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The Characteristics of Wood

Wood is naturally engineered with agood strength to weigh ratio. It has long hollow cells along the axis of the stem, which are bonded by lignin. The cell walls have a spirally woven mesh of cellulose micro fibrils in all a highly suitable structural material. (See also Biology module)

Hardwoods i.e. the broadleaved species are more complex than softwoods -the conifers. The latter have vertical fibre like cells which when cut show a fairly simple grain. Hard woods also have cells, some featuring as rays, which give greater decorative character to the wood. Also there is a different character between wood laid down at the beginning end of year. Early or Springwood comprises open cells suitable for conducting nutrients. Latewood or Summerwood comprises dense cells laid down later in the growing season giving strength Not to be confused with the above you get Heartwood near the pith at the center of the trunk and Sapwood in the outer layers from heartwood to bark. Heartwood cells cells are no longer functional as conducting tubes and become decay resistant and strong. Sapwood on the other hand has living cells carrying food reserves, is wetter than heartwood and more subject to decay.

This annual growth with a dormant season also gives rise to different ring patterns in wood coming from trees in our latitudes, which is not evident in tropical timber, which grows all year round.

Wood is dependant on water and therefore it is wet when freshly felled. As wood dries the fibrils move closer and there is shrinkage resulting in additional strength and stiffness.

Other features of wood are its density, ring width and frequency of knots. Wood density is the ratio of woody material to other material that makes up the volume of the timber. Density is also related to strength. For example heartwood is denser than sapwood and with a few exceptions broadleaves are denser than conifers. Ring width relates to the rate of growth of a tree. This can be related to inherent vigour, or the growing space allowed to the tree or to favourable or unfavourable climatic conditions. Knots result from branching, which means the grain around the knot, is distorted and if the branch dies the knot can fall out leaving a hole. All of these features will make the timber weak or strong. Less dense trees or trees with wide rings will be weaker and timber can break under stress near knots.

Wood also has properties, which makes it a durable material. It can last many years in a dry climate. For example the Norwegian Stave houses are going on a thousand years old. Also under specified conditions wood can absorb preservatives, which makes it a long lasting material in our climate. Wood also has a built in safety factor in the case of fire. A steel structure under intense heat will suddenly collapse while wood chars from the outside in lasting longer and acting more predictably. Our native oak is extremely durable and remains of wood stakes have been found on our ancient sites such as the Ceide Fields in Co. Mayo.

Finally wood has all sorts of decorative features through its grain, colour and even flaws such as knots and fungal or insect infections which make it attractive and interesting to the eye and so, the choice of material for so many products.