Timber may be protected for a time against the agencies of decay by covering the surface with paint, thereby preventing the entrance of fungus. By another process, certain oils or other materials which are poisonous to the insinuating threads of fungus, are forced into the pores of the wood. Creosote is often used for this purpose. Attacks are thus warded off, and decay is hindered. The preservative fluid will not remain permanently in wood exposed to weather conditions, but the period during which it affords protection and immunity extends over some years; but different woods vary greatly in their ability to receive and retain preservative mixtures.
The better seasoned, the less liable is timber to decay, because it contains less moisture to support fungi. It is generally supposed that timber cut in the fall of the year is less subject to decay than if felled in summer. If it is so, the reason for it lies in the fact that fungus is inactive during winter, and before the coming of warm weather the timber has partly dried near the surface, and fungi cannot pass through the dry outside to reach the interior. Timber cut in warm weather may be attacked at once, and before cold weather stops the activities of fungus it has reached the interior of the wood and the process of rotting is under way. When the agents of decay have begun to grow in the wood, destruction will go on as long as air and moisture conditions are favorable.
The bluing of wood is an incipient decay and is generally due to fungus. Some kinds of wood are more susceptible to bluing than others. Though boards may quickly season sufficiently to put a stop to the bluing process before it has actually weakened the material, the result is more or less injurious. The wood’s natural color and luster undergo deterioration; it does not reflect light as formerly, and seems dead and flat.
Decay affects sapwood more readily than heart. The reason may be that sapwood contains more food for fungus, thereby inducing greater activity. The sapwood is on the outside of timbers and is often more exposed than the heart. In some instances greater decay may be due to greater exposure. Another reason for more rapid decay of sapwood than heart is the fact that the pores of the heartwood are more or less filled with coloring matter deposited while the growth of the tree was in progress. The coloring matter, in many cases, acts as a preservative; it shuts the threads of fungus out. Sometimes the sapwood of a dead tree or a log is totally destroyed while the heart remains sound. This often happens with red cedar and sometimes with black walnut, yellow poplar, and cherry. Occasionally a tree’s bark is more resistant to decay than its wood. Paper birch and yellow birch logs in damp situations occasionally show this. What appears to be a solid fallen trunk, proves to be nothing more than a shell of bark with a soft, pulpy mass of decayed wood within.