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Black locust (Robinia pseudoacacia L.)

Names:

In many countries, it is simply called 'acacia'. This gives rise to misunderstandings as people may mistake it for the real acacia species (genus Acacia), growing in subtropical and tropical regions.

Geographic distribution:

Together with many other plants, locust has been introduced to Europe from North America by J. Robin, the royal chief gardener of Paris in 1601. According to geo-historical studies, it used to be present in Europe in the tertiary period. Its rapid spread in Hungary is unparalleled in Europe. Today, 20% our forests is made up of black locust, and no other European country has as much of it as Hungary does (345,000 ha). Thus, it is no wonder that black locust is considered to be a Hungarian species in the folklore. It prefers warm climates and nutrient-rich sandy soils (its climatic requirements are close to those of grapes). Romania, Italy, France, Serbia, Bulgaria, Slovakia and the Ukraine also have large stands of black locust. Presently, black locust forest areas shows fast expansion in China and South Korea.

Morphological characteristics:

Black locust grows 20-25 m tall with a 30-60 cm breast height diameter. The growth slows down after 25 years, and so it is customarily harvested at the age of 25-40 years. Its thick, greyish-brown bark has net-like crevices. Black locust develops relatively straight, cylindrical trunks in closed stands. The cultivation of special strains, like the straight "shipmast locust" variety, increasingly get greater attention.

Macroscopic characteristics

Black locust belongs to the group of ring-porous deciduous tree species.
The thin sapwood (about 2-6 annual rings) is bright yellow, the colour of the heartwood varies from yellowish-brown to greenish-grey. There are no extractives and extraneous materials in the sapwood, therefore its durability and mechanical properties fall behind those of the heartwood. The heartwood formation takes place in black locust in two manners: heart wood forming substances get deposited on the one hand, and tyloses penetrate into the vessels and clog them on the other. The most important of these substances are tannins, resins, pigments, gum and robinetins. Above all, tannins and robinetins are essential for the durability of black locust heartwood. Sapwood contains much moisture and many simple organic compounds, and so fungi and insects are attracted.
The wood of black locust, like that of ring-porous trees in general, has distinct annual rings and latewood/earlywood boundaries. According to our measurements, the ratio of latewood in trees from a 30-35 year old black locust stand amounts to 77%, and the average annual ring width is 3.0-3.5 mm.

Microscopic characteristics

The bulk of the xylem is made up of thick-walled libriform fibres. Their proportion is 58% with an average length of 1 mm.
The wide earlywood vessels (150-220 mikro-m) are arranged in a 2-3 vessels thick ring. They are heavily clogged by tyloses in the heartwood. Latewood vessels have smaller diameters (70-140 mikro-m) and are either single or trimmed in small tangential and radial groups. The ratio of vessels in the tissue is about 15%.
Rays amount to approx. 21%, and they are narrow (1-3 cells).
The longitudinal parenchyma is paratracheally arranged along the vessels and often assembled in longitudinal rows. Their share is only 6% section.
The longitudinal and the ray parenchyma often contain crystalline deposits.
Because of various clogging substances and tylosis, black locust is impermeable to liquids in all directions. Quarter-sawing is not a requirement for black locust staves.

Defects, damages, durability

The following defects require greater attention in black locust: uneven annual ring width, eccentric growth, false growth rings, in-grown bark and, last but not least, knots.
Due to its special tissue structure and chemical composition, black locust withstands attacks of biological agents well. The single most important fungus, harming living trees, is Fomes fraxineus Cooke, causing butt rot. Powder post beetle (Lyctus linearis) damage is frequent in the sapwood of stored or built-in wood.
According to our observations, excessive taper (above 1.5 cm/m) occurs in 20-24% of black locust sawlogs.
About 15-20% of the logs processed in the sawmills are bent in one direction. The butt taper in black locust is not very extensive, usually less than 50 cm. Since buttresses and pith-rot are often present, this log part shall preferably be removed.
The sawdust of black locust may cause allergic reactions upon skin contact.
According to the international standard EN 350-2, black locust is the only European species that can be put into the durability class 1-2. Because it does not require chemical treatment for outdoor applications, black locust can be considered a very environment-friendly material.
During a series of focussed investigations, the fungus-resistance of black locust was verified. We selected basidiomycetes fungi that have been also observed in naturally decaying black locust. Steamed and unsteamed black locust heartwood was exposed to such cultures for 12 weeks. Natural wood proved to be completely resistant: the weight loss was 0.1-0.6%. Conversely, steamed black locust has lost some of its resistance in contact with Grifola sulphurea and Irpex lactea (the weight losses were 2.1 and 4.4%, respectively). Accordingly, the steamed, and thus brown-coloured, black locust sawnwood is not recommended for outdoor use or where it is exposed to possible fungal infection.
According to Hungarian and foreign experience, the service life of black locust is estimated to be:

