As soon as it was brought into the air the process of decay began, and it became necessary to coat it with preservative. It stands today, 103 feet long, in the museum at Christiania.
Many other instances of under-water preservation might be mentioned.
The other extreme is also true. Wood which is kept perfectly dry will last indefinitely, as in the case of woodwork taken from the pyramids of Egypt, 3,000 years old, which is found to be perfectly preserved.
Fig. 233. A pile
But when wood is alternately wet and dry it decays rapidly. A pile driven into the bottom of a tidal river is a good illustration. If such a pile be divided into four sections (see [Fig. 233]), a is always in the ground, b is always in the water, c is alternately in air and water, d is always in the air. Sections a and b may be considered to be under the same conditions and should last the longest; c should decay first: d would last indefinitely if the atmosphere were always perfectly dry; but humidity and rain, air and heat combined finally bring about decay, and although this part of the pile will last longer than c it will in time decay. Section c should be coated with a preservative.
Various woods under the same conditions act very differently and according to no well understood law. For example, in contact with the soil black locust is our most durable wood. It is very hard, and its life under these conditions is estimated at from ninety to a hundred years. Red cedar comes next, though it is soft wood. Oak decays in a few years; chestnut, much softer, lasts two or three times as long. Our approaching timber famine has induced a study of this subject, since the preservation of wood is becoming an absolute necessity.
It has been found that certain materials put on the wood before it is placed in the ground prolong its life. Coal tar, wood tar, paint, and creosote all help, but creosote has so far proved to be the best. It is one of the by-products of coal tar and is being used extensively by the railroad companies for prolonging the life of ties.
Experiments with creosote have brought out some very interesting facts. It has been found that after being treated with the hot creosote all woods resist decay alike, regardless of their hardness or softness. Consequently, a treated cheap wood will last as long as an originally valuable one. This is a great gain, as it allows us to make use of wood like the poplar which would otherwise be practically of no value.