FRI BULLETIN No. 119

PROPERTIES AND UTILISATION OF EXOTIC SPECIALITY TIMBERS GROWN IN NEW ZEALAND

PART 11: AUSTRALIAN BLACKWOOD Acacia melanoxylon R. Br.

A. N. Haslett

This FRI Bulletin is of particular relevance to tree growers, and processors and users.

FOREST RESEARCH INSTITUTE, NEW ZEALAND FOREST SERVICE PRIVATE BAG, ROTORUA, NEW ZEALAND

1986 2

PREFACE

This series of booklets details the properties and potential uses of speciality timbers grown in New Zealand. Each booklet discusses one tree species or distinct group of related species under the following headings: The Tree, Wood Properties, Processing, Uses, and Glossary. The first booklet describes briefly the tests conducted, the properties evaluated, and the commercial significance of these properties.

The booklets are based on data obtained over many years of research and they represent work from many sections of the Wood Technology Division of FRI, along with input from Forest Service Utilisation Foresters and representatives of the forest industries.

ISSN 0111-8129 ODC 83:81 3

ABSTRACT

This booklet summarises the properties and recommended utilisation procedures for New Zealand­ grown blackwood. Blackwood timber is easy to process. The wood is similar to that of Australian­ grown material, with its attractive colour, and medium density and texture making it suitable for a range of high quality uses including furniture, cabinets, veneers, and others.

KEYWORDS: Australian blackwood, Acacia melanoxylon, wood properties, density, shrinkage, stability, mechanical properties, processing, sawing, drying, machining, uses.

THE TREE

Natural Distribution and Availability

Australian blackwood is a native of the south-eastern Australian mainland and Tasmania. It has been introduced to South Africa and over the last century small plantings have been made throughout New Zealand (less than 200 ha in total). Since being listed in the New Zealand Forest Service "Policy on Exotic Special Purpose Species" (1981) blackwood has been planted in increasing quantities in New Zealand, but substantial volumes of timber will not be available until trees currently being planted and tended mature.

Target Tree

The ideal final crop tree or 'target tree' for Australian blackwood (Fig. 1) should have a diameter at breast height (1.4m) of approximately 60 cm and a clear bole of 5-6 m to maximise the yield of full length clear wood from the bottom 5-6 m and to obtain short clear cuttings from higher up the stem. Preliminary research suggests that well-tended blackwood can be grown to this size in 30-40 years. Blackwood's high heartwood composition means that logs as small as 30 cm small-end-diameter can be sawn.

Blackwood has a tendency to develop forks and heavy branching so silvicultural techniques must be used to obtain a straight clear butt log. When grown in pure stands, pruning and the removal of competing leaders and/or branches (form-pruning) will encourage a single straight stem. Alternatively, blackwood may be grown in gaps in existing vegetation or in mixed plantings, to encourage the development of a single leader by forcing the trees to grow up towards the light.

Ignoring these silvicultural techniques will result in poorly formed trees which will yield sawn timber with severe cross-grain, which in turn will cause "pick-out" during machining and warp during drying.

Details on recommended silvicultural regimes may be obtained from Forest Service Advisory Officers.

Tony Haslett (B.For.Sc.Hons) is a scientist in the Wood Technology Division of the Forest Research Institute. He is co-ordinating utilisation research on speciality timbers, his specific interests being wood quality, drying and sawmilling. 4

FIG. 2- Typkal heartwood colour and figure in flat-sawn (lower) and quarter-sawn (upper) blackwood. The distinctive figure of FIG. ]-Target tree for Australian blackwood the grain is emphasised in the flat -sawn material.

WOOD PROPERTIES

General Description

Timber from New Zealand-grown blackwood resembles that grown in Australia in appearance and shows a similar large colour variation. In Australia this colour variation is accepted as being "natural" and so only a slight attempt is made at colour matching.

Colour: The heartwood varies considerably in colour from yellow to reddish-brown, to almost black. This wide colour variation is a result of combined genetic and site factors. Brown splotches, caused by an incomplete distribution of extractives, are scattered through the timber. Figure 2 shows typical heartwood colour in flat­ and quarter-sawn material. The sapwood has a distinct white to straw colour and is usually only 20-50 mm in width.

Grain and Texture: The grain is primarily straight, but areas of wavy grain give the wood a very attractive appearance (Fig. 2). Poor stem form can result in severe cross-grain which is difficult to machine successfully. The wood has a medium even texture with a distinctive figure caused by pronounced growth rings and tension wood.