Physical properties

Density (kg/m3)

• oven-dry 540-740-870
• air-dry (12% MC) 580-770-900
• green 800-900-950

Freshly felled black locust contains only 35-45% of water, therefore it burns well even without drying. The fibre saturation point of black locust was determined by various studies to be at 21.8-22.5%.

Shrinkage (%):

• longitudinal 0.1
• tangential 5.4-7.2
• radial 3.2-4.6
• volumetric 11.4-12.2

Porosity (%): 52

The ratio of the tangential and radial shrinkage (shrinkage anisotropy) is rather favourable in black locust. However, significant internal stresses were observed in black locust. They might have been introduced by the fast growth, the inhomogeneous growth ring structure, the high proportion of juvenile wood and the frequently occurring eccentric growth. The internal forces often cause various deformations and splits.

Thermal properties:
Because of its high density, black locust is rather hard to get ignited. The minimum radiation heat intensity necessary for ignition is 2.6 W/cm2. For designing structural fire resistance, the burning rate recommended for calculations is 1.0 mm/min for conifers, 1.3 mm/min for poplars, and a very favourable 0.5 mm/min for black locust.
According to our studies, the calorific value of oven-dry black locust in kJ/kg is :

• bark-free wood: 17,777
• bark 19,145
• bole (in bark) 18,047
• thick roots 17,223

Assuming a density of 700 kg/m3 (bark included), the volumetric calorific value is 12,633 MJ/m3, i.e., 2.5 tons (or 3.5 m3) of air-dry black locust wood would replace 1 ton heating oil. Due to its low initial moisture content, black locust needs to be stored as firewood for only one year, and it will not be damaged by fungi or insects if appropriately stored.

Mechanical properties

Black locust's strength and elastic properties are better than those of any other species grown in Central Europe.

The most important properties (in the longitudinal direction, unless otherwise indicated) are:

Bending strength (MPa)	103 - 136 -169
Bending modulus of elasticity (MPa)	9000 - 11300 - 13600
Compression strength (MPa)
longitudinally	62 - 72 - 81
across the grains	18.5
Tensile strength (MPa)	166,8
Shear strength (tangential)	11 - 13 -  16
Impact-bending strength (J/cm2)	12 - 14 - 18
Hardness (Brinell), (MPa)
end	67 - 78 - 88
side	28
Cleavage (MPa)
tangential	0.6 - 1.1
radial	1.12

The wear resistance of various woods is usually compared to beech. The resistance of some species are in decreasing order: black locust 0.37; beech 1.00; ash 1.53; oak 1.56.
Locust's wear resistance is unique among the European tree species..

Chemical properties

The elementary composition of the xylem is the following (%):

C 49.2; H 5.91; O + (N) 43.1; ash 0.79 (the estimated amount of nitrogen is 0.2-0.3%). According to our studies, the bark of black locust is rich in minerals. Its ash content amounts to 4.76% while that of the sapwood and the heartwood to 0.98% and 0.26%, respectively. Because of the large proportion of bark (20-25%) in locust firewood, 80-85% of the ash comes from the bark.

The cell walls of black locust consist of the following components: about 40-50% cellulose, 15-22% hemicellulose and 25-30% lignin.
Among the important extractives of black locust tannins can be mentioned.
The tannin content is 3-6% in the bark and 2-4% in the xylem; the respective values for oak are 5-10% and 3-7%.
In addition to tannin, dihydro-robinetin, with 2-5 weight-percentage in the heartwood, substantially contributes to the durability of black locust. This substance provides the typical greenish-yellowish-brow colour to black locust.
Crystalline deposits (calcium carbonate, calcium oxalate) can be found particularly in the rays, and occasionally in the vessels. This significantly increases tool wear when processing locust.

Assortments

In Hungary, about 8,500 ha of black locust forests are harvested annually. Our annual black locust crop (including intermediate cutting) was around 1.5-1.9 million m3 for the past few years. The rotation period varies with the quality of the stands, the national average being 31 years.

