Dry Rot, a kind of decay, often very rapid, to which timber is subject. Its name is misleading, since this decay is associated with the presence of water, such as is found in unseasoned wood. It has proved ruinous to many valuable edifices, and has been the cause of many serious accidents. The ends of joists are often affected by it, so that upon being burdened with even a slight additional load, they are ready to break off by the wall; and the process of destruction has often gone far without a suspicion being entertained of anything wrong. Dry rot is occasioned by Fungi, the mycelium of which penetrates the substance of the timber, destroying its texture, and reducing it to a fragile or even friable mass. Merulius lacrymans and Polyporus destructor are species very commonly productive of this mischief; the first being by far the most common and formidable. Its German name is Hausschwamm. Other fungi, however, produce the same effects; and there are some forms of mycelium not unfrequently occurring as dry rot, of which it is uncertain to what fungus they ought to be referred, since they have not been observed to develop themselves in any perfect form. The different modifications of appearance which the mycelium of the same fungus may exhibit in different circumstances are also imperfectly known. Very destructive ravages have been ascribed, without much probability, to different species of Sporotrichum, particularly in the naval yards of Britain; but the genus is altogether a doubtful one, and not improbably consists of mere forms of undeveloped mycelium. Several species of fungi are often present together in timber affected with dry rot. Merulius lacrymans first appears in small white points; a filamentous substance radiating from these gradually forms broad patches, sometimes many feet in diameter; from these long creeping shoots often proceed, and a network of filaments penetrates into every crevice, filling the whole mass of the timber with delicate filaments, which destroy the cohesion of its fibres. It often appears in the form of leathery laminae.
Of the causes of dry rot, stagnation of air, as behind a wainscot or under a floor, is certainly one of the chief, and a knowledge of this fact suggests means of prevention which may often be easily and most advantageously employed. Another principal cause is insufficient drying of the timber itself; and much of the prevalence of dry rot is not improbably due to the practice of felling trees in spring when the wood is full of sap. Any circumstance which may tend to render the sap acidulous greatly increases the liability to dry rot. The production of fungi takes place with unusual rapidity when by fermentation or otherwise an acidulous condition of organic substances is produced. A fermentation and chemical change in the albuminous constituents of the wood is not improbably the immediate cause of dry rot, providing a soil suitable for the vegetation of fungi.
For the prevention of dry rot various processes have been employed, the object of which is to fill the pores of the wood with some chemical substance. Active inquiry as to methods of preservation of timber began about the middle of last century, and the matter was rendered urgent by the premature decay of the ships of the royal navy at a time of long-maintained conflict. No satisfactory method was discovered, however, until the development of the railway and telegraph system led to further inquiry with the view of preventing the destruction of sleepers and telegraph poles. The various processes of kyanising (corrosive sublimate or bichloride of mercury), margarysing (sulphate of copper), burnettising (chloride of zinc), have been replaced by the effective method of creasoting, invented by Mr John Bethell in 1838. But without the use of any such means we have abundant evidence that well-seasoned timber, in favourable circumstances, may remain unassailed by fungi for many centuries.
See Britten's treatise on the subject (1875); Boulton on the 'Antiseptic Treatment of Timber' in the Proceedings of the Institution of Civil Engineers (1884); Murray on 'Dry Rot' in Architect (January 1885); Marshall Ward on 'Diseases of Timber,' Nature (1888); and Goeppert, Der Hausschwamm (Breslau, 1835).