Balsa Wood

Balsa wood is one of the most unique types of wood on the market. Although it is technically considered a hardwood, it is the softest and lightest wood commercially harvested and sold. Harvested from the Balsa tree, which grows throughout Central and South America, this wood has little structural mass. The tree, usually harvested at 60 to 90 feet in height (and can grow higher) and as much as six feet in diameter, is held up predominantly by the high amount of water in the wood’s cells. Trees reach a point where they are harvestable in as little as five years, with a maximum maturity of 15 years. After this point, the heartwood breaks down and becomes waterlogged, and the sapwood becomes hard.

The Ochroma Pyramidale tree, otherwise known as the Balsa tree, is largely harvested by hand. An axe is used to cut down the tree in much the same way that lumberjacks worked long before chainsaws came into being. The felled trees are dragged to the water by oxen, where they are formed into rafts and floated downstream to where they are converted to lumber.

Freshly cut balsa wood has a very high moisture content, 200% to 400%, requiring kiln drying of green timbers before it is converted to usable boards. Due to this high moisture content, there is a high tendency for the wood to warp and twist while drying. Once kiln-dried to a 17% moisture content over 4-1/2 months, it can be shipped to the finish mill, where it is converted into usable boards and sticks. While green wood warps, cups, and twists quickly when drying, the wood is relatively stable once dried.

The resulting wood is extremely light, flexible (especially across the grain), and vital for weight. Compared to other hardwoods, it is relatively expensive, primarily due to the large amount of hand work that has to go into harvesting it and kiln drying it. While balsa is sold in ¾” thickness for making surfboards, it is mainly sold in thin sheets and sticks for model building, along with some blocks for carving.

Balsa Wood for Models

Balsa was first used commercially in the 1920s. At that time, it was sought after as a replacement for cork. It is highly buoyant, explaining why it was once a popular wood for making surfboards. It has also been used for packing material, almost like Styrofoam, and as an insulating material. Seed fiber from the tree is used as stuffing for mattresses and cushions.

During World War II, balsa was the primary structural material to build the British de Havilland Mosquito aircraft. This was considered the best British aircraft made during World War II. Originally designed to be a light bomber, the plane was quickly adapted to other purposes, including as a submarine hunter. Its great advantage, speed, and agility were at least partly due to its lightweight, thanks to balsa; but at the same time, it was balsa that was its downfall, as the airplanes would rot in the hanger, especially in the damp climate of the tropics.

But airplanes weren’t the only places that balsa was found useful; it was also used for manufacturing boats. Today, some high-speed wood boats are still made from balsa, laminated with a hardwood veneer. The combination of the hard veneer coating and the light core helps make boats that are tough, light, and resilient to damage. The flexibility of the balsa is very good at withstanding the pounding of the waves on the ship.

But balsa wood found its real home in model building, more than anywhere else. This started in 1926 when a WWI US Navy aviator built a line of model gliders and airplanes out of his barn. This grew into the Nu Craft Toy company. Balsa is used for both model airplanes and boats. Its lightweight, strength, and flexibility make it the ideal material for model airplanes, which must withstand hard landings and crashes quickly.

But that’s not the only kind of model building that balsa is used for. There is a wide range of professional model applications where it is used, such as architects building models of their proposed projects. Before computer modeling became so sophisticated, balsa models of bridges were used for stress testing of new bridge designs. Even the film industry uses balsa extensively, making both models and full-sized furniture out of it. A balsa wood chair is less likely to injure an actor when it is broken over his head, and if he is knocked down into a table made of balsa, it will collapse nicely under him, cushioning his fall.

Another exciting use of balsa wood is in the manufacture of string instruments. These require lightweight, strong wood, and balsa fits the bill perfectly. Balsa is easy to shape and can be made into ribs for the inside of everything from violins to guitars. It’s hard to tell, but many fishing lures are made with a painted balsa wood body.

Making Balsa Wood Stronger

While balsa wood is vital for its weight, it is not as strong as other types of wood. However, there are several ways of strengthening it, such as making plywood out of it—thin balsa wood plywood, as thin as 1/64” thick and as thick as 3” thick. As with any other plywood, balsa plywood is considerably stronger than balsa boards, although it can be expensive. Surprisingly, the thinnest balsa plywoods are the most costly.

But the woodworker can also make balsa wood stronger. One is to use a wood hardener. This is a liquid product designed for use on rotting wood to protect it from further rot. It penetrates into the wood, where the hardener’s resins bind with the wood fibers, strengthening them.

To use a wood hardener to strengthen balsa, it is best to immerse the balsa pieces in the hardener, holding them with tongs to ensure they get fully submerged. Hold the wood pieces there until they stop bubbling, then remove them. At this point, the pieces can no longer soak up any more of the hardener. Set them on waxed paper to dry, flipping them over as per the instructions on the container. Be sure to dry both sides evenly.

Another method for strengthening balsa is to cut two mirror-image components out of half the desired thickness of the material. Then, use epoxy to glue the two pieces together. Even though the grain on both pieces will run in the same direction, adding the epoxy layer will strengthen the finished part.

Working with Balsa Wood

Even though balsa wood is considered a hardwood, it is in a category all by itself. Regular woodworking techniques, significantly cutting techniques, will likely create nothing but scrap on balsa. Because of its softness, care must be taken with it.

Balsa should always be cut with a sharp knife, such as a hobby or crafts knife. Most of the cutting is done with the blade’s tip, so ensure that the tip is sharp before using it. Due to how much of their use involves cutting with the tip, the blades of these knives can appear to be sharp long after the tip has become dull.

It is best to use a straight edge or other robust template as a guide to set the knife against when cutting. Do not apply too much pressure to the knife; rather, cut a series of strokes, each cutting farther through the wood. If too much blade pressure is applied, the wood fibers will crush rather than cut.

Another place where pressure can cause problems is clamping balsa wood. Due to its light density, the wood can crush easily, so normal clamping pressure will leave a mark on its surface. If clamps are needed, place cauls over the sides of the wood to be clamped and then apply the clamps to the cauls, using no more than light pressure.

Regular PVA wood glue, cyanoacrylate adhesives, and epoxy can be used to assemble balsa wood. The wood’s cell structure makes it extremely easy to glue. When gap filling is needed, epoxy is the best choice. The adhesive will be considerably more vital in these cases than the wood.

Joinery is essential when working with balsa, as the wood has limited strength. Mortise tenon and lap joints are the most common methods used, both of which can be cut with the hobby, as mentioned earlier, and craft knives.