Ochroma

"Balsa" redirects here. For other uses, seeBalsa (disambiguation).

Ochroma
Ochroma pyramidaleat the Large Tree Habitat atTropical Gardens of Maui,Iao ValleyRoad,Maui,Hawaii,United States
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 (Cav.exLam.)Urb.
Synonyms
Bombax pyramidaleCav. ex Lam. Ochroma bicolorRowlee Ochroma concolorRowlee Ochroma lagopusSw. Ochroma obtusumRowlee

Ochroma pyramidale,commonly known as thebalsa tree,is a large, fast-growingtreenative to the Americas. It is the sole member of thegenusOchroma.The tree is famous for its wide usage inwoodworking,due to its softness and its high strength compared to its low density. The namebalsais the Spanish word for "raft."

A deciduousangiosperm,Ochroma pyramidalecan grow up to 30 m tall, and is classified as ahardwooddespite 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 in 10–15 years, and do not usually live beyond 30 to 40 years. They are often cultivated in dense patches, with Ecuador 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 expeditionwhere 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.

Biology

Balsa onBota Hill,Limbe Botanical Garden,Cameroon

A member of the mallow family,Ochroma pyramidaleis native from southern Mexico to southern Brazil, but can now be found in many other countries (Papua New Guinea, Indonesia, Thailand, 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 (89 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.

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 open in the late afternoon and remain open overnight. Each may contain a pool of nectar up to 2.5 centimetres (0.98 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, thekinkajouand theolingo,may be the primary pollinators.

It isevergreenor dry-seasondeciduous,with large 30 to 40 centimetres (12 to 16 in), weaklypalmatelylobed leaves.

Being a deciduousangiosperm,balsa is classified as ahardwooddespite the wood itself being very soft; it is the softest commercial hardwood.

Cultivation

Ecuadorsupplies 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 (compared to about two to three per hectare 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.

Uses

Three different sizes of balsa wood stock

Balsalumberis very soft and light, with a coarse, opengrain.Thedensityof dry balsawoodranges from 40 to 475 kg/m³,with a typical density around 160 kg/m³.Balsa is the softest wood ever measured using theJanka hardness test(22 to 167lbf).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 therainforestswhere balsa trees grow, the cell walls of kiln-seasonedbalsa wood retain their strong structure ofcelluloseandlignin.

Because it is low in density but high in specific strength, 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 lineandradio-controlled aircraftvarieties 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.

Balsa is used to make woodencrankbaitsfor fishing, especiallyRapalalures.

Sticks of dried balsa are useful as makeshift pens forcalligraphywhen commercial metal nibs of the desired width are not available.

Balsa wood is often selected as a core material incomposites.Because O. pyramidale grows quickly and tolerates poor soils it is lower in cost per performance compared to polymer foams likeEPSwhile having better tensile strength than typical foams. For example, the blades ofwind turbinesare commonly constructed of many balsa plywood cores and internal spars covered with resin infused cloth on both sides. Intable tennisrackets, a balsa layer is typically sandwiched between two pieces of thinplywoodmade from other species of wood. Balsa wood is also used inlaminatestogether withglass-reinforced plastic(fiberglass) for making high-quality balsasurfboardsand for thedecksand topsides of many types ofboats,especially pleasure craft less than 30 m in length. On a boat, the balsa core is usuallyend-grainbalsa, 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.

Thefifthandsixthgenerations of theChevrolet Corvettehadfloor panscomposed of balsa sandwiched between sheets ofcarbon-fiber-reinforced plastic.

Norwegian scientist and adventurerThor Heyerdahl,convinced that early contact between the peoples ofSouth AmericaandPolynesiawas possible, built the raftKon Tikifrom balsa logs, and upon it his crew and he sailed thePacific OceanfromPeruto the PolynesianTuamotu Archipelagoin 1947. However, theKon Tikilogs were not seasoned and owed much of their (rather slight) buoyancy to the fact that their sap was of lower density than sea water. Thisserendipitouslymay have saved the expedition, because it prevented the seawater fromwaterloggingthe wood and sinking the raft.

Balsa wood is also a popular wood type used in the arts ofwhittling,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 traditionalpanningmethod of extracting gold from ore. When mixed with water a soapy solution is produced and this helps the lighter, unwanted material to wash away.

Gallery

• 
  Painting by Frances W. Horne from theFlora Borinqueña
• 
  Ochroma pyramidaleatTropical Gardens of MauiIao ValleyRd,Maui
• 
  Two balsa rafts and a kayak at Lagos de Montebello inChiapas, Mexico
• 
  Balsa construction in a "stick and tissue"free-flight rubber scalemodel airplane

See also

• Tilia,another tree producing lightweight lumber (especiallyTilia americana)
• Paulownia

External links

• Media related toOchromaat Wikimedia Commons