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There are two methods you can use to soften or plasticize wood. Steaming is a popular means of bending wood in order to make items like chair parts or staircase banisters, and is also sometimes used in the production of canoes, musical instruments, and baskets. The alternative is 'kerfing', which entails making a series of cuts in the wood. The downside of kerfing is that it weakens the wood, but it is ideal for decorative purposes.
How to Steam Wood
Wood with a relatively high moisture content responds best to steaming, and you should choose a hardwood over a soft wood. If the wood is not already moist, then you can add moisture by pre-soaking it. Woods that are damp cope better with heat transfer.
Wood is steamed using a steam box, which is connected to a steam generator using a hose. Steam boxes may be made from either PVC or wood, and need to be large enough to accommodate the whole piece of wood which is to be steamed. Large boxes will feature racks in order to support the wood. The steam box will be nearly airtight, but do need to have one or two small holes in order for steam to escape.
A steam generator can be created with a simple kettle, however you can purchase purpose made products as well. To use a kettle as a steam generator, you need to ensure the hose fits snugly over the end of the kettle, and is well secured. The steam needs to flow uninterrupted in order to steam the wood properly. Most importantly, be very careful when handling any steam generator – if the steam touches your skin it can cause severe burns.
It takes around one hour to steam an inch of wood (aim to heat the wood to 212 degrees Fahrenheit). This uses a lot of water (far more than you probably think) so make sure to monitor water levels in the steam generator.
When the desired temperature has been reached, and the wood has been steamed for long enough, position it on a bending clamp, and secure with straps. Start bending the wood right away, before it begins to cool. Work slowly and carefully, working your way from one end of the wood to the other. Once the desired shape has been achieved, secure the wood in position, and leave to cool and set for around 24 hours.
How to Bend Wood with Kerf Cutting
The term “kerf” refers to the width of the teeth in a saw blade. Kerf cutting entails making deep cuts into a piece of wood, resulting in wood that bends. To be effective, the wood needs to be cut almost all the way through, and the cuts should be made quite close together. When kerf cutting wood, take note of the teeth width because a gap of this width will remain.
After the cuts have been made, you can attempt to bend the wood. If done properly, the wood should bend in the same way a straw bends. If the wood resists being bent, or feels as if it may snap, you have not cut deeply enough into each kerf. When bending the wood, the kerf cuts should be on the inside of the curve; the spaces will close as the wood bends.
Some Final Tips
When choosing wood for steaming, try to select pieces with a straight grain. Cross-grained wood is more likely to crack. Greenwoods are apopular choice, although white oak, red oak and hackberry bend well too. Soak the wood for at least a day before you treat it.
Kerf cutting is labour intensive and requires careful concentration. Use a sharp saw and work slowly and methodically. Check the cuts regularly so that you don’t accidentally cut all the way through the wood. Use a wide saw for a steep angle, or a narrower saw for more gentle bends.
For both kerf cutting and steaming, it is a good idea to try the process on some test pieces of wood so that you get used to how long to treat the wood, or how deep to make the cuts in the case of kerf cutting. If the wood cracks, then you know you need to steam it for longer, or make more cuts.