Balsa

(Ochroma pyramidale)

Like many children of a certain generation, my introduction to balsa came in the form of a model airplane kit. Its low weight and high strength made it the perfect choice for miniature wing spars and other model parts.  Historically, though it was its high buoyancy which first attracted our interest and use (Balsa is Spanish for raft). Thor Heyerdahl’s Kon-Tiki expedition used replicated balsa rafts to argue that the original people of South America populated Polynesia.

Balsa grows in the humid forests of Central and South America. Ecuador provides ideal habitat and most balsa production comes from that country. It is a pioneer specie, taking advantage of clearings and open space both manmade and natural. Fast growing, it can attain heights of 10 to 12-feet just 6 months after germinating. Mature trees can grow to 100-feet with diameters of 4-feet. Most trees are harvested in the first 6 to10 years as the mature trees produce an inferior product.  he heart is reddish brown and is not usually cut into lumber. The off-white heart is the commercially valuable product and is the balsa we are familiar with. Ecologically, balsa is considered a ‘nurse’ specie. The large palmate leaves provide shelter from the tropical sun for slower growing forest trees.

Balsa’s light weight derives from its unique anatomy.  The cells of the tree are large and thin-walled yielding a high ratio of open space to solid wood. Balsa is also low in lignin: the heavy glue-like substance which connects the trees cells together. These features yield a tree where only 40% of its volume is solid material.

The tree must compensate for this lack of structural integrity. Balsa pumps its cells full of water, in this manner producing a rigid structure. Green balsa can have 5 times as much weight by water as it has actual wood. Balsa must be carefully kiln dried to remove all of this water.

Balsa machines easily using sharp cutting tools. Dull tools will produce a fuzzy surface. Obviously, it has low nail holding capability and gluing is the recommended method for joining pieces of balsa. The lumber is coarse grained and stains and paints easily, although multiple coats of finish are necessary due to its porosity.

Originally used for rafts and buoys, demand surged during World War I as it was used as a cork substitute. Designers and engineers have since taken advantage of its high strength / low weight properties. Wind turbine blades and 5th generation Corvette floor pans have utilised laminated balsa panels. Surfboards, table tennis paddles, structural models and the famous British World War II Mosquito bomber incorporated balsa into their designs. It carves well and is considered a fine wood for whittling. So balsa is more than a convenient wood for model airplane enthusiasts. It has a glamorous side as well; most of the breakaway furniture and set pieces in the movies are made from balsa. Not bad for a fast growing and short lived tree once considered a nuisance weed.


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