This section is from the book "The Building Trades Pocketbook", by International Correspondence Schools. Also available from Amazon: Building Trades Pocketbook: a Handy Manual of reference on Building Construction.
The best timber is obtained from mature trees, the fibers of which have become compact and firm, both by the drying up of the sap and by the compressive action of the bark. There is a great difference both in appearance and strength between the heart-wood, or duramen, and the sap-wood, or alburnum; in the former, the fibers are firm and dense, and possess a deep color; in the latter, they are open, porous, and filled with sap, and are usually of a pale color. The heartwood is much stronger, and less liable to decay and to the attacks of insects, than sap-wood.
The medullary rays consist of vertical layers or sheets, radiating from the center, and connecting the pith with the bark, as shown at i and j, Fig. 1. They are not, however, continuous, but are broken by the interweaving of the fibers. Those rays extending from the pith to the bark are called primary rays; those extending through only a portion of the stem are called secondary rays. The medullary rays are prominent in oak, beech, and sycamore, but are not so well defined in birch, chestnut, and maple. It is the presence of these medullary rays, sometimes called silver grain, that gives so much beauty to quartered oak.
Fig. 1 represents a section of an oak or ash tree of 13 years' growth, the coarse texture formed of the large sap vessels being shown at a, the closer texture at 6, the bark at c, the primary medullary rays at i, and the secondary ones at j.
In good lumber for building purposes, the heart-wood should be sound and mature, the sap-wood, or layers next the bark, being entirely removed. The wood should appear uniform in texture, straight in fiber, be free from large or loose knots, flaws, shakes, or any kind of blemish. When the rings are close and narrow, they denote a slowness of growth, and are usually signs of strength. When freshly cut, the wood should smell sweet; a disagreeable smell is a sign of decay. The surface, when sawed, should not appear woolly, but be firm and bright, and it should not be clammy and choke the saw. When planed, the wood should have a silky appearance; the shavings should come off like ribbons and stand twisting around the fingers. When the wood appears dull and chalky, and the shavings are brittle and friable, it is not first-class stock. Good lumber should be uniform in color; when blotchy or discolored it signifies a diseased condition.
There are various defects in timber which may be caused either by the nature of the soil in which it grew or by accidents due to storms, etc.
Heart-shakes are cracks or partings of the fibers, radiating from the center of the tree. They are common in nearly all classes of timber, and are caused by the shrinkage of the inner layers incidental to loss of vitality; the cracks are wider towards the heart.
Star-shakes are cracks radiating from the center, but differ from heart-shakes in that they are wider towards the bark; they are caused by the rapid drying of young timber while it is full of sap.
Cup-shakes are curved splits which separate the layers, and are caused by severe wind storms.
Rind-galls are curved swellings, caused generally by resinous layers forming over a wound where a branch has been broken off.
Foxiness is a yellow or red coloring, signifying the early stages of decay.
Dry-rot is a fungus growth, and can be discovered by a black-and-blue tinge; the end wood is crumbly and crisp. Timber thus affected is of no permanent value, as the rot continues until the fiber becomes powder.
Twisted fibers are caused by the twisting tendency of winds blowing generally in one direction; such timber possesses little strength, owing to the oblique direction of the fibers.
Knots are either stubs of branches or the gnarly growth formed where the branches are lopped off. Knots may be small and sound, in which condition they are not objectionable, or they may be large and loose; if large, the strength of the timber is very much reduced, and, if loose or dark in color, they will ultimately fall out, loose knots being the stubs of dead branches.