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Spotted gum

Image showing the tree Corymbia maculata.
Image showing the wood of Corymbia maculata.
Corymbia maculata

Spotted gum is a species that grows well on favourable sites, usually attaining 35-45 m in height and 1-1.3 m diameter at breast height (dbh), with exceptionally large trees reaching 70 m and exceeding 3 m dbh. On poorer sites it may be 20-35 m in height and 0.7-1.2 m diameter. This species naturally occurs in open-forest to tall open-forest formation on the east coast of Australia from the Victoria-New South Wales border to the Maryborough District in Queensland. Trial plantings have been established in the south-west of Western Australia for pole timber.

Wood descriptionThis description can include heartwood and sapwood colour, grain, figure.

Heartwood is light brown to dark brown, and sapwood is pale and up to 8 cm wide. The texture is moderately coarse, with an interlocked grain, and the frequent presence of wavy grain produces an attractive ' fiddleback' grain. The wood is slightly greasy and gum veins are common.

Wood density

Green density is the density of wood in the living tree, defined as green mass divided by green volume, and useful for estimating transport costs. It varies with season and growing conditions.

Air-dry density is the average mass divided by volume at 12 per cent moisture content (this is the average environmental condition in the coastal capital cities around Australia).

Basic density is oven-dry mass divided by green volume. This measure has the advantage that moisture content variations in the tree during the year are avoided.

Green density is about 1150 kg/m3, the air-dry density about 970 kg/m3, and basic density about 790 kg/m3.

Drying and shrinkageAs wood dries, it shrinks more in the tangential direction (i.e. parallel to the growth rings) than it does in the radial direction (i.e. at right angles to the growth rings). The figures given are shrinkage from green to 12 per cent moisture content, before steam reconditioning treatment, and with some species after stream reconditioning. Reconditioning is essential for recovering collapse which may have occurred during the drying process, and is essential for species such as the ash-type eucalypts of eastern Australia.

Tangential and radial shrinkage before reconditioning are 6.1 and 4.3 per cent respectively, and after reconditioning 5.0 and 3.7 per cent respectively.

WorkabilityWith south-west and plantation-grown trees, comments are made on the comparative ease or difficulty of turning, nailing and bending, on susceptibility to splitting and other working properties. With semi-arid, arid and desert species, a more complex survey was made and reported in Siemon and Kealley (1999). The properties assessed were turning, machinability, boring, screwholding, stability, sanding, gluing and finishing. A semi-quantitative score was used: very poor = 1, poor = 2, average = 3, good = 4, and excellent = 5. This book uses the descriptive terms rather than numbers.

The timber is not difficult to work. Unseasoned wood is somewhat corrosive to aluminium nails and screws, and the high extractives content can be a problem when gluing phenolic-type adhesives. For good bonding a pressure of about 1000 kPa and temperatures above 200 C are usually needed. A low moisture content, preferably between 8 and 10 per cent, is also desirable.

Durability

The CSIRO Durability Classes are based on the performance in ground of outer heartwood when exposed to fungal and termite attack.

ClassLife Years
1More than 25
215 to 25
38 to 15
4Less than 8

The ratings are not relevant to above-ground use. In late 1996, CSIRO published revised ratings, which include termite susceptibility. Ratings are now available for about seventy species for decay, and for decay plus termites.

Durability Class based on the 1996 CSIRO assessment is 2 for decay, and 2 for decay + termites combined. Sapwood is Lyctus-susceptible.

Strength group and properties

Minimum values (MPa) for strength groups for green and seasoned timber come from Australian Standard AS2878-1986 'Timber - Classification of strength groups'. In grading structural timber, each species is allocated a ranking for green timber of S1 (strongest) to S7, and for seasoned timber SD1 (strongest) to SD8.

MOR is modulus of rupture or bending strength, MOE is modulus of elasticity or 'stiffness', and MCS is maximum crushing strength or compression strength. Hardness refers to the Janka hardness test and is a measure of resistance to indentation.

Minimum values (Mpa) for green timber

Strength Property S1 S2 S3 S4 S5 S6 S7
MOR 103 86 73 62 52 43 36
MOE 16300 14200 12400 10700 9100 7900 6900
MCS 52 43 36 31 26 22 18

Minimum values (Mpa) for green timber

Strength Property SD1 SD2 SD3 SD4 SD5 SD6 SD7 SD8
MOR 150 130 110 94 78 65 55 45
MOE 21500 18500 16000 14000 12500 10500 9100 7900
MCS 80 70 61 54 47 41 36 30

Where test data were available, they are shown in bold print. Most values are from Bootle (1983), Wood in Australia. Types, properties and uses. (McGraw-Hill), or Julius (1906), 'Western Australian timber tests 1906: The physical characteristics of the woods of Western Australia'.

Where no strength data were available, air-dry density was used in accordance with the Australian Standard AS2878-1986 Timber - Classification of strength groups to predict the strength group. Consequently, the strength values quoted are from the above two tables.

Green and dry strength groups are S2 and SD2 respectively. The more important strength properties are given in the table below.

Property Units Green Dry
Modulus of Rupture MPa 99 150
Modulus of Elasticity MPa 18000 23000
Max Crushing Strength MPa 50 75
Hardness KN 8.0 11.0
AvailabilityTimber from many species is available only in limited quantities, from near the areas where the trees grow naturally (or in plantations). There are other species such as red tingle and yellow tingle whose timber is rarely commercially available because the areas of occurrence are predominantly in conservation areas. Other species such as Goldfields timbers are only available in limited quantities because of their scattered occurrence and the fact that the industry is in the early stages of development.

The timber is not readily available in Western Australia, although common in New South Wales and southern Queensland.

UsesVarious past and potential uses are given as a general guide, but the list is obviously not conclusive. In particular, there is increasing interest in specialty timbers, and the semi-arid, arid and desert area species have considerable potential for this use.

The uses are as heavy engineering construction and mining timbers, where shock resistance is important, house framing, flooring, tool handles, piles and poles, shipbuilding, agricultural machinery and
plywood.