Pterocarpus soyauxii (PROTA)
|Geographic coverage Africa|
|Geographic coverage World|
|Dye / tannin|
|Essential oil / exudate|
Pterocarpus soyauxii Taub.
- Protologue: Oliv., Hooker’s Icon. pl. 24: t. 2369 (1895).
- Family: Papilionaceae (Leguminosae - Papilionoideae, Fabaceae)
- African padauk, African padouk, barwood, African coral wood (En).
- Padouk d’Afrique, padauk d’Afrique, bois corail (Fr).
- Ndimbu, nkula (Po).
Origin and geographic distribution
Pterocarpus soyauxii occurs from south-eastern Nigeria east to eastern DR Congo and south to northern Angola.
The wood of Pterocarpus soyauxii is a valuable multipurpose hardwood. Because of its resistance to water it is locally used to make canoes and because of its beautiful reddish colour it is favoured for carving and sculpturing, furniture, cabinets, knife and tool handles, traditional hair combs, walking sticks and musical instruments. The wood has a high resonance quality as its damping of vibrations is low and formerly large telegraph slit drums and war drums, as well as xylophones, were made from it in DR Congo and Gabon. It is currently used also for ‘Western music’ xylophones and increasingly tried for the back and sides of guitars. Because of its high durability the wood is excellent for construction, carpentry, outdoor joinery, flooring, staircases, railway sleepers and boats but also for veneer, inlay, billiard tables, toys, joinery, dowels, shuttles, bobbins, spindles, sporting goods and paddles. As the wood is resistant to marine borers it was used in temperate regions for marine constructions such as piers and sluice gates. In the Herault region in France it was used for years in the construction of waterwheels for irrigation. The wood is also used as fuel.
The heartwood is the source of the so-called true barwood dye. In Africa nowadays, the dye is still used to colour red fabrics, fibres and clothes, including the tail-like ornaments made from raffia fibre in Cameroon and worn on the back by women of the Bulu people. In DR Congo, in the former kingdom of Kuba, at the confluence of the rivers Kasai and Sankuru, the dyes of the famous ‘Kasai velvets’ include Pterocarpus soyauxii reds with a more violet shade, obtained by combining the red dye with tannin-rich plants and a mordant of iron-rich mud.
A pomade is made by mixing the red wood powder with oil and its use as a body cosmetic is widely applied in DR Congo (‘ngula’). The roots can be prepared and used in the same way as the heartwood and yield a dye of equal or better quality. Pulverized bark, mixed with palm oil, is also used as a cosmetic pomade. The leaves are eaten as a cooked vegetable.
The powdered wood, baked with a slice of lime, is used in Gabon on wounds and, mixed with palm oil, raffia oil or vegetable butter (e.g. from seeds of Tieghemella africana Pierre), to treat skin diseases, ringworm and yaws. Probably partly due to its blood-red colour and the associated symbolism, it is also used in ritual ceremonies for circumcision, initiation, marriage, delivery and widowing. The bark contains a kino type resin (‘dragon’s blood’) which is very astringent and used to ward off skin parasites in ethnoveterinary medicine. In Gabon the resin is used (usually in combinations with parts of other plant species) as an enema to treat dysentery and against toothache, gonorrhoea and excessive menstruation. In Congo and the Central African Republic a bark decoction is drunk to treat dysmenorrhoea, uterine haemorrhage, dysentery and haemorrhoids. A pulp obtained by scraping the inner surface of the bark is applied as a wet dressing against inflammations, oedemas, incipient hernia and whitlow. Decoctions, draughts or vapour-baths of leaves and bark are taken against broncho-pulmonary affections.
Production and international trade
In Gabon Pterocarpus soyauxii is among the 10 most important export timbers. In Gabon, where the standing stock was estimated at 15 millions m³ in 1999, the minimum diameter limit for exploitation is 70 cm. Between 2000 and 2003, Gabon exported 120,000 m³ of African padauk logs each year, whereas in 1997 the export was only 57,000 m³. Cameroon, where the export of logs is prohibited, exported 6,500 m³ of sawn timber in 2003. The domestic use of the timber is high. In North America African padauk is available as lumber and veneer, and prices are high. In Europe sawn timber and veneer are available on a limited scale only.
