Properties and characteristics of hardwoods or softwoods

Properties of timbers required for joinery and cabinetwork

The selection of timbers for furniture, veneers, joinery, etc., depends on a number of factors, chiefly, the appearance characteristics and the working qualities of the wood. In addition to this, consideration must be given to density, mass, various strength properties, shrinkage and durability.

Appearance characteristics

Colour - depends on the substances and chemicals in the cells and cell walls. For example, Jarrah grows in soils rich in bauxite, giving it its deep red colour.
Grain - refers to the direction of the wood elements and will vary according to the position the board occupied in the tree and the conversion method used to cut it out of the log. Grain may be described as 'cross', 'spiral', 'wavy' etc.
- Quartersawn boards are cut out of the log with their wide faces at right angles to the growth rings so that the edges of the growth rings are or the surfaces of the rays are seen if they are distinct. Oak is a timber that has particularly large rays so is often quartersawn to display this feature.
- Back sawn boards are generally cut with their width at right angles to the rays, (tangential to the rings) showing growth ring markings or distinctive features such as ring-porous vessel lines and soft tissue
Texture refers to the relative size and arrangement of the cells, and may be described as 'fine', 'medium' or 'coarse', 'uneven' or 'uniform'. The differentiation between coarse and fine texture applies mainly to hardwoods and is made on the dimensions of the vessels and the width and which have large vessels or pores and broad rays, are said to be of coarse texture, whereas timbers such as coachwood, which have small vessels and narrow rays, are fine-textured.
Figure is the overall pattern or design produced on the longitudinal surface of the wood and is due to variations in texture, grain and colour. This term is usually applied to timbers, which possess unusual or attractive designs due to the natural arrangements of the elements, method of conversion or irregularities in the tree such as knots, burls and crotches

Working and other qualities

While trees can be grouped into pored and non pored woods, nature has provided many different species of timber having special properties or characteristics. These are governed by the type and arrangement of cells and the variety of chemical inclusions such as sugars, starches, resins, gums, oils, tannins, silica and colouring matter.

Working qualities refer to the ease or difficulty of sawing, planing, chiseling, carving, shaping, bending, turning, gluing and nailing. These are influenced by the density, texture, grain, hardness and durability of the wood and the presence of materials such as silica (blunts tools quickly) or oils (can cause problems with finishes). Desirable timbers such as figured timbers or burls can be challenging to work with tools.

Density or weight depends on the size of the cells, the thickness of the cell walls and the amount of lignin, gums and resins present. Density is usually quoted as 'light', 'medium', or 'heavy' and in kilograms per cubic metre (kg/m3) for timber dried to 12% moisture content.
Hardness is the ability to resist indentation, bruising and abrasion. Hardness and softness are closely related to density as they vary, according to the size of the cell, the thickness of the wall and chemical inclusions. Remember that balsa, classed as a hardwood (pored wood), has a density of only around 112 kg/m3 so that it is, in fact, very light and soft.
Durability is the ability to resist decay, borers, termites and weathering. Durability depends largely upon moisture content and the presence in the cells and cell walls of chemicals that are resistant to insect and fungus attack. Sapwood generally contains less resin and gum and more plant food than heartwood; therefore, it is usually less durable. A durable timber is not necessarily hard or tough; for example, Californian Redwood is one of the most durable timbers in the world.
Strength is directly related to the type of cell structure, size of cells, thickness of walls, amount of lignin and direction of growth. Strength does not necessarily follow weight: Oregon has one of the highest strength-to-weight ratios for building timbers yet it is soft, light and easy to work with.

Fiddleback( a type of figure) in Blackwood

Oregon trees (AKA Douglas Fir)

Birdseye maple.

It is not known what causes the phenomenon. Research into the cultivation of bird's eye maple has so far discounted the theories that it is caused by pecking birds deforming the wood grain or that an infecting fungus makes it twist. However, no one has demonstrated a complete understanding of any combination of climate, soil, tree variety, insects, viruses or genetic mutation that may produce the effect.

