This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
This is the result of the evaporation or drying out of the water which is held in the cell walls of the wood in its natural state, and the shrinkage which naturally follows. If wood were perfectly regular in structure, so that the shrinkage could be the same in every part, there would be no warping, but wood is made up of a large number of fibers, the walls of which are of different thicknesses in different parts of the tree or log, so that in drying one part shrinks much more than another. Since the wood fibers are in close contact with each other and interlaced, thus making the piece of wood rigid, one part can not shrink or swell without changing the shape of the whole piece, because the piece as a whole must adjust itself to the new conditions; consequently the timber warps.
Fig. 6. Section Showing Formation of a Knot.
In Fig. 7, if the fibers in the lower portion of the piece near the face CDG happen to have, on the average, thicker walls than those in the upper portion, near the face ABFE, the lower part will shrink more than the upper part. The distance CD, originally equal to the distance AB, becomes smaller and the shape of the whole piece changes as shown in Fig. 8.
The only way in which warping can be prevented is to have the timber thoroughly dried out before it is used, as after it is once thoroughly seasoned it will not warp unless it is allowed to absorb more moisture. All wood which is to be used for fine work, where any warping after it is in place will spoil the appearance of the entire job, must be so seasoned, either in the open air or in a specially prepared kiln.
The wood of the conifers which is very regular in its structure shrinks more evenly and warps less than does the wood of the broad-leaved trees with its more complex and irregular structure. Sap-wood, also, as a rule shrinks more than does heartwood.