Finally, the pieces should be accurately cut to length so the timber can be firmly held in place during bending and holes for bolts or screws should be pre-bored to allow for greater working time when the timber is hot. Softening the timber Timber can be softened for bending by heating the pieces at air pressure to approximately 100 degrees centrigrade in a steam-tight box called a steaming chamber. Very long periods of steaming are not desirable as they over-soften the timber resulting in a greater chance that it will buckle when bent. Prolonged steaming also great increases the moisture content of the wood, thus accentuating difficulties such as shrinkage, checking and warping during drying. In comparison to time, the temperature at which the wood is steamed is a critical factor that will influence the results of bending significantly and this should be kept constant at 100 degrees centrigrade.
Steamers are generally inexpensive to run, relying on gas or even on timber off-cuts as a fuel.
The steaming procedure can also be used to colour the timber. This can be achieved through fumigation with ammonia while the wood is being steamed. The ammonia affects the tannins in the wood which ultimately darkens the colour of the timber. The principles of wood bending In its natural state wood has a rather limited capacity to bend as it exhibits elastic properties only over a limited stress range. When timber is bent there is a stretching or tensile force on the convex face and a compressive force on the concave face.