All timber is categorized as either a hardwood or a softwood. The two terms can be misleading since softwoods, such as parana pine and yew, are much horde! denser and tougher than hardwoods such as balsa — a classic example. An extreme comparison, but it demonstrates that you cannot rely on the simplistic view that all hardwoods are hard and all softwoods soft. Fortunately, most species have already been identified for us.
As a tree grows, a new ring of sapwood builds up around the previous year’s growth. The heartwood increases in area, while the sapwood remains much the same thickness throughout the tree’s life. A growth ring is the layer of wood produced in one growing period. Rapid growth in spring is called `earlywood’. Large cells make up this wider, paler part of the growth ring. Denser cells produced in summer (`latewood’) form a darker ring, adding support to the tree.
Growth rings in hardwoods are either ‘ring porous’ or ‘diffuse porous’ depending on their cell-structure formation. Ring-porous timber has alternating layers of open cells and tightly grouped cells. Open cells are formed as the tree grows during spring and summer, while tighter cells are formed during autumn and winter, as growth slows down. Examples of ring-porous hardwoods are oak and ash. Diffuse-porous timber occurs where there are no clear changes during the growing season, resulting in cells of a more uniform size and formation. Examples of diffuse-porous hardwoods are maple and beech. Ring-porous timbers have a more open grain than that of diffuse-porous hardwoods, which are therefore more consistent when it comes to planing or sanding.
Characteristics of hardwoods
Broad-leaved trees are called hardwoods, with marl), species losing their leaves in winter, especially in temperate climates. The hardwood structure relies on each cell or fibre being very long and needle-shaped. Lying side by side, these tend to make the hardwood timbers more elastic than softwoods. Growth rings are often hard to distinguish, especially in diffuse-porous timbers, such as maple. Durability tends to be greater than most softwoods, and the variety of colours offered by hardwood species is vast. Hardwoods grow in both tropical and temperate climates and may be deciduous or evergreen. Common hardwoods include oak, mahogany, maple, walnut, beech and many others.
Characteristics of softwoods
Coniferous or cone-bearing trees are called softwoods and have needle-pointed leaves. A coniferous tree matures in about a quarter of the time taken by a hardwood tree. The term softwood means that the tree cells are hollow and spindle-shaped. Along the sides of the cells are small holes that act as connecting passages through which food passes on its way to the leaves. Softwoods are generally paler in colour than hardwoods, with clearly visible growth rings. They are generally easier to work than hardwoods, although there are exceptions. Softwoods grow mostly in temperate climates and include spruce, fir, pine, yew, giant redwood and the mighty sequoia.
The characteristics of hardwood and softwood cells
Colour and grain
Hardwoods range enormously in colour from purples, reds and oranges through greens to browns and blacks. The same cannot be said of softwoods, which are predominantly pale yellows and browns. Apart from colour, grain is probably the primary aesthetic reason for choosing a particular species. Some timbers have large pores or open grain (ring porous), such as oak, chestnut, wenge and teak. Others have a fine grain or small pores (diffuse porous), polishing more easily with a deep lustre (maple, sycamore, holly and tulipwood). Species such as sapele and utile have interlocking grain. This gives a striped effect, with planing tearing the grain on alternate stripes. The ultimate beauty in any species is perhaps that produced from a quartersawn board, revealing the medullary rays. English oak is famous for this figuring.
Which wood to choose?
There is a greater range of hardwoods available for furniture-making than there are softwoods. Although the latter tend to be much cheaper to buy, they are less dense and durable than hardwoods, and so tend to be used less for woodworking.
When choosing timber, consider the environment to which it will be exposed. For outdoor use, for example, iroko, cedar and European oak are all excellent, although they should still have a protective finish. Not only are teak and greenheart ideal for outdoors, but they can withstand water immersion without detriment to their strength. It is advisable to use teak or cheaper iroko when there is a risk of iron corrosion: ferrous metals can cause nasty black stains in timbers such as oak and chestnut, which have a high tannic acid content. Metal fittings should be plated.
For decorative indoor furniture, walnut, mahogany, yew, oak or maple will look impressive. Sports equipment is often made of ash and willow, whereas balsa is best for making models. Musical instruments frequently use exotic timbers such as rosewood or ebony. Some woods, such as cedar of Lebanon, have a strong aroma and are highly prized for box making.
