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.