What is Wood?
A tree grows in thickness by the activity if a single layer of cells called the cambium.
This produces sapwood or xylem on the inside and bark or phloem on the outside. As the cambium continually divides, the first formed xylem cells become progressively detached from the cambium, undergoing chemical and physical changes to form the distinctive hart wood.
What is done to wood to control it in use?
As wood is an organic and hydroscopic material susceptible to moisture changes it will always endeavour to reach a state of equipoise with the surrounding atmosphere, giving up and taking water like a sponge, until the balance is achieved.
The cells are arranged in circular manner as annular rings about the centre of the tree. As wood dries it shrinks, and as it shrinks it distorts in relationship the these rings.
The larger the section the longer it will take to season, and the more distortion is likely to take place. So wood is cut into convenient sizes before seasoning. How this is done will affect the appearance of the finished board.
The log is cut either:
1: Through and through: this will provide wavy edged boards with a tendency to split at will down the middle , but the widest available.
2: Billet Sawn or square one edge: The first cut down the centre restricts splitting and there is less cahnace of warping.
3: Boxed hart: this produces much more radial cut wood, the most stable.
4: Tangential: this will tend to warp.
5: Radial: this is the most stable.
6: Quarter sawn: this produces most radial sawn boards but is very expensive to do.
Seasoning is the controlled drying of wood. Growing trees contain up to 50% free water. In domestic situations this must be brought down to 12% or less.
Traditionally seasoning was done in the open air. The cut boards were replaced upon each other with 3/4' sticks between each boards. The whole stack was then protected from the sun and rain but open to the wind and left for approximately two years for every 1' thickness of the boards.
This is a long and therefore expensive process. To speed the process timber is now kiln dried. Hot air and the controlled reduction of the humidity within an enclosed kiln speeds this process up to about 6-10 weeks.
In drying all wood will shrink. Some woods will shrink more than others.
Approximate comparative movement of timbers:
Wood name: Moisture content: humidity % Tangential shrinkage Radial shrinkage
Range 90%-60% mm per m. mm per m.
Afromosia 15-11 13.0 6.5
Beech 20-12 31.2 16.9
Chestnut 17.5-12.5 13.0 6.5
Elm 20.5-12 23.9 15.6
African mahogany 20-13.5 13.0 9.1
C American mahogany 19-12.5 13.0 10.4
English oak 20-12 26.0 15.6
Sitka spruce 19-12.5 13.0 19.1
Yellow pine 17-11 15.0 9.1
Characteristics and uses of some home grown timbers (UK): Hardwoods (deciduous).
Alder: Wood soft, durable under water but not when dry. Used for piles, barrels, clogs easily turned for toys ,broomheads, hat-blocks and other small articles. Charcoal used for gunpowder amking in the past.
Apples: Wood brown or reddish, compact and homogeneous but apt tp warp and split when drying. Used by turners, cabinet makers, sculptors and for engraving. Used for making mallets heads and golf clubs. A good fuel wood.
Ashes: Wood pale, greyish-white, very strong, tough and pliant. It cleaves well and does not splinter. It is easily 'bent' when steamed. A valuable timber used for all kinds of tool-handles, sports-equipment, hoops, walking-sticks, furniture, ladder-poles, carts, wheels etc. Ash is not durable when wet but an excellent fuel wood that makes good charcoal.
Aspen: Wood light, porous and easy to split but does not splinter. It stands up to hard wear. Used for cart-bottoms, trucks, also for light packing cases and chip boxes. Used for interior work in furniture and particularly for matches, as it does not burn too quickly. Pulp makes a high quality paper.
Beech: Wood fine-grained, smooth, free from knots, and is easily cleaved. Used for furniture, coach-building, tool and brush handles, wood paving, sabots and for many domestic truned articles such as chair legs. Good for fuel and charcoal. Can be bent but less successfully than ash. Used for plywood.
Birch: Wood hard and strong, but small and not very durable particualrly outdoors. Used mostly in Scandinavia for plywood, furniture, veneers, skis, flooring and also many turned articles. Twigs used for brooms and brushes. The bark for roofing, boxes, plates and for tanning. Good fuel wood.