Formerly the heartwood of Pterocarpus soyauxii was exported in great quantities in standard blocks as red dyewood from Cameroon and Gabon to Europe and North America and used in the wool-cloth and printed cotton industries. Because of its colour fastness, it continued to be used to dye wool-cloth until the beginning of the 20th century in the United States, and in England and France (particularly Elbeuf) to produce brick reds and red-browns or to give a ground for greys and blacks dyed with logwood. As a dye for cotton-cloth, Pterocarpus soyauxii with a tin mordant gave the traditional red colour of the printed bandanas that were so popular in England in the 19th century. The resinous consistency of the dye gave the handkerchiefs added weight, characteristic of these items.
The heartwood is bright red when freshly cut, becoming orange-red on exposure and darkening to purple-brown, and distinctly demarcated from the whitish to brownish yellow sapwood 6–10(–20) cm wide. The grain is straight to interlocked, texture coarse. The wood has a faint aromatic scent when freshly cut.
Pterocarpus soyauxii wood is moderately heavy, the density is (650–)675–815(–900) kg/m³ at 12% moisture content. The shrinkage rates are moderately low, from green to oven dry 2.2–3.8% radial and 4.1–6.2% tangential. The wood dries well but moderately slowly, with little risk of distortion. Once dry, it is very stable.
At 12% moisture content, the modulus of rupture is 101–218 N/mm², modulus of elasticity 10,800–15,900 N/mm², compression parallel to grain 54–79 N/mm², shear 7–8 N/mm², cleavage 11–18 N/mm and Janka side hardness 6850–8320 N.
The wood is moderately difficult to work. Stellite-tipped sawteeth are recommended. The wood takes a good finish, but sometimes with some tearing of interlocked grain. Slicing does not cause problems, and the wood holds nails and screws satisfactorily; however, pre-boring for screwing is advisable. The gluing properties are good. The dry sawdust may cause irritation to skin, nose and bronchia. The heartwood is durable and also resistant to fungi, Lyctus beetles, termites and marine borers; it is moderately difficult to impregnate with preservatives. The sapwood is less durable and moderately difficult to impregnate. The heartwood is rich in extractive substances by organic solvents. The ash, lignin and cellulose contents are moderate. Pentosane content is very low, similar to coniferous wood. Silica content is also very low. The vibration damping factor of the wood is 0.004–0.007 at frequencies of 200–500 Hz. The wood has little moisture uptake; in given conditions, its moisture content is nearly half that of ‘standard’ woods.
The heartwood of Pterocarpus soyauxii contains the red biflavonoids santalin A, santarubin A and santarubin B, isoflavonoids including pterocarpin, formononetin and prunetin, the isoflavanequinone claussequinone and the isoflavanes vestitol and mucronulatol. The low damping factor, shrinkage coefficient and moisture uptake of padauk heartwood appear to be linked to its specific composition in extractive substances, and sapwood has much higher values of these physical properties.
The wood is also rich in tannins, which contribute to mordanting in the dyeing process. In the Colour Index barwood is cited as a source of natural red No 22. Santalin is a histological dye, comparable in use to haematoxylin. In combination with an acid Fe or Al mordant, it selectively dyes cell nuclei, elastic tissues and striations in voluntary muscle fibres.
The leaves have a high ascorbic acid content even after cooking. The bark of Pterocarpus soyauxii showed antifungal activity against some pathogenic fungi.
Adulterations and substitutes
The wood of Pterocarpus osun Craib from southern Nigeria and Cameroon, Pterocarpus tessmannii Harms from Equatorial Guinea, Gabon and DR Congo, and Pterocarpus tinctorius Welw. from DR Congo and Angola are also marketed as African padauk. The dye of Pterocarpus soyauxii may be substituted for by the dyes of other insoluble redwoods, the best known of which are sandalwood (Pterocarpus santalinus L. from southern India), narrawood (Pterocarpus indicus Willd. from the Philippines and Myanmar) and camwood (Baphia nitida Lodd. from West Africa).
