Page:Cassells' Carpentry and Joinery.djvu/50

34 of many woods are more or less changed, especially in the case of beech, the colour of which is changed from a dull white to the familiar reddish tint. After it is taken out the wood is piled under cover in the ordinary manner and allowed to dry; this, in small thin material, usually takes three weeks or a month. The drying time might be considerably shortened by utilising the space above the boiler as a drying loft. A temperature of 120° F. to 180° F. (obtainable above most boilers) would get the drying over in a day or two, but the material should not be transferred to such a position direct from the steam-box; let it have a few days' ordinary drying first. The apparatus illustrated by Fig. 132 is also suitable for steam-bending purposes.

Fig. 132.—Barrel for Seasoning Small Timber.

Shrinkage During Seasoning.—During seasoning a large proportion of the moisture evaporates, causing the fibres to shrink and the timber to become less in bulk and weight. Timber is considered fit for carpenters' work when it has lost one-fifth of its weight, and for joiners' work when it has lost one-third. It also becomes lighter in colour and more easily worked. The shrinkage is scarcely perceptible in the length, but is very considerable in the width, measuring circumferentially on the annual rings (see and, Fig. 126). Radially, or in the direction of the medullary rays, the shrinkage is only slight, as shown by the board, Fig. 126. If the log is whole, the shrinkage causes shakes and wind-cracks; if cut up into planks or quartering, the shrinkage is determined by the position of the annual rings, and, with care, shakes are not caused. The wood curls or bends breadthwise, with the edges turning on the side which is farthest away from the heart. This is illustrated at, , and , Fig. 126. This circumstance must always be considered in fixing timber in position.

Bethell's Process.—There are a number of preservative processes other than seasoning which are of value in increasing the durability of timber. Bethell's process, also known as creosoting, consists in placing pieces of seasoned timber in closed wrought-iron cylinders, from which, and also from the pores of the wood, the air is extracted. Oil-of-tar, known as creosote, is then forced into the cylinders and pores of the wood, at a temperature of about 120°, and under a pressure of 60 lb. to 170 lb. per square inch, according to the porosity of the wood and the purpose for which it is required. The quantity forced into the wood varies from 3 lb. per cubic foot in some hard woods to 12 lb. in soft woods.

Bouchere's Process.—This consists in placing a reservoir, containing 100 parts in weight of water to 1 part of sulphate of copper, in a position about 40 ft. or 50 ft. above the timber, and connecting it by a flexible tube to a cap which is fixed tight to one end of the piece of timber under treatment. The pressure is sufficient for the fluid to force out the sap at the other end and take its place in the pores of the timber.

Burnett's System.—By this system a fluid is prepared in the proportion of 1 lb. of chloride of zinc to 4 gal. of water. The timber is sometimes laid in a bath of this fluid until it has absorbed sufficient; or the solution is forced under pressure into the timber. The value of the above processes lies in the preservation of the timber from dry and wet rot, and, in the case of the latter two systems, from most insects, so