| Ochroma | |
|---|---|
_(14723405443).jpg) | |
| Ochroma pyramidale inColombia | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Clade: | Angiosperms |
| Clade: | Eudicots |
| Clade: | Rosids |
| Order: | Malvales |
| Family: | Malvaceae |
| Subfamily: | Bombacoideae |
| Genus: | Ochroma Sw. |
| Species: | O. pyramidale |
| Binomial name | |
| Ochroma pyramidale | |
| Synonyms[2] | |
Bombax pyramidaleCav. ex Lam. | |
Ochroma pyramidale, commonly known asbalsa, is a large, fast-growingtree native to the Americas. It is the sole member of thegenusOchroma, and is classified in the subfamilyBombacoideae of the mallow familyMalvaceae.[1] The tree is famous for its wide usage inwoodworking, due to its softness and its high strength compared to its low density. The namebalsa is the Spanish word for "raft"[3] and the Portuguese word forferry.
A deciduousangiosperm,Ochroma pyramidale can grow up to 30 metres (100 feet) tall, and is classified as ahardwood despite the wood itself being very soft; it is the softest commercial hardwood and is widely used because of its light weight.
Balsa trees grow extremely fast, often up to 27 metres (90 feet) in 10–15 years, and do not usually live beyond 30 to 40 years. In terms of volume (as opposed to height) they may be the fastest growing tree known; Streets mentions one individual which grew 11.2 m (37 ft) tall and 17 cm (6.7 in)diameter at breast height during a period of fifteen months.[4] Balsa, like mostrainforest trees, does not makeannual rings, but this growth is equivalent to rings 7 cm (2.8 in) wide. They are often cultivated in dense patches, withEcuador supplying 95% or more of the commercial balsa. The wood from these trees is highly valuable due to its high strength-to-weight ratio, which is achieved through a kiln-drying process that leaves the wood's cells hollow and empty.
Balsa wood is popular for light, stiff structures in model bridge tests, model buildings, and construction of model aircraft. It is also used in the manufacturing of wooden crankbaits for fishing, makeshift pens for calligraphy, composites, surfboards, boats, "breakaway" props for theatre and television, and even in the floor pans of theChevrolet Corvette. Balsa wood played a historical role in Thor Heyerdahl'sKon-Tiki expedition where it was used to build the raft. Balsa wood is also popular in arts such as whittling, and in the making of baroque-style picture frames due to its ease of shaping.
A member of the mallow family,Ochroma pyramidale is native from southern Mexico to southern Brazil, but has been introduced to many other countries, including Papua New Guinea, Indonesia, Thailand, and the Solomon Islands. It is apioneer plant, which establishes itself in clearings in forests, either man-made or where trees have fallen, or in abandoned agricultural fields. It grows extremely rapidly, up to 27 metres (90 ft) in 10–15 years. The speed of growth accounts for the lightness of the wood, which has a lower density thancork. Trees generally do not live beyond 30 to 40 years.[5]
Flowers are produced from the third year onwards, typically at the end of the rainy season when few other trees are in flower. The large flowers, up to 20 cm (8 in) in diameter,[6] open in the late afternoon and remain open overnight. Each may contain a pool of nectar up to 2.5 centimetres (1 in) deep. Daytime pollinators includecapuchin monkeys. However, most pollination occurs at night; the main pollinators were once thought to be bats, but recent evidence suggests that two nocturnal arboreal mammals, thekinkajou and theolingo, may be the primary pollinators.[5]
It isevergreen or dry-seasondeciduous, with large 30 to 40 centimetres (12 to 16 in), weaklypalmately lobed leaves.
Being a deciduousangiosperm, balsa is classified as ahardwood despite the wood itself being very soft; it is the softest commercial hardwood.
Ecuador supplies 70% or more of commercial balsa. In recent years, about 60% of the balsa has beenplantation-grown in densely packed patches of around 1000 trees per hectare (400 per acre) (compared to about two to three per hectare/around one an acre in nature). The trees are harvested after six to ten years of growth in Ecuador. The remaining volume of balsa is harvested from plantations in Papua New Guinea; the climate is different, therefore harvesting occurs at 4-5 years of age.
