Wood is so commonplace that we take it for granted. Wood has lasting qualities like no other material – it is both warm and pleasing to the touch and its wealth of colour and texture is a delight to the eye. The diversity of colour and texture and the physical properties of stiffness and strength in proportion to weight give each species of timber a character of its own, offering both a challenge and a source of inspiration to the creative mind. The antiquity of woodworking techniques is reflected in the design of products over the centuries, yet despite its link with the past wood is also very much a modern material. It is versatile and has a range of applications.
How trees grow
The great value of trees is that they are a renewable resource. Wood as a material is adaptable and has an infinite variety of types and uses.
The structure of a typical tree has a main stem carrying a crown of leaf-bearing branches. The root system both anchors the tree in the ground and absorbs water and minerals that sustain the tree. The cell system carries the sap from the root system to the leaves.
Carbon dioxide is taken in through the pores in the leaves which is then dispersed to the growing parts of the tree as well as being stored by some of the cells. Sap is drawn through the tree structure by a process of evaporation from the leaves.
Cells in a tree generally run longitudinally through the tree through it’s main trunk and along its branches. Fine textured or coarse textured wood is determined by the type of cell structure. Cells deposited in the outer layer known as the Cambium layer are what makes the tree grow. As new bark is formed the old bark splits and the girth of the tree increases. Sapwood which either conducts or stores nutrients is created as the new wood cells develop. As the sapwood is built up it is no longer used to conduct water. With a gradual chemical change it is converted into the spine of the tree which is called heartwood. As the tree continues to grow more and more heartwood is added whilst the quantity of sapwood remains relatively the same.
Modern forestry methods can cut down, top and debark trees, such as spruce which is a softwood, in a matter of minutes. Conifers are relatively fast growing which makes it possible to control supply with demand. Hardwoods however are not and are slow growing. Currently the demand for hardwood outstrips the rate at which they can be replenished.
Most wood comes from the trunk of the tree rather than from its branches. Trees are felled and the wood transported as logs to the sawmills. Paper products and manufactured boards are usually used from the trimmings. Band or circular saws then cut the logs into sawn timber. The types of cut are known as plain sawn, rift sawn or quarter sawn defined by the position of the trees growth rings within the board. The plane of the saw in relation to the annual growth rings determines the stability of the wood.
Freshly cut timber contains a very high percentage of moisture and is known as ‘greenwood’. Water is captured within the cell walls and the process of removing it is by drying or ‘seasoning’. There is a two stage process; water loss from cell cavities and water loss from cell walls. Wood shrinkage occurs when water is lost from the cell walls. The ‘equilibrium moisture means that moisture loss will stop when the balance of moisture contained within the wood is in balance with the relative humidity of its surroundings. If the surrounding humidity is lower then the wood will lose water and shrink, if it is higher then the wood will absorb moisture and swell.
The traditional method of drying wood is air-drying. Boards are either stacked with evenly spaced battens between them or nail down firmly in situ. To dry out softwood boards 25mm thick can take 6 months depending on the relative humidity. Ideally the stack or location of the timber drying out should have the same humidity as the environment in which the wood is to be used. Kiln dried wood is usually used to reduce the moisture content below air-dry level. Once in an exposed outdoor environment it will take up moisture again.
The stability of the wood varies from one wood species to another. As wood dries it shrinks. Boards can change, become distorted and move as shrinkage takes place. The orientation of the growth rings dictates the amount a board will distort. A board bends or cups when the inner rings shrink more than the outer rings.
The most common softwoods available are spruce, pine and fir. These are readily available in a sawn finish and can be planed to a required dimension and texture. During planning usually 3mm or more will be lost from each board.
Wood is graded according to the allowable number of knots, evenness of grain and other criteria. Structurally used timbers are stress graded. Knot free timber is often referred to as ‘clear timber’ and is available if requested. Wood can be affected by one or more of the following:
Honeycomb checks, shakes, end splits and surface checking are splits that occur between annual-growth rings and are due to the growth pattern of the tree and shrinkage through drying out.
Bowing, warping, and twisting occur when a piece of timber is not stacked or held down securely during the drying out period.
Dead or encased knots are causes when new growth rings grow over the remains of dead branches which have left stumps. The knot has a tendency to fall out and if it remains makes the surface of the wood irregular and hard to work.
Ingrown bark is where the bark can be seen inside the board effecting the appearance and strength of the wood.
Possible future paragraphs on this page to include:
The properties of wood
Fixings and Fittings
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