Hardwoods explained
From the Spring/Summer 2009 edition of Bush Telegraph Magazine.
In the last issue of Bush Telegraph, we took a look at what makes wood. This issue, we take a closer look at hardwoods and some of the best grown in NSW State forests.
Broad-leaved trees, like eucalypts and red cedar, are hardwood trees. Most Australian native timber trees are hardwoods. The wood of these trees is made up of two distinct types of cells vessels and fibre cells.
Sap is carried upwards in large ducts known as vessels or pores. These start as wide cells with large cavities, arranged one above the other. In some cells the end walls break down to create long pipes running considerable distances.
Vessels can usually be seen with the naked eye. Timbers with vessels are sometimes called pored timbers (hardwoods), and the arrangement of the vessels in a cross-section is a useful aid to identifying different timbers.
Strength in broad-leaved trees is imparted by other types of cells, called fibres. These are similar to conifer tracheids but are shorter in length (commonly about one millimetre long) and usually thicker-walled. Fibres make up the bulk of the wood in broad-leaved trees and, like tracheids, the walls of these cells are made of cellulose and neighbouring cells are held together by lignin.
Other wood cells
Among the other types of cells that occur in wood are ray cells. These store food in the stem and are found in all timbers. Unlike the other cells of sapwood which are arranged vertically, ray cells are arranged horizontally, extending radially outwards towards the bark. These cells give wood a distinctive sheen when it is radially split.
Often rays are only one cell wide and several cells high and quite difficult to see without a magnifying lens. However, in some trees the rays are very large and give the wood characteristic patterns, such as the patterns seen in oaks, and other timbers like silky oak and she-oak.
Properties of wood
Although the wood of all trees consists essentially of cellulose fibres held together by lignin, different trees produce woods with many different properties that make them suitable for different purposes.
The density of wood depends on the thickness of the fibre walls in relation to the size of the cells hollow centre. Light timbers (for example balsa) have cells with very thin walls and a large hollow while in contrast, dense timbers (like ironbark) have cells with thick walls and narrow hollow.
The colour of wood is determined by various substances, such as polyphenols, which are deposited in the cell walls. In most woods the heartwood is distinctively coloured, commonly red, pink, brown or yellow and sometimes even green and purple. While heartwood is usually evenly coloured, in some woods the colour is unevenly distributed, giving a streaky appearance that can be highly decorative, like walnut, for example. Some woods have pale coloured heartwood not easily distinguished from sapwood.
The figure or pattern of wood is the ornamental appearance produced by the interaction of such features as colour; the arrangement of the vessels, latewood and earlywood. Other properties vary considerably between species and may have an important bearing on the way the timber is used. These properties include shrinkage as the wood dries; fibre length, important when wood is used for paper making; hardness or resistance to marking; durability against attack by insects, fungi and other agents; strength properties, that can vary markedly between seasoned wood (wood that has been dried) and green wood (wood with a high moisture content, such as wood just cut from a living tree); and flexibility. See our website for more information on forests, trees and timber.
Some best of NSW commercial hardwoods
Well over 100 different species of trees grown in NSW State forests are used for timber. Around 40 are used commercially. These include:
Blackbutt (Eucalyptus pilularis)
An important native hardwood grown in coastal forests. It produces pored hardwood timber, light brown in colour and coarse in texture. The basic density of the timber is 720 kg/m3. The timber is hard, strong and tough, making it a durable timber used for general building construction, flooring, poles and railway sleepers.
Spotted gum (Corymbia maculata)
An attractive large hardwood of the east coast, as well as a timber species it is often planted as an ornamental tree. It produces a pale to dark brown heartwood with paler sapwood, course in texture. The basic density of the timber is 740kg/m3. The timber is used for piles, poles, flooring and heavy engineering construction. It is the main Australian species for handles subject to high-impact forces, such as axe handles.
Messmate (Eucalyptus obliqua)
A large hardwood, common in Tasmania, Victoria and the tablelands of New South Wales and Queensland. The heartwood is pale brown with pale yellow sapwood. The basic density of the timber is 630kg/m3. The timber is used for furniture, flooring and panelling.
Sydney blue gum (Eucalyptus saligna)
Another hardwood species that grows in coastal areas of New South Wales. It produces a pored timber, pink to red in colour and with a coarse texture. The basic density of the timber is 690 kg/m3. The timber is easy to work, moderately durable and strong, making it suitable for use in general construction, flooring, weatherboards and panelling.
Flooded gum (Eucalyptus grandis)
Also known as rose gum because the heartwood is pink to pale red brown. It is a fast growing hardwood plentiful in northern New South Wales and southern Queensland. The basic density of the timber is 510kg/m3. The timber often features the work of the scribbly borer. This insect leaves a small trail in a scribbling pattern on the timber. The timber is used for joinery, panelling and furniture. It is good for fruit boxes as the shooks are not easily affected by fungal staining.