Density: In general, the density of New Zealand-grown blackwood is within the range considered suitable for furniture and cabinet work. The species can be classed as a "medium-density ". A comprehensive survey of 70-year-old trees revealed that whole-tree basic density ranged from 465 to 670 kg/m 3 (mean: 593 kg/m 1) . These values are similar to those for Australian-grown blac'.cwood. Within a tree, basic density varied little vertically. However, for the radial profile, basic density increas­ ed steeply by over the first 150 mm then levelled off, and finally decreased slightly in the sapwood. Green and air dry density were 1040 kg/m 3 and 680 kg/m 3 respectively. 3 Young (15-year-old) fast-grown blackwood may have an average basic density as low as 400 kg/m , with over 500Jo sapwood by volume. 5

Strength

New Zealand-grown blackwood is at least as strong and stiff as tawa (Beilschmiedia tawa), and Australian-grown blackwood. The New Zealand timber has above average hardness which is important for furniture timbers, and it is moderately tough. However, these strength properties vary considerably between trees, reflecting the wide variation in density (for example, the tree of lowest strength had a Modulus of Rupture 200Jo lower than the between tree mean). Furthermore within-tree trends in strength properties parallel the trends in density. There is little variation in strength up the tree, but strength increases noticeably in the rjidial direction, to reach a maximum at about 15 cm from the pith, then decreases slowly. This wide variation of strength properties need not detract from blackwood's high suitability for furniture. However, it may reduce its suitability for demanding end uses such as impact handles.

TABLE 1: Strength properties of blackwood compared with tawa

Species/Source Modulus of Modulus of Compression Hardness Rupture Elasticity parallel (MPa) (GPa) (MPa) (kN) Green 1207o Green 12% Green 12% Green 12%

Black wood New Zealand 76.4 129.9 9.95 14.4 29.4 62.5 NA 6.60 Australia! 75.0 115.0 9.27 13.2 29.8 60.3 4.23 4.89 Taw a New Zealand2 69.8 114.4 10.2 13.2 31.4 39.2 4.28 4.76

NA = data not available ' Bolza, E. and Kloot, N.H. I%3: The mechanical properties of 174 Australian Timbers. Division ofForest Products Technological Paper No. 25. CSIRO, Australia. 1 Bier, H. 1983: Strength properties of small clear specimens of New Zealand grown timbers. New Zealand Forest Service, FRI Bulletin No.41

Shrinkage, Dimensional Stability, and Related Properties

Blackwood has a low shrinkage. Shrinkage from green to 12% m.c. is 3.6% tangentially, 1.8% radially. Because blackwood is more impermeable than radiata , it has better short-term stability. However, radiata pine has superior long-term stability probably because of the very low shrinkage intersection point for blackwood.

Australian-grown blackwood has a reputation for dimensional instability, which can result in glue-line failures. To minimise this problem, timber should be dried to the correct moisture content and correctly glued, artd wide cross-sections should be avoided. These recommendations apply equally to New Zealand-grown blackwood.

TABLE 2: Dimensional stability (in%) of blackwood and radiata pine

Blackwood Radiata pine

Shrinkage Intersection Point 25.4 28.7

e.m.c. at 90% RH 19.2 21.2

e.m.c. at 60% RH 12.0 12.3

Long-term movement tangential movement from 60% RH to 90% RH 2.7 2.0 radial movement from 60% RH to 90% RH 1.3 1.0

·short-term movement tangential swelling after 24 h at 95 OJo RH 1.6 2.2 m.c. increase after 24 h at 95% RH 4.6 7.4 6

PROCESSING

Sawmilling and Grade Recoveries

Blackwood is not difficult to successfully and can be sawn with band normally used for radiata pine, provided the saw pitch is 45-70 mm. An acceptably smooth finish can be obtained at a range of feed speeds but, because the density of blackwood is higher than that of radiata pine, slower feed speeds are advised. As would be expected, blackwood's higher density results in blunting properties that are somewhat higher than those of radiata pine, but they are no reason for concern.

Growth stress can cause some splitting of logs and movement of sawn-timber, but stress levels of blackwood are low compared to some other (e.g., ash eucalypts, tawa). Logs may be either flat- or quarter-sawn but flat-sawing is preferred because it does not require large logs or specialised sawing equipment, and it produces a more figured board (Fig. 2). However, the poor long-term stability of blackwood means that the wide flat-sawn boards may cause problems with subsequent movement in use.

