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Wood

- wood is the material forming the trunks and branches of trees. Timber (lumber in America) is cut from the trunk. It is the most amazing building material, capable, properly stressed, of spanning over seventy feet. Its true value has perhaps tended to be underestimated because it is seldom obvious in polite architecture.

Timber is traditionally classified as hardwood, taken from broad leaved deciduous trees, or softwood, from conifers. Once wood began to be imported from outwith Europe, the description proved innacurate. Good douglas fir can be as hard as mahogany, while balsa is a hardwood. The circular rings evident in the cross section of a trunk, are caused by the contrast of the more open texture of earlywood grown in spring/summer, and latewood grown later in the season. Earlywood is more porous, latewood tougher and more fibrous. Timber from slow grown trees contains less latewood than timber from fast grown trees. Rapid growth means wider growth rings, and loss of strength, at least 8 rings per 25mm is preferred. A feature of the cut end of most trees is a ring of pale wood - sapwood - beneath the bark. The centre is a darker colour and is known as heartwood. Water and minerals travel up the tree through the sapwood which contains living cells. Heartwood has no living cells. Because sapwood is more permeable, it is more succeptible to decay. Shrinkage does occur in timber, but nearly always in section, very rarely along a length. Most good quality woods are better (and easier) worked green, but where precision is required in for example, door construction, the wood should be well seasoned.

Most of the problems of swelling, shrinkage and decay associated with timber stem from moisture content, which can vary between 12-25% over a season depending on where and how it is used. Wood is mainly composed of cellulose (65%) which gives strength, and lignin (35%) which gives hardness. The common analogy is that of a frozen sponge where cellulose represente the fibres, lignin the ice. Timber is sold by the cubic metre, the price varying depending on quality and species. A metre cube however is a lot of wood, and when considering the amount of timber in say, a sash and case window, price should not be an argument for accepting poor quality. Timber can be pretreated against rot and insect attack. The most common treatment is CCA, copper chrome/arsnic which sounds alarming, but is generally held to be safe, because the chemicals used do not leech out, various brand names exist for this process, cellcured, tanalised etc.

Glossary
Cupping - the curvature that occurs in the cross section of a piece of timber ie in the end of a board.
Extractives - small amount of substances aditional to the major components of wood which give timbers their own colour and odour. Resin is the best known.
Figure - the pattern revealed on the surface of wood after it has been sawn.
Free stuff - timber with no knots or other imperfections.
Grain - the direction of the fibres which make up the wood in relation to the trunk.
In straight grained wood the elements lie parallel to each other and to the trunk, as a result of which the wood is easliy split. It is usual to work timber with the grain, ie plane or adze the timber so that the grain slopes away from the direction in which the tool is pushed. Working against the grain can cause the tool to jam and damage the surface of the timber.
Knot - a knot is formed when a branch becomes embedded in the trunk. While they can increase the resistance of timber to splitting, they can seriously effect the bending strength of joists, floorboards etc.
Ray - a fissue radiating from the centre of the heartwood outwards, In which protein is stored.
Seasoning - seasoning is the process of drying timber which contains a great deal of water when growing. It is dried mainly, to minimise shrinkage and to make it more resistant to fungal attack, but it also makes it lighter and easier to handle, better for painting etc. Seasoning is quite an exact science undertaken in two basic ways, "air seasoning" in which the timber is referred to as being "in stick" and, "kiln drying." While most conservationists prefer timber to be seasoned naturally, one advantage of kiln drying is that it does kill off any boring insects that may be lurking within the material. (Rather confusingly, sticking is nothing to do with seasoning, it is a joinery term for cutting a moulding)
Sawing - logs can be sawn into boards or planks in two basic ways, either by cutting straight through from one side to the other, known as plain or flat sawn, or to cut in a radial direction where the rings meet the board at an angle of more than 45 degrees. Plain sawn timber is more prone to cupping and is less durable when used as flooring. The circular saw was invented in 1777.Pit saws where the log was positioned over a pit with a man above (lifting) and one below (guiding) was the traditional method of sawing timber along its length. The man below got filthy, and while the man above worked harder he was "top dog" - the origin of the expression.
While on the subject of sayings, timber was first sawn on trestles, the log was balanced at its centre, one end was raised and sawn, and then the process was repeated to the other end, hence - "see saw".
Shakes - splits along a ray in a piece of timber. They do not usually affect the strength of the material but can cause distortion, particularly if the timber suddenly becomes wet.
Cup shakes are splits separating the annual rings.
Waney edge/waney timber - boards or pieces of timber which, instead of being cut square, show the original curve of the log from which they are cut.