- Evergreen, sometimes deciduous large tree up to 55 m tall; bole straight and cylindrical, branchless for up to 20(–30) m, up to 140(–200) cm in diameter, slightly to prominently and highly buttressed; bark grey-brown to brown, peeling off in thin irregular flakes, exuding a red gum abundantly on slashing; crown dome-shaped, open; twigs brown hairy when young.
- Leaves alternate, imparipinnately compound with 7–17 leaflets; stipules linear, up to 2 cm long, hairy, falling off early; petiole 1–3.5 cm long, rachis 3.5–16.5 cm long, densely brown hairy; petiolules 3–5 mm long, shallowly furrowed; leaflets alternate to nearly opposite, obovate to elliptical, 2.5–9 cm × 1.5–4 cm, base rounded to obtuse, apex usually abruptly acuminate and mucronate, leathery, glabrous, with closely set, fine and obscure lateral veins.
- Inflorescence an axillary or terminal much-branched panicle 10–35 cm long, densely brown hairy; bracts linear, falling off at anthesis.
- Flowers bisexual, papilionaceous; pedicel 3–19 mm long, hairy; calyx campanulate, c. 7.5 mm long, densely hairy, with 5 triangular teeth 1–2.5 mm long, upper 2 more or less connate; corolla with clawed petals, bright yellow or orange-yellow, standard almost circular up to 13 mm × 10 mm, wings up to 12 mm long, keel up to 9.5 mm long; stamens 10, connate into a sheath up to 8.5 mm long, the upper stamen sometimes free; ovary superior, 1-celled, stalked, white hairy, style up to 4 mm long, glabrous, stigma terminal.
- Fruit a circular, flattened, indehiscent pod 4.5–9 cm in diameter, on a stalk up to 1 cm long and with a papery, finely veined wing with wavy or plaited margin, finely hairy, glossy brown, 1-seeded.
- Seed kidney-shaped, flat to slightly thickened, 12–16 mm × 5–7 mm, smooth, red when fresh, turning dull brown to black.
Other botanical information
Pterocarpus is a pantropical genus belonging to the tribe Dalbergieae, comprising 21 species of which 12 occur in Africa, 6 in America and 5 in Asia. Several Asiatic and African species were important commercial sources of red dyes but most species are now much more valuable for timber. The bark of most species yields a reddish medicinal resin of ‘kino’ type.
Pterocarpus tessmannii Harms is often confused with Pterocarpus soyauxii. It has similar wood and dye uses and properties, but grows in wetter parts of the forest. It can be distinguished by its winged, sickle-shaped to oblong fruits 11.5–13.5 cm × 4–5 cm.
Wood-anatomical description (IAWA hardwood codes):
- Growth rings: 2: growth ring boundaries indistinct or absent.
- Vessels: 5: wood diffuse-porous; 13: simple perforation plates; 22: intervessel pits alternate; 23: shape of alternate pits polygonal; (25: intervessel pits small (4–7 μm)); 26: intervessel pits medium (7–10 μm); 29: vestured pits; 30: vessel-ray pits with distinct borders; similar to intervessel pits in size and shape throughout the ray cell; 43: mean tangential diameter of vessel lumina ≥ 200 μm; 46: ≤ 5 vessels per square millimetre; 58: gums and other deposits in heartwood vessels.
- Tracheids and fibres: 61: fibres with simple to minutely bordered pits; 66: non-septate fibres present; 69: fibres thin- to thick-walled; 70: fibres very thick-walled.
- Axial parenchyma: 77: axial parenchyma diffuse-in-aggregates; 80: axial parenchyma aliform; 82: axial parenchyma winged-aliform; 83: axial parenchyma confluent; (85: axial parenchyma bands more than three cells wide); 86: axial parenchyma in narrow bands or lines up to three cells wide; (89: axial parenchyma in marginal or in seemingly marginal bands); 91: two cells per parenchyma strand; 92: four (3–4) cells per parenchyma strand.
- Rays: (96: rays exclusively uniseriate); (97: ray width 1–3 cells); 104: all ray cells procumbent; 116: ≥ 12 rays per mm.