A major sawing study of 70-year-old untended trees gave good sawn-timber grade recoveries (although the sawn conversion was only 450Jo). Over 65% of the sawn-timber obtained was in the clear, dressing, or clear-cutting grades. Use of cuttings grades will maximise grade and financial returns. For example, blackwood cuttings less than 200-mm-long are often recovered in Australia.

Veneer and

After conditioning at 70°C flitches can be sliced into 0.5-mm-thick veneer without difficulty. Similarly, blackwood is moderately easy to peel. Care should be taken to minimise iron/tannin staining (which can result from contact between wet veneer and iron).

Veneers may split and buckle during drying.

Drying

Blackwood is a relatively easy hardwood to dry; boards less than 50-mm-thick may be kiln dried from green without significant degrade. The major problem associated with kiln drying from green is that drying time varies greatly- quarter-sawn heartwood takes twice as long to dry as flat-sawn heart­ wood. For this reason, it is recommended that all material, particularly that 50-mm-thick or more, be air dried in well-constructed, protected stacks to about 30% · m.c. before kiln drying on the schedule below. Air drying of 25-mm boards to 30% m.c. takes 12-20 weeks in Rotorua (depending upon the season), with final kiln drying taking approximately 4 days.

Blackwood has a low shrinkage and only a slight tendency to collapse, even when dried at elevated temperatures. Preliminary air drying will eliminate collapse, making final reconditioning unnecessary. Spring and twist were a problem in the material studied but these could also be attributable to poor stem form - warp should be low in material from silviculturally tended trees. Good stacking is essential.

TABLE: 3 Recommended drying procedure for New Zealand-grown blackwood

Air dry 25-mm-thick 3-5 months to 30% m.c. 50-mm-thick 5-7 months to 300Jo m.c:

Kiln dry 25- and 50-mm-thick Temperature (0 C) Dry bulb Wet bulb Mean m.c. 30-40% 65 60 Mean m.c. 20% 70 60 Final conditioning 75 74 (4 h/25-mm thickness) 7

TABLE 4: Correction figures for an electrical resistance moisture meter calibrated for Douglas

If meter reading is 10 11 12 13 14 15 16 17 18 19 20 21 22

average corrected moisture content will be 11 12 12 13 14 15 16 17 18 18 19 20 21

Durability and Preservation Preliminary results suggest that the heartwood of New Zealand-grown blackwood could be classified as durable and the sapwood as perishable. Both the heartwood and sapwood are readily treated by boron diffusion.

The sapwood is susceptible to attack by the common house borer (Anobium punctatum) and the powder post beetle (Lyctus brunneus). However, the low likelihood of infestation by these two insects suggests that preservative treatment against them is probably not warranted.

Working Properties

Straight-grained material has good machining properties but material with cross-grain is prone to severe picking-out. Because cross-grain occurs frequently in current supplies of blackwood, it is ad­ visable to machine lengths individually.

Blunting: Moderate, according to density. Boring: Good; no problems with roughness or tear-out. Nailing: Must be pre-drilled before nailing. : Straight-grained lengths will give an excellent finish with a 30° cutting angle and slow to moderate feed speed. Material containing cross-grain incurs severe picking-out and fuzzy-grain. For this type of material, the cutting angle should be reduced and lengths individually hand-machined. Sanding: Sanding occasionally produces a stringy surface. Blackwood dust is especially irritating to the skin and nasal passages of some people. Sawing: Hardness can cause burning during sawing, so tungsten-carbide-tipped saws are an advan- tage. Screwing: No problems. moulding: Good, although burning can occur. Turning: An excellent turnery timber; turns well at high speed, but with some chipping; cutters must be sharp.

Gluing

Glue-line failure can occur because of the long-term instability of blackwood. Care should be taken to ensure that wood is at the moisture content appropriate for the end-use and that the surfaces to be glued are freshly machined and dust free. It is not advisable to glue boards over 150 mm wide.

A wide variety of glues can be used successfully. However, it should be noted that casein glues will discolour the wood at the glue line.