An example of the complex grain patterns found in a Burl.

Burls form on tree roots, trunks and branches as a result of the abnormal development of bud growth cells. They appear as wartlike lumpy growths, roundish in shape and covered in coarse bark. Burls contain twisted, compact wood that is tougher than the wood from the rest of the tree. Source

Burl growing on a tree.

Opinions vary as to the causes of burls but most experts agree on several factors, the first being disease. Although burl formation has not yet been fully analysed it seems able to be attributed to several diseases caused by bacteria, fungi or viruses with mould infestations being held as the most common cause. Infections just below the tree bark seem to trigger these abnormal growths with the tree's cells continuing to grow at the same rate as the rest of the tree but in a drastically changed direction. Source

Spalted maple.

Spalting is caused by certain white-rot decay fungi growing in wood, primarily hardwoods such as maple, birch, and beech. The fungi create zone lines in the wood where territories of competing fungi meet.

producing-spalted-wood.pdf

What exactly is a burl?

Burls are also known as 'burrs' in the US and in the UK - the dashboards of Jaguars are traditionally made from Burr Walnut with the lovely swirly patterns typical of burls or burrs. Burls form on tree roots, trunks and branches as a result of the abnormal development of bud growth cells. They appear as wartlike lumpy growths, roundish in shape and covered in coarse bark. Burls contain twisted, compact wood that is tougher than the wood from the rest of the tree.

Environmental stress and physical trauma also contribute to burl formation. The former can include a response to injuries from fire or flood or to irritation from constant wind or water erosion. Physical trauma can include insect damage, wounds from animals or injuries from logging. In many cases it will be a combination of factors that will cause burl growth and basically anything that can injure a tree has the potential to cause it.

There are some trees, however, with Eucalypts being one of them, that are genetically predisposed to form burls. These burls are more like big clumps of unformed bud clusters which could resprout into a branch or even a whole tree if the primary stem were injured. These burls display wild grain patterns that are prized by woodworkers and furniture makers who use them for decorative accents. Burls, deviating from the close straight grain of the trunks, display grain that is typically wavy, curly, swirled or bird's eye and for this reason is much prized by woodworkers.

The flip side of a burl's beautiful grain is that it is extremely difficult to work with because of the structural weakness that the abnormal growth generated. As mentioned previously, burls contain twisted compact wood whose cells do not grow in the nice straight lines of the wood in the main part of the trunk so when a burl is put on a lathe to be turned into a bowl and a large amount of mass is removed from its centre it can behave in very unpredictable ways.

All the competing stresses and tensions in its dense twisted grain are suddenly released with the result that it will skew off centre, warp and twist into a very un-bowl-like shape. The turner then has to rest the burl/bowl for some time to let it settle and then do another turning to re-shape the bowl and make it round again.

This process may have to be repeated several times over many months to achieve a bowl or vase that is stable and sound and will not warp or twist out of shape. So a burl has to be harvested, dried, turned and rested several times, sanded and finally polished - a process that can take a year from start to finish. Woodworkers have lots of burls in different stages on the go at any one time but it is still a time-consuming process. Source

Weathering

Generally, the function of finishes on timber is two-fold, firstly to improve the durability of the building and secondly to add to the aesthetics. Unfinished, unprotected timber will inevitably weather as a result of gradual changes to its physico-chemical structure brought about by temperature and moisture content variations. The rate is generally slow, at about 6mm per century.

The weathering process leads to a slow breaking down and wearing away of surface fibres, change in colour and roughening of the surface. Under extreme conditions, timber may deform, check, split and pull away from fasteners. The extent of weathering will vary with timber species and ambient conditions.

Although unprotected timber has been used externally for centuries, the weathered effect is not always desirable. In most applications timber needs protection from the elements of water, cold, heat and ultra-violet light to promote a long service life. In addition to protection from the elements, finishes may provide a decorative effect.

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