Balsa wood is very soft and light, with a coarse, opengrain. Thedensity of dry balsa wood ranges from 40 to 475 kg/m3 (2½ to 30 lb/ft³), with a typical density around 160 kg/m3 (10 lb/ft³).[7][8][9] Balsa is the softest wood ever measured using theJanka hardness test (22 to 167lbf).[10] The wood of the living tree has large cells that are filled with water. This gives the wood a spongy texture. It also makes the wood of the living tree not much lighter than water and barely able tofloat. For commercial production, the wood is kiln-dried for about two weeks, leaving the cells hollow and empty. The large volume-to-surface ratio of the resulting thin-walled, empty cells gives the dried wood a large strength-to-weight ratio because the cells are mostly air. Unlike naturally rotted wood, which soon disintegrates in therainforests where balsa trees grow, the cell walls of kiln-seasoned balsa wood retain their strong structure ofcellulose andlignin.[11]
Because it is low in density but high in specific strength (strength per weight), balsa is a very popular material for light, stiff structures inmodel bridge tests, model buildings, and construction ofmodel aircraft; all grades are usable for airworthycontrol line andradio-controlled aircraft varieties of the aeromodeling sports, with the lightest "contest grades" especially valuable forfree-flight model aircraft. However, it is also valued as a component of full-sized light woodenaeroplanes, most notably theWorld War IIde Havilland Mosquito.[11]
Balsa is used to make woodencrankbaits for fishing, especiallyRapala lures.
Sticks of dried balsa are useful as makeshift pens forcalligraphy when commercial metal nibs of the desired width are not available.[12]
Balsa wood is often selected as a core material incomposites. Because balsa grows quickly and tolerates poor soils, it is lower in cost per performance compared to polymer foams likeEPS while having better tensile strength than typical foams. For example, the blades ofwind turbines are commonly constructed of many balsa plywood cores and internal spars covered with resin infused cloth on both sides. Intable tennis rackets, a balsa layer is typically sandwiched between two pieces of thinplywood made from other species of wood. Balsa wood is also used inlaminates together withglass-reinforced plastic (fibreglass) for making high-quality balsasurfboards and for thedecks and topsides of many types ofboats, especially pleasure craft less than 30 metres (100 feet) in length. On a boat, the balsa core is usuallyend-grain balsa, which is much more resistant to compression than if the soft balsa wood were laid lengthwise.
More than 90% of the world's Balsa wood volume is prepared into end grain panels for the composites industry, mostly used as structural cores in the wind turbine blades, where strength, rigidity, durable and environmentally sustainable materials are sought after.
Balsa is also used in the manufacture of "breakaway" wooden props such as tables and chairs that are designed to be broken as part of theatre, movie, and television productions.
Thefifth andsixth generations of theChevrolet Corvette hadfloor pans composed of balsa sandwiched between sheets ofcarbon-fibre reinforced plastic.[11]
The Norwegian scientist and adventurerThor Heyerdahl, convinced that early contact between the peoples ofSouth America andPolynesia was possible, built the raftKon Tiki from balsa logs, and on it his crew and he sailed thePacific Ocean fromPeru to the PolynesianTuamotu Archipelago in 1947. However, theKon Tiki logs were not seasoned and owed much of their (rather slight) buoyancy to the fact that their sap was of lower density than sea water. Thisserendipitously may have saved the expedition, because it prevented the seawater fromwaterlogging the wood and sinking the raft.[13]
Balsa wood is also a popular wood type used in the arts ofwhittling,[11] andsurfing. In the making of picture frames, balsa was often used in a baroque style because of the ease of shaping the design.
In parts of Africa and south America the leaves of the balsa tree are used to enhance the traditionalpanning method of extracting gold from ore. When mixed with water a soapy solution is produced and this helps the lighter, unwanted material to wash away.[14][15]
Media related toOchroma at Wikimedia Commons
