Fig. 28.) Thin straps of iron with one tapered edge are run between intermeshing cogs and crimped, after which they may be cut off any length desired.  The time for driving S-irons of either form is when the cracks first appear.

[Illustration:  FIG. 28.—­Control of season checking by the use of S-irons. 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].