Interesting - a few facts about wood.

Growth of Wood

A cross section of an exogenous ("outward growing") tree shows it to be made up of several concentric rings, called "annual rings," from their generally being deposited at the rate of 1 a year. At or near the centre is a column of pith, from which radiate thin lines called "medullary rays," which, in some woods, when suitably cut, afford a handsome figure called "silver grain."As the tree increases in age, the inner layers are filled up and hardened, becoming what is called duramen or "heartwood" ; the remainder, called alburnum or "sapwood," is softer and lighter in colour, and can generally be easily distinguished. The heartwood is stronger and more lasting than the sapwood, and should alone be used in good work. The annual rings are generally thicker on the side of the tree that has had most sun and air, and the heart is therefore seldom in the centre.

Felling

While the tree is growing the heartwood is the strongest; but after the growth has stopped, the heart is the first to decay. It is important therefore that the tree should be felled at the right age. This varies with different trees, and even in the same tree under different conditions. The induration "hardening," of the sapwood should have reached its extreme limits before the tree is felled, but the period required for this depends on soil and climate. Trees cut too soon are full of sapwood, and the heartwood is not fully hardened. The ages at which the undermentioned trees should be felled are as follows: - Oak, 60 - 200 years; 100 years is best: Ash, Larch, Elm, 50 - 100 years; Spruce, Scotch Fir, 70 - 100 years. Oak bark is sometimes stripped in the spring, when loosened by the rising sap. This practice improves the timber. The best season for felling is either midsummer or midwinter when the sap is at rest.

Squaring

Directly the tree is felled it should be squared, or cut into "scantling" ( size to which it is to be cut), in order that air may have free access to the interior.

Features

These depend greatly on the treatment of the tree, the time of felling it, and the nature of the soil in which it has grown. Good timber should be from the heart of a sound tree, the sapwood being entirely removed, the wood uniform in substance, straight in fibre, free from large or dead knots, flaws, shakes or blemishes of any kind. If freshly cut it should smell sweet; the surface should not be woolly, nor clog the teeth of the saw, but be firm and bright with a silky lustre when planned; a disagreeable smell indicates decay, and dull chalky appearance is a sign of bad timber. The annual rings should be regular in form; sudden swells are caused by rind-galls (curved swellings caused when branches are lopped off); closeness and narrowness of rings indicate slowness of growth, and are generally signs of strength. When the rings are porous and open, the wood is weak and often decayed. The colour should be uniform throughout; when blotchy, or varying much from the heart outwards, or becoming pale suddenly towards the limit of the sapwood, the wood is probably diseased. Good timber is sonorous when struck; a dull heavy sound indicates decay within. Among samples of the same timber the heavier are generally the stronger. The worst position for knots is near the centre of the balk (squared timber) required, more especially if forming a ring around the balk at one or more points. Though the sapwood should be entirely removed, the heart of trees having most sapwood is generally strongest and best. The strongest part of the tree is usually that containing the last formed rings of heartwood, so the strongest scantlings are got by removing no more rings than those which include the sapwood. Timber that is thoroughly dry weighs less than green; it is also harder and more difficult to work. 

Defects

The principal natural defects in timber, caused by vicissitudes of climate, soil etc.... are:- "Heartshakes" : splits or clefts in the centre of the tree; these are common in nearly every kind of timber; in some cases hardly visible, in others extending almost acrossthe tree, dividing it into segments; one cleft right across the tree does not necessarily mean too much waste as it divides the squared trunk into 2 substantial balks; if however 2 clefts cross at right angles or twist along the length of the trunk it is virtually impossible to obtain scantlings or planks. "Starshakes": in which several splits radiate from the centre of the timber. "Cupshakes" : curved splits separating the whole or part of one annual ring from another; when they occupy only a small portion of a ring they do no great harm."Rind Galls" : perculiar curved swellings caused generally by the growth of layers over the wound remaining after a branch has been been imperfectly lopped off. "Upsets" : portions of the timber in which the fibres have been injured through crushing. "Foxiness" : a yellow or red tinge caused by incipient decay. "Doatiness" : a speckled stain found in Beech, American Oak and others. "Twisted Fibres" : are caused by the action of a prevalent wind turning the tree constantly in one direction, timber injured in this way is not fit for squaring as many of the fibres would be cut through. In selectingbalks and deals it should be remembered that most defects show better when the timber is wet. Balk timber is generally specified to be free from sap, shakes, large or dead knots and other defects and to be die-square. The best American Yellow Pine and the crown timber from the Baltic have no visible imperfections of any kind.