Steam Bending

Australian-grown blackwood is reputed to have good properties and there is no reason to expect New Zealand-grown material to be any different. 8

Staining and Polishing

There are no problems with staining, and blackwood takes a fine even polish. Spots of dark ex­ udate which sometimes appear do not seem to affect finishes as they are easily sanded off. Polyester and other catalysed finishes are sometimes retarded in their curing.

USES

With its medium even texture, attractive appearance, and good machining properties, blackwood is an excellent substitute for rimu and tawa, and for numerous speciality timbers currently being imported. Compared to the ash eucalypts also included in the Special Purpose Species Policy blackwood is easier to saw and dry.

New Zealand-grown blackwood should be similar to that grown in Australia and so its major use will probably be in high-quality furniture and cabinet making, either in solid-wood or veneer-overlay form. Decorative veneer is an excellent use for this valuable timber species. Other uses will probably include turnery, carving, gun stocks, and .

TABLE 5: Acceptability of blackwood for various end-uses

Furniture *** Handles ** Cabinet-making *** Knobs *** Veneers *** Engineering Turnery *** Exterior joinery Carving ** Weatherboards Gun stocks ** Boat-building ** Flooring (decorative) ** Firewood ** Panelling **

not suitable * suitable but not preferred ** moderately suitable *** highly suitable 9

FIG. 4- Spinning wheel made from blackwood

FIG. 3- Interior doors manufactured from black

FIG. 5- Wall unit of Australian blackwood showing variation in wood colour 10

GLOSSARY OF TERMS

Brittleheart: Wood of abnormal brittleness resulting from compression failures caused by growth stress. Commonly occurs in the heartwood of hardwood species.

Collapse: A flattening or buckling of wood cells during drying resulting in excessive and/ or uneven shrinkage.

Compression failure: A deformation or fracture of the wood fibres across the grain resulting from excessive compression parallel to the grain, either by direct end-compression or by bending. It appears as a minute fracture or crinkling of the fibres, and is often difficult to detect until the timber is machined.

Density: The weight per unit volume at a specified moisture content.

Durability: The natural ability of a timber to resist decay.

Equilibrium moisture content (e.m.c.): The moisture content at which timber neither gains nor loses moisture when exposed to a constant condition of temperature and humidity.

Flat-sawn/sawing: Timber sawn/sawing so that the annual growth rings, as seen on the end-section, form an angle of less than 30 degrees with the board face.

Growth stress: Stress developed in the wood of standing trees. It can lead to the formation of brittleheart, end splitting of logs and sawn timber, and warp of sawn timber.

Hardwood: Wood from flowering tree species.

Heartwood: The non-living central zone of the tree. This may contain varying quantities of chemicals, the presence of which can impart a darker colour to the wood.

Kiln: A chamber for drying wood where temperature, humidity, and air circulation can be controlled.

Moisture content (m.c.): The amount of moisture in wood expressed as a percentage of the weight of wood substance.

Quarter-sawn/sawing: Timber sawn/sawing so that the annual growth rings, as seen on the end-section, form an angle of more than 60° with the board face.

Radial direction: The direction along, or close to, a line passing through the pith, across the growth rings, to the circumference of a tree or log.

Sapstain: The discolouration of the sapwood by fungi which do not decay the wood. It is sometimes called bluestain because of the characteristic colour of the fungal hyphae.

Sapwood: The outer, living layers of wood in the tree.

Shrinkage: The loss in wood dimension that occurs when the cell walls lose moisture.

Shrinkage intersection point (SIP): The moisture content at which any further removal of moisture results in shrinkage. Silviculture: The growing and tending of forests.

Softwood: Wood from coniferous tree species.

Stability: The degree to which timber retains its shape and size despite changing environmental con­ diti.ons (commonly referred to as "dimensional stability"). 11

Tangential direction: The direction along (parallel to) the growth rings on the end-grain.

Tension wood: Abnormal wood formed in leaning or crooked stems of a hardwood. When sawn it often gives a woolly surface. It has abnormally high longitudinal shrinkage.

Through and through sawing: Sawing logs by making a number of consecutive cuts parallel to each and in the same as the longitudinal axis.

ABBREVIATIONS

e.m.c. equilibrium moisture content m.c. moisture content MP a megapascal GP a gigapascal kN kilonewton SIP shrinkage intersection point RH relative humidity

Illustration of Queen Anne bedroom suite made of Australian blackwood on following page. Chest of drawers, bedside tables, and headboard made of Australian blackwood. Note the rich colour and attractive figure of the flat-sawn boards.