The following assortments are produced:

• sawlogs,
• lower quality (short) logs,
• mine timber,
• masts, poles, chips, etc.
• firewood.

Only about 18-20% of the harvest is suitable for sawnwood.
The popularity of locust firewood significantly influences the assortment composition. Logs and firewood are often the only assortments because of the great market demand.

Workability

Due to the relatively small diameter of the industrially used timber (on average 23-24 cm), the knot-free portion is rather small. The mean distance between larger knots is 60-70 cm, and rotten knots are quite frequent. This makes the production of knot-free timber longer than 1m considerably difficult.
Black locust is not easy to saw due to its hardness and strength characteristics; the cutting resistance (power requirement) is 20-30% greater than of oak and so the tool wear.
Timber does not require any special protection (e.g., spraying) on the log yard.

In the past few years, about 150 to 220 thousand m3 of black locust timber was annually converted to lumber in Hungary.
If properly steamed, the wood loses its unpleasant greenish-yellowish colour and gradually browns. After steaming, black locust wood can be machined more easily then before, and there will be less grain tear and splits. Black locust lumber is usually steamed in tight stacks immediately after sawing. Steaming is most efficient if it is carried through in pressurised cylinders (autoclaves), where the cycle lasts 20-30 hours at 110 °C.
Black locust can be properly steamed also in traditional steam chambers without saturating pressure. In this case the cycle lasts 60-180 hours. This method is milder, causes less cracking, and results in a lighter brown colour.
Locust is easy to dry (the schedules are similar to those of beech). The inherently low initial moisture content of timber is of great advantage. In our experience, if the wood was sawn up 1-1.5 months after felling, the net moisture content of the sawnwood was found to be 30-35% (50-60% with oak lumber).
Accordingly, black locust lumber, floor and furniture material can be kiln dried right after sawing, and the sawn wood can be dried without the danger of quality loss, i.e. there is no need for pre-drying.
Steam-softened wood bends well for furniture parts.
With black locust, machining tools have to be replaced more frequently when compared with other, more commonly used species (oak, beech).
Black locust can be properly bonded with the usual glues and bonding techniques. However, the specific features of black locust (e.g., clogged pores) have to be taken into consideration when choosing bonding parameters.
The usual surface treatment techniques apply to black locust. The variegated colour can be smoothened by proper priming and dyeing.

Utilisation

Black locust can be used as a supplementary species in manufacturing particle boards.
The Mohács Fibreboard Factory has successfully solved the utilisation problems of black locust. Interestingly, (wet) fibreboards retain the typical yellowish-greenish colour of black locust wood.

Hungarian furniture industry has first started using black locust as hidden structural components (e.g., the framework of upholstered furniture, clothes-hanger rods, etc.). Locust kitchen chairs (petite seats) and desk frames were introduced in the middle of the 70's, and subsequently, black locust also appeared as raw material for living room chairs and tables. Its use as material for indoor furniture was hindered both by its colour multitude and difficult machinability. These problems have been solved by steaming, dyeing and the use of modern carbide-tipped tools, so much so that today even edge-profiled solid wood panels are made of it. Owing to its durability, black locust is currently the most important raw material for outdoor furniture. Significant quantities of hardwood flooring, staircase and railings are also made of it in Hungary. It plays an important part in structural applications, glulam beams, residential housing, tool-sheds, fence components and panelling. In addition to solid wood flooring, laminated floors are also manufactured (e.g. at Barcs or Zalahaláp) where the surface layer exposed to wear is made of black locust. Cartwright traditionally makes use of black locust. Tool handles are produced in large amounts of it. Attempts seem to be successful to manufacture large glued-laminated railway ties for switches. Traditionally, wine barrels have been made of oak during the past centuries. At first (some 15-20 years ago), there was a strong aversion against the introduction black locust vats. Yet, black locust turned out to be an excellent resource for tight barrels! The wood is impermeable to liquids, regardless of sawing direction, and black locust staves rarely break during bending. The characteristic odour and colour of black locust do not have a negative effect on the wine quality. At last, the utilisation of black locust as energy source has to be mentioned. Currently, 600 to 700 thousand m3 of black locust firewood is used in Hungary annually. Also, promising experiments are conducted into establishing short-rotation energy wood plantations.