Classification

Timber is generally divided into 2 classes called "pine" woods and "hard" woods. The chief characteristics of the two classes are: Pine wood (coniferous timber) in most cases contains turpentine; it is distinguished by straightness of fibre which favourable quality makes it especially useful  when pieces are required to bear either a direct pull or a transverse load, or for purposes of planking; the lateral adhesion of the fibres is small, so it is much more easily split along the grain than hardwood which makes it less well fitted to resist thrust or shearing stress. In hard wood ( non-coniferous timber) there is no turpentine; the degree of distinctness which can be seen in hard wood depends on the difference of texture of several parts of the wood such difference tending tio produce unequal shrinking in drying; consequently those kinds of timber in which the medullary rays and the annual rays are distinctly marked are more liable to warp than those in which the texture is more uniform. Within the class soft wood are: Firs, Pines, Spruce, Larch and all cone bearing trees. In the hard wood class are: Oak, Beech, Ash, Elm, Mahagonay etc..... Carpenters generally give the name "Fir" to all red and yellow timber from the Baltic, "Pine" to similar timber from America and "Spruce" to all white wood from either place.

Market Forms

The chief forms into which timber is converted for the market are as follows:- A "log" is the trunk of a tree whith the branches lopped off; a "balk" is obtained by roughly squaring the log. "Planks" are parallel-sided pieces 2-6inches thick, 11 inches broad, and 8 - 21 feet long; "deals" are similar pieces 9 inches broad and not exceeding 4 inches thick; "whole-deals" is sometimes the name given to deals 2 inches or more thick; "cut-deals" are less than 2 inches thick; "battens" are similar to deals but only 7 inches broad.

Seasoning 

The objective of seasoning timber is to expel or dry up the sap remaining in it, which otherwise putrifies and causes decay. One effect is to reduce the weight - timber is considered 'seasoned' when it has lost 1/5th of it's weight and is then fit for carpenters work and 'dry' when it has lost 1/3rd of it's weight when it is fit for joiners work. Natural seasoning is carried out by stacking the timber in such a way that the air can circulate freely around each piece whilst at the same time protecting it by a roof from the sun, rain,draughts and high winds. The timber must be kept clear of the ground during the process by the use of bearers. The time for natural seasoning differs with the size of the pieces, the nature of the timber and it's condition before seasoning. Following are some examples of the time needed for seasoning: pieces 24 inches and upwards square require, Oak, 26 months, Fir, 13 months - pieces 20 to 24 inches square require, Oak, 22 months, Fir, 11 months - 16 to 20 inches square require, Oak,18 months, Fir, 9 months - pieces 12 to 16 inches square require, Oak, 14 months, Fir, 7 months - pieces 8 to 12 inches Square require, Oak, 10 months, Fir, 5 months and pieces 4 to 8 inches square require, Oak, 6 months, Fir, 3 months. There are several processes for artificially drying timber which speed up the process significantly. 

Decay 

To prevent wood from decay it should be kept dry and well ventilated; clear of the influence of damp earth or damp walls, and free from contact with mortar, which hastens decomposition. 'Rot' in timber is decomposition generally caused by damp, which proceeds by the emission of gases, mostly carbonic acid and hydrogen. There are two types of rot 'dry-rot' and 'wet-rot'. The chief difference seems to be that wet-rot occurs where the gases evolved can escape and through this the sappy portions are decomposed. Dry-rot on the other hand occurs in confined places where the gases cannot get away but enter into new combinations which enable fungi to form which then feed on and destroy the timber. Dry-rot is almost always caused by lack of ventilation; confined air, without much moisture encourages the growth of fungus which eats into the timber, renders it brittle and reduces the fibres to powder. The principal parts of buildings where it is found are - warm(ish) cellars and inadequately (under) ventilated wooden floors. The ends of timber built into walls are almost certain to be affected by dry-rot unless they are protected by metal shoes, lead or zinc. The same result occurs by fixing woodwork to walls before they are dry. Painting or tarring cut or unseasoned timber has the same effect.There is a danger about dry-rot in that the germs of the fungi producing it are spread easily and in all directions in a building once dry-rot establishes itself without necessity for any actual contact between affected and sound wood.

Suitability

It is useful to have a general idea of the merit of woods relative to the purpose for which they are destined. Following is a listing of properties and the woods most associated with them:

Elasticity and Toughness - oak,beech,elm, lignum-vitae, walnut, hornbeam.

Even Grain (for Carving or Engraving) - pear, pine, box, lime tree.

Durability (dry) - cedar, oak, yellow pine, chestnut.

Building (boats & ships) - cedar, deal, fir, larch, elm, oak, locust, teak.

Wet construction (piles, foundations etc...) - elm, alder, beech, oak, chestnut, ash, spruce, sycamore.

Frames - ash, beech, birch, pine, elm, oak.

Furniture (Common) - beech, birch, cedar, cherry, pine, whitewood.

Furniture (Fine) - amboyna, black ebony, mahogany, cherry, maple, walnut, oak, rosewood, satinwood, sandalwood, chestnut, tulip-wood, ebony.

Extracted from Spons Mechanics Own Book - a manual for craftsmen and amateurs,  sixth edition by E & F N Spon Ltd, 125 Strand, London. Published 1901