[ bending wood ]

Musical instruments have some of the most complicated shapes of any objects made in wood, and as a result, require techniques that are rather uncommon in most areas of woodwork. One such technique is that of bending wood. There only a few ways that wood can be bent without causing undue damage.

1. Kerfing.

In this technique, lots of parallel slots are sawn quite close together and almost all the way through the thickness of the material. This weakens one side so much that the wood can be bent cold. This technique is often used to make the linings for a guitar. The resulting strip is very weak, as it has become virtually a succession of separate little blocks, but it is ideal in the case of guitar linings.

2. Heating

. Some woods possess a property most wonderful for luthiers. They behave almost exactly like thermoplastics, so that on heating they can flow into a different shape and then set in that shape. This is rather like re-heating a bowl of cold spaghetti. The individual strands or fibres become more flexible and can slip over one another. On cooling, friction increases and the material sets in its new shape. Before the invention of electricity, luthiers would bend wooden parts round the flue of a stove. Just like ironing your shirt, if the iron is too cool, nothing very much happens, while if it is too hot, it leaves nasty scorch marks. Through the wonders of electricity, we now have thermostatically controlled bending irons to make life easier. They look like small metal cans and the heat can be very accurately controlled. Steam and dry radiant heating work just as well, and are sometimes used.

3. Soaking.

Wood and paper are related, so it is easy to see what happens when wood is soaked in water. The change is less extreme in wood than in paper, but the timber cells absorb water and swell up, becoming more floppy. This allows the plank to be bent cold with less damage than would otherwise happen. This technique was sometimes used by harpsichord makers, to make bentsides, when they would then attach the plank to the rigid framework of the spinet to hold it in shape. Interestingly, the wood does not hold its shape as in hot bending, and even centuries later, if it removed from the rigid support, the panel will become more or less straight again.

4. Forming

This is a technique more common to furniture makers, but is sometimes used for instruments as well. The principle is this: when materials become very thin, they become increasingly flexible. You need only think of a 12 inch ruler. In one orientation it is very stiff, but if you flex it where it is thin, it bend very easily. So it is with wood. If the material is sawn into lots of almost veneer- thin pieces, then all the stiffness is lost. The wood is now like the pages of a book - each can slide easily over its neighbours. The trick is to glue the "pages" back together in the curve that you want. This is done over a solid shaped block, which is not surprisingly called a former. When the whole pack of veneers is set, the new shape is very strong indeed. The only clue that the material is not a single piece can be found if you look at the edge. Here the grain is too stripy and does not flow continuously as in a piece of solid wood. This is the way that most modern bentwood chairs are made