There are various other methods besides kiln-drying (referred to in Chapter III.) of seasoning and of hastening the drying process. Wood is sometimes soaked in water before being seasoned. This assists in removing the soluble elements of the sap, but it is doubtful whether the process improves the quality of the wood. Smoking and steaming are also resorted to. Small pieces can readily be smoked, which hardens the wood and adds to its durability,—a method which has been known for centuries,—but care must be taken not to burn, scorch, or crack the wood.
Fig. 694.
Decay and Preservation.—Timber decays fastest when alternately wet and dry, as in the piles of a wharf, fence-posts, and the like, or when subjected to a hot, moist, close atmosphere, as the sills and floor-timbers situated over some damp and unventilated cellar. Fig. 694 shows the decay caused by alternate wetness and dryness, while the parts above and below are still sound.
Wood lasts the best when kept dry and well ventilated. When kept constantly wet it is somewhat softened, and will not resist so much, but it does not decay. Recently, upon cutting a slab from the outside of a large log taken from the bed of a river, where it had lain for one hundred years or more, the interior proved as sound and clear as could be found in any lumber-yard. Undoubtedly, however, such long submersion lessens the elastic strength of timber after it is dried. That is not, however, an extreme example of durability. Wood has been taken from bogs and ancient lake-dwellings after being preserved for ages. Piles were taken from the Old London Bridge after about 650 years of service. Piles placed in the Rhine about 2000 years ago have been found quite sound during the present century; and piles are now regularly used, as you doubtless know, for the support of the most massive stone buildings and piers, but only where they are driven deep in the ground or below the low-water line. Many examples of the durability of wood kept dry are found in European structures. Timbers put into the roof of Westminster Abbey in the reign of Richard II. are still in place, and the roof-timbers of some of the older Italian churches remain in good condition.
Thorough seasoning, protection from the sun and rain, and the free circulation of air are the essentials to the preservation of timber.
Many preparations and chemical processes have been tried for the preservation of wood.
Creosote is one of the best preservatives known. Insects and fungi are repelled by its odour. The modern so-called "creosote stains" are excellent, not very expensive, and easily applied. They are only suitable for outside work, however, on account of the odour.
Coal-tar and wood-tar or pitch, applied hot in thin coats, are also good and cheap preservatives for exposed wood-work.
Charring the ends of fence-posts by holding them for a short time over a fire and forming a protecting coating of charcoal is another method which has been extensively used.