The object in preservation of timber is to save it from rot, whether wet or dry. There are many different ways and patented methods of trying to prevent these forms of decay; but, before proceeding to explain them, it will be as well to say a few words on wet and dry rot.

Wet rot is really the decomposition of the wood, especially the sappy portions; and it takes place while the tree is standing, and also when the gases can escape; whereas dry rot occurs in the converted timber, and is generally brought about by fermentation, the result of confinement and lack of circulation of the air, which causes a fungus, similar to a cobweb, to encircle the wood and suck its very nature out, until it is reduced to a powder.

Wood is most subject to dry rot when in damp and warm situations, with no circulation of air. Ground-floor joists, with no ventilation, or with misplaced ventilation, are soon attacked by it. The ventilation should be placed where the air can circulate underneath the whole floor. The corners are generally ignored, and the result is soon apparent. The same result arises from timbers built into damp walls without a passage of air around them. Oilcloths also favour this form of decay, which is very difficult to get rid of. When once in a building it soon spreads all over. Dry rot, of course, renders timber dead as far as sound goes. A log may be good outside, while the inside is a mass of dry rot. This can be detected by the fact that it will not conduct a sound from end to end; or, in boring it, after the outside skin is penetrated, there is nothing but powder to resist the pressure.

The simplest method of preservation is, of course, to paint or tar the work, care being taken that the wood itself has previously been thoroughly seasoned, or the paint will keep the moisture in, and be itself the cause of decay. The same thing applies to charring the ends of posts, joists, etc., and creosoting. This latter is a process by which a mixture of oil and tar is forced (under pressure in tanks) through the pores of the wood. This is the most common, and perhaps the best, way of preserving timber which is required to withstand damp, especially sills and posts resting on or in the ground; but, at the same time, it is an objectionable method, because it makes the timber of a dark brown colour, almost black, and very greasy; and, moreover, the whole of the piece must be treated in the same way, so that nothing can afterwards be done to make it look respectable. It will take neither paint nor whitewash. In this respect it is that charring is far superior, though not so generally used, as it is only necessary to char that portion of the timber which is required to withstand more than ordinary moisture.

Among the patent processes the most common are: -

Burnet's, by which the timber is soaked in a watery solution of chloride of zinc, in the proportion of 4 gallons of water to 1 lb. of chloride of zinc. The advantage of this process is that it is colourless, and protects the heart; whereas the creosote is more greedily taken by the sappy part. This mode is very much used for wood-blocks for paving.

Boucheries method consists of forcing sulphate of copper through the pores, but not in a tank, as in creosoting, the liquid being in this case poured into a hole at the top of the log, and the weight of the liquid coming from a tank above forces the other through. The disadvantage of this method is that the wood has to be cut into to allow the liquid to get a start.

Peters' (of Derby) Carbolenium is a mixture similar to creosote, which is put on the ends of joists. It can be applied with a brush before the joist is placed in the building; and it is said to be a great success. It is also used on all wall-plates and other members generally, whenever built within or into the walls.

Kyanising consists of injecting corrosive sublimate, dissolved and diluted with water, into the wood. It is said to be efficacious in preventing rot, and is especially used for seasoning railway sleepers, having an advantage over creosoting, in that it does not make the wood so inflammable.