This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
6. A general knowledge of the properties and kinds of timber, the laws of growth and development, the characteristics of good timber, its natural defects, its conversion into building material, its shrinkage during the process of drying, modes of drying, etc. should be possessed by the student, in order to enable him to intelligently judge of the value of the material which he handles, and to provide for its disposition and arrangement so as to secure the most stable and permanent construction.
7. To suit the purposes of the carpenter and joiner, the tree should not be cut until it has reached the period of mature growth; soon after this growth is reached, the heart begins to decay, hence the necessity of cutting, or felling, as it is called, at the proper time. After felling, the trunk and the larger branches are cut into suitable lengths, in which condition it is classified as timber, but when cut up into planks, etc. it is generally called lumber.
8. There are three divisions of trees, each being classified in accordance with its respective mode of growth: (1) The exogenous, or outward growers, in which the stem increases by the formation of annual layers deposited around the outside of the preceding layer; such as oak, chestnut, pine, hemlock, etc. (2) The endogenous, or inward growers, in which the woody matter is formed on the inside of the stem, of which the palmetto tree is an example. (3) The aeroge-nous, or summit growers, in which the stem is produced by the lower stalks of the leaves growing together, such as tree ferns.
9. The first division, or the exogenous method of development, is the one that furnishes the timber for the woodworker. On examining the section of a young oak tree, such as shown in Fig. 1, which is a microscopic enlargement of a section of a stem of 3 years' growth, we find three well defined kinds of tissue forming it; the pith, the woody layers, and the bark. At the core, or heart, at a, is seen the medulla, or pith, composed of cellular tissue, a net-like fabric of cells, resembling a honeycomb; at b, the medullary sheath surrounds the pith and is composed of spiral vessels and fiber ducts for the conveyance of the sap; this constitutes the inner layer of the first year's growth; at c are the wood cells, or fiber tubes, composing the successive annual layers which are formed in a series of concentric rings; each annual layer is called a zone, or circular belt, as shown in Fig. 1, at 1, 2, and 3.
10. Observe that there is a well defined line of separation between each pair of zones; about one-half of the width of the zone is occupied by bundles of fiber tubes containing large sap vessels h, h, whose walls are pitted and dotted as shown, while the remainder of the width is filled with fiber tubes of much closer texture. In ash, chestnut, and oak, the vessels h, h are easily observed.
The line of separation is caused by the suspension of the growth of the stem during the winter. When the tree is young, the tissue is open and spongy, and filled with various fluids, but in process of time, it becomes thickened and firm and the ducts close, the thickening of the tissue commencing with the first formed layer. For this reason the best timber is secured from mature trees, the fibers have become compact and firm, and when once well seasoned, are less sensitive to changes of temperature. Both young and second-growth timber are unfit for all purposes where strength, durability, and "staying " qualities are concerned, the last named quality being a most essential one to the carpenter and the joiner.
11. Much difference as to strength and appearance exists between the mature, compact, inner layers of the heart wood, called duramen, and the outer layers of the sap wood, called albumen. In the former, the fibers are firm and dense and possess a deep rich color, while in the latter the fibers are open, porous, and filled with sap and usually of a pale color. The sap wood possesses little strength, and the sap it contains is largely composed of a sugary substance which invites the attack of insects and hastens decay.
12. The age of a tree may be ascertained by counting the zones, when these are visible, but various causes occur which interrupt the growth, such as severe frosts, etc., causing a line of separation, and several lines may thus be formed in one year, which may lead to an erroneous estimate.
That part of the tree which is more fully exposed to the sun will be found to have wider layers than the other part; hence, the pith, or heart, is seldom in the center of the stem.
Soft woods usually possess wider zones than the hard woods, and it will be observed that much difference exists between the width of the zones in the same tree.
As in the animal kingdom, there are three periods of existence, infancy and youth, vigorous life, and declining vitality and decay; so a similar series exists in the vegetable kingdom.
When the tree is most vigorous, it produces the largest zones. In the case of the oak, this occurs between the twentieth and the thirtieth year, after which its productive power is gradually lessened, and as it grows older, the zones become smaller.
13. Between the inside of the bark and the woody layers is located the cambium layer, as shown at d d, Fig. 1, which consists of a cellular tissue like the pith, but contains, in addition to the cell sap, the rich life-giving secretion called protoplasm, without which the tree cannot live. This layer possesses the property of building up the woody formation by the product of ever increasing cells, and only by the vital energy of this layer, also called the thickening zone, can the tree increase in diameter.
14. The bark consists of three distinct layers. The inner or bast layer e e is composed of woody fibers combined with a cellular tissue, which retains a flexible, rubber-like elasticity and allows it to expand as the woody layers are produced by the cambium. The central layer f f is comprised of prismatic cells and tubes filled with juices, while the outer layer g g is composed of a corky substance of cellular structure. During the growth of the tree there is a continual distending and separating of the fiber and cellular tissue composing the bark, which is renewed and strengthened by means of the cambium layer.