|
Durability
Wood is naturally a very durable substance. If not attacked by living organisms,
it will last for hundreds or even thousands of years. Samples of wood used by the ancient Romans have been
found virtually in their original condition when a combination of circumstances protected them against
attack. The most important of the organisms attacking wood are the fungi that cause so-called dry rot,
which actually occurs only when the wood is damp. The sapwood of all trees is susceptible to
this type of decay, but the heartwood of a few species is naturally resistant to these fungi.
Walnut, redwood, cedar, mahogany, and teak are among the well-known woods that are extremely durable. Other
woods are resistant to various types of attack. Greenheart and teak are particularly resistant to the attack
of marine borers, and so are often used for underwater construction for wharves. A number of woods are
comparatively resistant to termites, including redwood, black walnut, mahogany, and several types of cedar.
In most of these cases, the woods are aromatic, and the resistance is probably due to the resins
and similar chemicals they contain.
Wood may be preserved by protecting it chemically against deterioration. The most
important method of treatment has long been impregnation with creosote or zinc chloride. This method is
still one of the best, although a number of newer chemicals, notably several containing copper compounds,
have been introduced for the same purpose. Wood can be protected against weathering by suitable surface
coatings, applied by brushing, spraying, or dipping. Surface applications yield little penetration, however,
and therefore do not prevent deterioration under attack by insects, fungi, or borers.
|
