The timber used in engineering and building works is obtained from a class of trees which grows by the deposit of successive layers of wood outside under the bark, while at the same time the bark becomes thicker by the deposit of layers on its under side.
The cross section of such trees (see Fig. 407) consists of several concentric rings or layers, each ring consisting in general of two parts - the outer part being usually darker in colour, denser and more solid than the inner part. The difference between the parts varies in different kinds of trees.
These layers are called annual rings, because one of them is, as a rule, deposited ever year. Sometimes, however, a recurrence of exceptionally warm or moist weather will produce a second ring in the same year.
In the centre of the tree is a column of pith p from which planes, seen in section as thin lines m m, radiate toward the bark b, and in some cases similar lines m m converge from the bark toward the centre but do not reach the pith.
These radiating lines are known as "medullary rays" or "transverse septa." In many woods they are not discernible by the eye, but when they are of large size and strongly marked, as they are in some kinds of oak, they present, if cut obliquely, a beautiful figured appearance, known as "silver grain " or " felt."
As the tree increases in age the inner layers are filled up and hardened, becoming what is called "heartwood," the remainder being called "sapwood." The latter is softer and lighter than heartwood and can generally be easily distinguished from it.
Good timber should be from the heart of a sound tree - the sap entirely removed. The wood, uniform in colour and substance, straight in fibre, free from large or dead knots, flaws, shakes, or blemishes of any kind. The annual rings should be regular in form; close and narrow rings indicate strength, porous and open rings are signs of weakness. Good timber is sonorous when struck. A dull heavy sound betokens decay.
For practical purposes timber may be classed as : -
Soft Wood, including fir, pine, spruce, larch, and all cone-bearing trees.
The following are the most common forms in which timber is sold : -
Logs, being trunks of trees with the branches lopped off.
Balks or square timber, being the trunks roughly squared, generally by the axe, sometimes by the saw.
Planks, being parallel-sided pieces 2 to 6 inches thick, 11 inches wide. and from 8 to 21 feet long.
Deals, Similar pieces 9 inches wide, and not more than 4 inches thick.
Battens, being like deals, but only 7 inches wide.