TIMBER PRESERVATION. The art of protecting timber from decay or the ravages of insects is of very ancient origin. It is probable that the Egyptians, who were experts in the preserva tion of organic bodies, also preserved wood by artificial means. Pliny mentions that the ancients employed essential oils, such as oil of cedar, olive oil, oil of spikenard, or vinegar. An even more universal method was to char wood to protect it from decay, a well-known instance of which is the charred piles under-pinning the temple of Diana at Ephesus. The beginning of the 19th century saw the introduction of modern methods of injecting wood with either salt or oil solutions. Later, a further impetus was given to wood preservation by the introduction of railways, requiring large quantities of wooden sleepers, and by the discovery of teleg raphy, which necessitated large numbers of telegraph poles. There is at the present time an urgent need, other than that of direct monetary importance, for prolonging the life of timber, and that is to conserve the world's timber supply by every possible means, which include's, amongst others, the antiseptic treatment of tim ber. Antiseptic treatment was introduced into America about 185o, and gradually spread to most civilized countries of the world, though in India and Australia, where extremely durable timbers are available, such as teak and jarrah, the treatment of railway sleepers has only been undertaken in quite recent years.
Causes of Decay.—Timber may deteriorate from several causes; amongst the more common are decay due to fungi, the attack of insects and marine-borers, and mechanical defects such as abrasion, crushing, splitting or other types of fracture. One of the commonest forms of decay is known as "dry rot," produced by several species of fungi, of which the most important is Meru lius lachrymans. The number of insects which attack wood is very great, but probably those which do most damage are the death watch beetle (Xestobium rufo-villosum), injurious to struc tural timber; the powder-post beetle (Lyctus sp.) and the common furniture beetle (Anobium punctatum), destructive to furniture. A number of marine molluscs (Teredo, the shipworm) and crusta ceans (Limnoria, the gribble, and Chelura) are most destructive to wood construction in sea-water. In the tropics the greatest enemy to timber are the termites known as white ants.
Antiseptics Used in Wood Preservation.—Antiseptics used in the preservation of wood may be divided into two broad classes, namely, oils and salts, of which the former are mostly derived from coal-tars, though wood-tar products and certain essential oils must be added to the list. The oils are in certain processes mixed with salts, as for instance in the Card process, while, recently, considerable attention has been paid to mixtures of coal-tar creo sotes and earth oils. Certain salts attracted attention as possible antiseptics for the treatment of timber, as early as 1705, when Homberg used mercuric chloride, although this salt is more com monly associated with Kyan, after whom the process is named. Of all salts which have been used in treating wood that which has best stood the test of time is chloride of zinc, discovered by Boucherie, but known as Burnettizing, after Sir William Burnett. Copper sulphate was used as long ago as 1840, but has since fallen out of favour; the most recent to be seriously considered are fluoride salts, which were first exhaustively tested by an Austrian engineer, named Malenkovic. Between 1904 and 1908 an entirely new process was brought out by Powell, which consisted in boiling timber in molasses, to which was added a small percentage of arsenic for use where white ants were prevalent.
It was Bethell who started on practical lines the treatment of wood with coal-tar products in 1838, though it is certain that the possibility of injecting these oils into timber was attempted before this date. Probably no name is better known in connection with timber preservation than that of Bethell, who used a product of coal-tar commercially known as creosote, which is at present by far the most popular antiseptic in use. It should here be noted that true creosote is a wood-tar product, though the word is now commonly and erroneously used for the coal-tar product. Coal tar creosotes differ greatly in composition, depending on the quality of the tar and the coal from which they are produced, and various commercial grades are prepared to meet different require ments. For standard products the British Engineering Standards Association's Specification, No. 144/, 1921, dated Aug. 1921, should be consulted.