Encyclopædia Britannica, Ninth Edition/Dry Rot

DRY ROT, a disease in timber, apparently infectious, which occasions the destruction of its fibres, and reduces it eventually to a mass of dry dust. It is produced most readily in a warm, moist, stagnant atmosphere, while common or wet rot is the result of the exposure of wood to repeated changes of climatic conditions. In both diseases, however, a kind of spontaneous combustion or decomposition goes on in the wood; water, carbonic acid gas, and probably carburetted hydrogen are evolved, and a pulverulent substance, or humus, remains. Though the growth of fungi undoubtedly accelerates the progress of dry rot, it would seem that the true origin of the disease is the incipient decomposition of the sap in wood, and that by virtue of this decomposition the fungi obtain a nidus for their growth. The most formidable of the dry rot fungi is the species Merulius lacrymans, which is particularly destructive of coniferous wood; other species are Polyporus hybridus, which thrives in oak-built ships, and P. destructor and Thelephora puteana, found in a variety of wooden structures. The nature of ships cargoes has a considerable influence on the duration of their timbers,—hemp, pepper, and cotton being highly favourable, and lime and coal unfavourable, to the development of dry rot. The commonest precaution against the occurrence of that disease is to deprive the wood of its moisture by exposure to the open air, or, in other words, to season it. Charring, steaming, boiling, and smoke-curing are other modes of desiccation which have been resorted to. At one time a Mr Lukin attempted the rapid seasoning of logs of green oak at Woolwich dockyard by heating them in pulverized charcoal; but the process, though it lessened the weight and dimensions of the wood, started its fibres from one another. He then sought to replace the moisture of heated wood by the products of the distillation of pitch-pine saw-dust; before, however, the operation was judged to be complete, an explosion took place, which proved fatal to eight workmen, and wounded twelve; the experiment, therefore, was not repeated. Davison and Symington's patent process of artificial drying, which has been found to yield good results, consists in exposing the wood to a current of air moving at the rate of about 48 miles an hour, and having a temperature of 110° to 112° Fahr.

The felling of trees when void of fresh sap, as a means of obviating the rotting of timber, is a practice of very ancient origin. Vitruvius directs (ii. cap. 9) that, to secure good timber, trees should be cut to the pith, so as to allow of the escape of their sap, which by dying in the wood would injure its quality; also that felling should take place only from early autumn until the end of winter. The supposed superior quality of wood cut in winter, and the early practice in England of felling oak timber at that season, may be inferred from a statute of James I., which enacted "that no person or persons shall fell, or cause to be felled, any oaken trees meet to be barked, when bark is worth 2s. a cart-load (timber for the needful building and reparation of houses, ships, or mills only excepted), but between the first day of April and last day of June, not even for the king's use, out of barking time, except for building or repairing his Majesty's houses or ships." In giving testimony before a committee of the House of Commons in March 1771, Mr Barnard of Deptford expressed it as his opinion that to secure durable timber for ship-building, trees should be barked in spring and not felled till the succeeding winter. In France, so long ago as 1669, a royal decree limited the felling of timber from the 1st October to the 15th April; and, in an order issued to the commissioners of forests, Napoleon I. directed that the felling of naval timber should take place only from November 1 to March 15, and during the decrease of the moon, on account of the rapid decay of timber, through the fermentation of its sap, if cut at other seasons. The burying of wood in water, which dissolves out or alters its putrescible constituents, has long been practised as a means of seasoning. The old "Resistance" frigate, which went down in Malta harbour, remained under water for some months, and on being raised was found to be entirely freed from the dry rot fungus that had previously covered her; similarly, in the ship "Eden," the progress of rot was completely arrested by 18 months submergence in Plymouth Sound, so that after remaining a year at home in excellent condition she was sent out to the East Indies. It was an ancient practice in England to place timber for thrashing-floors and oak planks for wainscotting in running water to season them. Whale and other oils have been recommended for the preservation of wood; and in 1737 a patent for the employment of hot oil was taken out by a Mr Emerson. Common salt, but for the attraction of its impurities for moisture, might be advantageously used; indeed the Dutch ship-builders, having observed that the busses in which herrings were stowed away in pickle lasted longer than any other craft, adopted the practice of filling up with salt, not only the vacant spaces between the planks, but also holes bored for its reception in the large timbers.

Among the many processes for the prevention of dry and wet rot in wood by impregnating it with material capable of precipitating its coagulable constituents in a permanently insoluble and imputrescible form, the following may be enumerated:—Kyan's (1832), in which, according to Sir Humphry Davy's suggestion, a solution of corrosive sublimate is employed; Sir W. Burnett's (1836), M. Breant's (1837), Margary's (1837), and Payne's (1841), which consist respectively in the use of zinc chloride, copperas, copper sulphate, and copperas followed by sodium carbonate; and Bethell's (1838), for the treatment of the wood with crude creasote or oil of tar. The application of solution of copper sulphate, containing about a quarter of a pound of the salt to each gallon of water, according to Margary's patent, has been found very efficacious in the case of timber not liable to the solvent action of water; but of all processes the most satisfactory is Bethell's. In this the wood is injected with heavy tar-oil in cylinders 6 feet in diameter and 20 to 50 feet in length, at a temperature of 120° Fahr., and under a pressure of 150 lb to the square inch, so that ordinary fir timber absorbs on the average 8 to 10 lb of the liquid per cubic foot. Timber thus prepared has been found not only durable, but also exempt from the attacks of insects and other pests.

J. Papworth, An Essay on the cause of the Dry Rot in Buildings, 1806; Bowden, A Treatise on the Dry Rot, 1815; Wade, A Treatise on the Dry Rot in Timber, 1815; Chapman, On the Prevention of Timber from Premature Decay, 1817; M'Williams, Essay on the Origin and Operation of the Dry Rot, 1818; Burnell in Journal of the Society of Arts, June 1, 1860, vol. viii.