Those who wish to have a thorough knowledge of the causes of decay in timber, and the remedies devised for their prevention, should consult the valuable little book by Thomas Allen Britton on "Dry Rot in Timber." The subject is one which appeals to everybody having any dealing with wood for constructive purposes, and is far too commonly neglected. A brief summary of the main facts can only be given here.
The forms of decay which chiefly interest the builder are known as " wet" and "dry" rot, though both are indirectly due to the presence of moisture. In the former, it assists the decomposition of the tissues of the wood, especially those of the alburnum (sap-wood); in the latter, by aiding the growth of certain cryptogams, which obtain their nutriment from the substance of the wood. The reduction' of the natural moisture in the wood itself, which is effected by proper seasoning, and the prevention of the access of external moisture by a coating of some impervious substance, such as paint or tar, tend to prevent wet rot. The same means will also tend to prevent dry rot, but with the bitter there is the peculiarity that an excess of moisture is unfavourable to the growth of the fungus which feeds on the wood; also, that when the circumstances are favourable, such as a moderate degree of moisture, which most woods possess in themselves, and the existence of a damp, warm, stagnant atmosphere, no mere coating of paint will prevent the mycelium of the dry-rot fungus, from penetrating to the interior of the wood. This once effected, its destruction is rapid. There are several species of fungi which attack wood; the most common appear to be the Merulius lachry-mans and the Polyporus hybridus.
The former attacks chiefly fir and pine, and the latter oak.
The following processes have been suggested from time to time for preserving timber: -
This appears to be one of the most successful means yet adopted for preserving wood from dry rot, and even wet rot, or the attacks of the white ant and Teredo navalis. It consists in impregnating the substance of the wood with the oil of tar called creosote, from which the ammonia has been expelled, the effect being to coagulate the albumen, and thereby prevent its decomposition, also to fill the. pores of the wood with a bituminous substance that excludes both air and moisture, and which is noxious to the lower forms of animal and vegetable life. In adopting this process, all moisture should be dried out of the pores of the timber. Fir or pine, while warm from the drying-house, may be immersed at once in an open tank containing hot creosote oil, when it will absorb about 8 or 9 lb. per cub. ft. For hard woods, and soft woods which are required to absorb more than 8 or 9 lb. of creosote per cub. ft., the timber should be placed in an iron cylinder with closed ends, and the creosote, which should be heated to a temperature of about 120° F. (49° C), forced in with a pressure of 170 lb. to the sq. in. The heat must be kept up until the process is complete, to prevent the creosote from crystallizing in the pores of the wood.
By this means the softer woods will easily absorb 10 to 12 lb. of the oil per cub. ft. The most effective method, however, is to exhaust the air from the cylinder after the timber is inserted, then to allow the oil to flow in, and when the cylinder is full to use a force-pump, with a pressure of 150 to 200 lb. per sq. in., until the wood has absorbed the requisite quantity of oil, as indicated by a gauge which should be fitted to the reservoir tank. The oil is usually heated by coils of pipes placed in the reservoir, through which a current of steam is passed. The quantity of creosote oil recommended to be forced into the wood is, for railway sleepers, telegraph poles, and other purposes on land, 8 to 10 lb. per cub. ft.; for piles, jetties, and other marine works, 12 lb. Into oak and other hard words it is difficult to force, even with the greatest pressure, more than 2 or 3 lb. of oil.
This consists in impregnating the timber with a solution of 1 oz. copper sulphate to 100 of water, as follows: - A water-tight cap is placed on one end of the log to be saturated, and the solution is introduced within it by a flexible tube. The pressure required, not being more than 15 to 20 lb. on the sq. in., may be obtained by simply raising the tank to a height of 30 or 40 ft. from the ground. On this pressure being applied, the sap runs in a stream from the opposite end of the log. A piece of prussiate of potash rubbed on the end of the log will show if the solution has penetrated the entire length or not, for on coming in contact with the sulphate of copper it leaves a deep brown mark on the wood. Mar-gary in his process also used sulphate of copper in the proportion of 1 lb. of the salt to 8 gal. water, in which the wood was merely steeped until thoroughly saturated, which was supposed to take 2 days for every inch in the thickness of the wood. Boucherie also used the impure pyrolignite of iron, which was. found not only to preserve the wood from decay, but also to harden it.
A solution of 1 lb. chloride of zinc to 4 gal. water for timber, and 1 lb. to 5 gal. for canvas, cordage, etc, in a wooden tank. These were the proportions originally specified; 1 lb. of the salt to 9 or 10 gal. water, are now more frequently used. Timber requires to be immersed for about 2 days for each inch in thickness, and afterwards taken out and left to dry for about 14 to 90 days. Canvas, ropes, etc, require to be immersed in the solution for about 48 hours, then taken out and dried. The process on wood may be more expeditiously performed by forcing the solution into the pores with a pressure of 150 lb. to the sq. in. The advantage of this process is that it renders the material to which it is applied incombustible.
The timber is immersed in a saturated solution of corrosive sublimate (bichloride of mercury) in a wooden tank, put together so that no metal of any kind can come in contact with the solution. 1 lb. corrosive sublimate to 10 gal. water is used when a maximum strength is required, and 1 lb. to 15 gal. when a minimum, according to the porosity of the timber; with the latter proportion, 1 1/2 lb. will be sufficient for a load of timber of 50 cub. ft. Corrosive sublimate dissolves best in tepid water. The time required to saturate the timber depends on its thickness; 24 hours are usually allowed for each inch in thickness for boards and small timber; large timber requires 2 to 3 weeks.