THE REAL WOOD GUIDE TIMBER PRODUCTS LOGS, SAWN WOOD VENEERS AND PANELS Commercial timbers are divided into two basic groups, softwoods and hardwoods. The terms are slightly misleading because they do not always accurately describe the physical nature of wood. However they are the generally used descriptions for the coniferous, needle bearing gym nosperms (softwoods) and the broadleaved angiosperms (hardwoods). Commercial softwoods mainly grow in cooler temperatures or at higher altitudes, although they also occur widely in sub tropical and Mediterranean climates. Most of the general purpose utility and building timbers used in Britain are softwoods - Spruce, Pine, Douglas Fir, Hemlock and Larch. Hardwoods grow in a wide range of climatic zones. There is a greater variety of hardwood species ranging from the extremely light and soft -like Balsawood - to the world's strongest and most durable woods - like Ekki (Africa), Greenheart (South America) and Balau (Asia Pacific). In the moist tropics where temperature and seasonality are less varied, the growth of trees is less marked by those seasonal variations in density which are more obvious in the growth rings of trees in temperate climates. European Oak and Ash show annual growth rings whilst Mahogany and other species from the tropics grow more evenly throughout the year. Individual species show physical characteristics in such a range of properties as hardness, strength, elasticity, durability, abrasion resistance, colour, grain, texture, movement in various levels of atmospheric humidity and so on. LOGS A log is a section of the trunk of the tree. It is made up of an outer layer of bark below which is the outer living part of the trunk which is known as sapwood. This surrounds the inner mass of the trunk called the heartwood. Because the sapwood contains food material, it is less durable than heartwood. The log is the starting point for all wood processing. The UK imports about 47,000 cubic metres of temperate logs and 55,000 cubic metres of tropical logs. SAWN TIMBER When a log has been cut into square edged form it is known as lumber or sawn wood. There are specialised terms for certain general size groups -for example big timbers for sea defence and heavy construction of say 15Omm x l5Omm up to 45Omm x 3OOmm are sometimes called dimension stock. Timber which has been sawn presents a comparatively rough surface. A smooth surface is obtained by planing to provide machined timber. Wood cut from a log has a very high moisture content and must be allowed to dry or season. This initial drying process is normally done by stacking to permit free air circulation. As the wood dries it shrinks. "Green" lumber cut from the log is cut oversize to allow for this. Air drying in the UK only permits wood to dry to about 10% moisture content which is too high for timber used in heated buildings. Additional drying to much lower levels is done in kilns. The UK imports about 8 million cubic metres of softwood lumber, 300,000 cubic metres of temperate sawn hardwood and 650,000 cubic metres of tropical sawn hardwood annually. VENEERS Logs of acceptable shape, species and quality can be converted into very thin veneers. There are two forms of production. Rotary veneer comes from spinning a log against a knife on heavy duty lathes - rather like unrolling a carpet. Rotary veneers are usually between 1mm to 3mm in thickness and are used for panels like plywood and blockboard. Veneers can also be prepared by slicing thin sheets, commonly about 0.6mm thick, from flitches. Flitches are flat sided slabs cut from the log to present faces which offer particular grain patterns. Sliced veneers are usually intended for decorative surfacing of furniture and panelling, and extend the use of selected well figured logs. There are a variety of ways of slicing to expose different grain patterns. The bulk of veneer production is for plywood and panel manufacture. UK imports of all types of veneer are about 50,000 cubic metres per annum with the bulk coming from Europe. PLYWOOD Plywood is usually constructed of rotary veneers glued together so that the direction of the grain in one veneer is at right angles to the adjoining veneer. There are usually an odd number of veneer layers in plywood construction. This layered construction makes for high strength, virtually eliminates risks of splitting, and removes the weakening effects of defects, like knots, which can affect the strength of solid timber. Plywood has a high strength to weight ratio. Depending on the type and quality of adhesives, plywood can be made to meet exposed outdoor use - exterior grade. Or it can be bonded for less exacting indoor uses. The most common size of plywood as with other panels used in the UK is 2440mm (8') x 1220mm (4'). Plywood thicknesses range from 4mm up to 25mm. Annual imports of plywood by the UK are about 1.1 million cubic metres, of which a little under half is estimated to be of tropical wood. BLOCKBOARD Blockboard is a panel consisting of a core of sticks of solid timber with either one or two layers of veneers on both face and back. The most common thicknesses are 18mm and 15mm. Other similar panel constructions are imported on a much smaller scale and include laminboard where the core is made of veneers bonded together at right angles to the face and back veneers, and battenboard where the core sticks are much wider than in blockboard. UK imports total about 120,000 cubic metres per annum, of which some 60% is from tropical countries. PARTICLEBOARD Also known as chipboard, this panel product consists of small flakes of timber bonded together with adhesive. It offers high recovery of wood in its construction and is widely used as a base for veneering and other overlays in wall panels and furniture. The particleboard used in the UK is made of temperate softwoods. Production of particleboard within the UK has increased substantially to about 55% of total needs of 2.8 million cubic metres per annum. FIBREBOARD Among the most common forms of fibreboard are hard board and MDF (medium density fibreboard). Both forms are made from compressed wood fibres and are usually made from residues and waste wood. Hard board is of comparatively low strength but is a valued material for area coverage at low cost. It is usually produced in thin sheets up to 6mm/8mm. In recent years there has been a significant increase in world production of MDF which is a much stronger panel of higher density. It comes in a range of thicknesses and has the capability for lighter structural use in small cross section, and can be machined and shaped like solid wood. It is forecast that MDF will be used in increasing volume. The UK uses about 165,000 cubic metres of hard board per annum which is imported, and 240,000 cubic metres of MDF of which about 11 ,000 cubic metres is imported, but where UK production is expected to rise. WAFERBOARD AND OSB Waferboard and Oriented Strand Boards can be compared with particleboard but constructed of larger wood flakes. They are designed for more arduous usage and are suitable for exterior use. Volumes imported by the UK are at present fairly modest. Wood is versatile. It is available in a variety of different species which are all individual with their own range of appearance, colour, strength and end use suitability. Versatility also comes in the variety of forms in which wood can be purchased. As rough sawn timber which the user can shape to his own needs. As veneers to give fine overlays. As panels to cover wide areas and to do a variety of jobs indoors and out. Next Section: 'Further Processed Products'