Timber suppliers tend to refer to a species not by its Latin name but by its general name, such as ‘oak’. This one species, however, covers a range of oaks including American red and white, Japanese and English — each with its own characteristics. Mahogany is a widely used species with many different sources so identification can be difficult. The species includes Brazilian, Honduran, Cuban, American and several types of African mahogany. Besides these, there is also sapele — not strictly a mahogany but widely used as a substitute — meranti from Malaysia, and also lauan, which is stained to look like mahogany. They are used particularly for general joinery work such as window-and doorframes.
There are endless timber characteristics and properties. If a species is new to you, buy a small amount first and experiment before buying more. Some woodworkers find certain timbers can produce an allergic reaction, so purchasing a large quantity could be a costly mistake. Some timbers are also very hard on cutting tools. For example, teak has calcium pockets and grit within its fibres, which can dull a keen edge even tungsten carbide-tipped saw blades. Dense tropical timbers, such ebony, require exceptionally sharp tools.
Defects in timber may be obvious — knots and cup shakes on the end of a board, for example — and may occur naturally or be man-made. Some are more difficult to spot than others. Some natural defects are sought-after, as they increase the beauty of the wood.
Knots show the emergence of branches in a growing tree. They occur mostly in softwoods, which are often graded by the number and size of any knots present. Small, live knots are not usually a problem, but dead knots are often loose and can drop out. Large knots create weakness and make a board difficult to work. Resin may eventually seep out from knots, especially on timber used externally: seal these before final finishing
with a shellac sealer. Small knots in timber such as English oak can be desirable, enhancing the grain’s character and increasing the price.
Checks and splits
Checks and splits in timber are usually the result of poor seasoning, where the timber is allowed to dry too quickly and shrinkage occurs. Major splits tend to occur at the ends and edges of boards, while checks (tiny splits) may appear anywhere across the face. Air-dried oak is particularly prone to this defect. End splits can be reduced during seasoning by sealing board ends with paraffin wax.
Shakes are cracks that occur within the tree, but are longer and wider than checks. Star shakes are radial cracks that appear around the outside of a log, following the line of medullary rays at 90 degrees to the growth rings. They are caused when the outer log shrinks while the centre remains more stable, and are the result of a log being left too long before conversion. Heart shakes result from internal shrinking and radiate outwards from the centre of the log. These may be caused by over-maturity of the tree or disease. Cup or ring shakes occur when growth rings separate. They form in the growing tree when subjected to high winds or a lack of food.
Cupping, twisting and bowing
Cupping is a common problem in crown-cut softwood. Boards tend to cup away from the curve of the growth rings as the timber dries out, both faces contracting at different rates. Twisting and bowing often result from bad sawing or poor stacking once the log has been converted. Some trees grow in a natural spiral, leading to twisting. Bowing along the length is usually the result of stacking boards badly. Stresses resulting from these distortions can make machining the affected timber difficult.
Sometimes formed when the bark of a tree is damaged, growths are produced when the wound heals. Known as burrs, or burls, they can grow on the trunk, the roots or individual branches. Characterized by dense, swirling grain, burrs are highly prized and generally cut into veneers or used for woodturning. The wild interlocking grain makes burrs extremely difficult to use for furniture-making.
Insect and fungal attack
Sapwood in new timber is prone to insect attack and bark should be removed before boards enter the workshop. Old buildings with poor ventilation and heating can lead to damp conditions that give rise to fungi and wood-boring insects. Constructional timbers are susceptible to attack from both wet and dry rot. Furniture stored in these conditions is also prone to attack from various beetles and must be treated with a suitable preservative.
Word Discription :
Air-drying Once a trunk has been converted, logs are stacked in stick’ in the open air to dry naturally, but protected from the rain. Crown-cut Crown-cut boards display almost flat or slightly curved growth rings in the end grain. Faces show flame effect of growth rings. Figure The grain pattern revealed in a piece of timber, usually when planed. It usually refers to unusual effects characteristic of certain woods. Heartwood Hard, dense cells at the centre of a tree, providing the most stable timber. Kiln-drying The process by which the moisture content of timber is reduced by more than is possible by air-drying. Timber is seasoned in an oven using a mixture of hot air and steam. This is essential for wood destined for internal use. Medullary rays Flecks evident on the face of some quartersawn timber.
Sapwood New wood growing furthest from the centre of a tree, providing least stable timber. Seasoning The process of removing moisture from the cell walls of wood.
Quartersawn Planks cut from a tree radially, where growth rings are at least 45 degrees to the face. This technique exposes the best figure.