- Storied structure: 118: all rays storied; 120: axial parenchyma and/or vessel elements storied; 121: fibres storied.
- Mineral inclusions: 136: prismatic crystals present; 142: prismatic crystals in chambered axial parenchyma cells.
Growth and development
In the Lope reserve in Gabon the trees flower from December–February and the fruits, produced in great quantity, ripen from January–April. Sometimes the tree is deciduous just before flowering. The winged fruits are dispersed by wind, but also by animals. Germination and seedling growth are rather fast. In plantations the annual increment of the wood in Nigeria has been estimated at 40 m³/ha. In trial plantations of 1.5 ha in Côte d’Ivoire in 1964 and 1968, striplings were transplanted at spacings of 4 m × 4 m and 5 m × 5 m. The first 7 years, annual height growth varied between 1.6 m and 2.7 m. The mean annual volume growth, including thinnings, was 20–30 m³/ha over a period of 15 years. The average annual diameter growth of the 150 largest trees per ha was 2.5 cm when 17 years old. Pterocarpus soyauxii fixes nitrogen in its root nodules.
Pterocarpus soyauxii occurs scattered or in small groups in evergreen and deciduous forest, from sea-level up to 500 m altitude. It prefers a moist but well-drained deep soil, an average annual rainfall of 1500–1700 mm and an average annual temperature of 23°C.
Propagation and planting
Propagation by seed is easy. The seeds are dried in the sun and the wings are removed. In Congo germination started 3 days after sowing and 92% of the seeds germinated within 30 days. In a test in Nigeria, 86% of the seeds with fruit wall removed and soaked overnight in water germinated in 7 days. The germination is hypogeal. Planting into the field is done about 40 days after sowing and is easy. Propagation by non-woody cuttings in normal topsoil gave 83% success. Seedling growth showed a greater response after inoculation of the soil with fungi from the rhizosphere of the mother tree than after inoculation with a similar spore number of fungi from a fallow field.
Pterocarpus soyauxii requires much light for good growth. Stump regrowth is weak and uneconomic for wood production.
Logs are liable to brittle heart. For dye extraction, preferably old and hollow trees are cut from the forest and the heartwood is lumbered out. Often trees are felled and left for 2–3 years lying on the forest floor before taking the heartwood for dyeing purposes. The roots are also harvested for dye extraction.
Handling after harvest
Freshly felled logs of Pterocarpus soyauxii usually do not float in water, and consequently cannot be transported by river. For dye production the heartwood is split into billets and chips which are dried and subsequently pounded into powder. A little oil is added to the pulverized material, which is moulded into cakes for stocking and for local sale. For dye export the heartwood is traded in standard blocks or bars (‘barwood’), making quality control easier.
Since the colorants present in the wood are difficult to dissolve in water, special methods of extraction were developed in the dyeing industry in 19th century Europe, boiling the wood for 1.5–2 hours in a solution of 45° alcohol or in water and carbonate of soda (30 g per 100 g of fibre to be dyed). This solution was then diluted with water and used as the dye bath.
Pterocarpus soyauxii is rather widespread in Central Africa and current exploitation rates do not seem to endanger the species. To safeguard genetic variability, protection measures of some natural forest where Pterocarpus soyauxii occurs are recommended.
Pterocarpus soyauxii is a useful multipurpose tree from forest areas in Central Africa, producing good-quality timber, dye, vegetables and medicine. Over-exploitation endangers natural populations. More research is needed on growth requirements. Solutions to make better use of and more profit from the chips, sawdust and bark as by-products of the timber, e.g. for dye-extracts and medicinal applications, should be encouraged. As it is quite a fast growing, nitrogen fixing and light demanding tree, it might be useful for agroforestry purposes (e.g. as shadow tree in coffee plantations). The feasibility of establishing commercial plantations also deserves research.
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Correct citation of this article
Jansen, P.C.M., 2005. Pterocarpus soyauxii Taub. In: Louppe, D., Oteng-Amoako, A.A. & Brink, M. (Editors). PROTA (Plant Resources of Tropical Africa / Ressources végétales de l’Afrique tropicale), Wageningen, Netherlands. Accessed 11 April 2019.
- See the Prota4U database.