Steam bending is the process of using steam on wood in order to make it more flexible. The wood can then be bent around a mold, which will give it the desired shape. Steam bending is a common method employed in woodworking, and is frequently used in fields such as shipbuilding, furniture making, and musical instrument construction.
The science behind steam bending is that the heat softens the wood's hemicelluloses, which are polymers found in it. Since these hemicelluloses have resin-like properties, when they soften, the piece of wood can be bent. Some of the more common woods used in steam bending include oak, ash, mahogany, and walnut. In general, hard woods tend to bend more easily than soft woods.
A steam box can be used for steam bending. This is a specially designed container into which wood is placed, in order to achieve maximum results. The size of steam boxes can vary according to the type of wood that is to be used. Many times, wood is soaked in a water solution before being added to the steam box. This makes the wood moister, and when the heat in the steam box draws this moisture out, it becomes more flexible.
Steam boxes are usually not airtight. Steam enters the box through one end, and on the other end, a small hole is drilled so that excess pressure can escape. The amount of time a piece of wood is placed in the steam box generally varies according to its size and type. A general rule, however, is that the wood needs to be steamed at a ratio of one hour for every inch (2.54 cm) of thickness of the timber.
After the wood has been steamed, it is generally removed from the steam box and placed into a jig-type device. This fixes the wood in place, allowing it to be bent in the desired direction. Molds may also be used in order to give the wood a specific shape, and are often employed when the same shape has to be made many times. Sometimes it is necessary to bend the wood as quickly as possible once it is removed from the steam box, since the steam-induced flexibility of the wood may dissipate as the wood becomes dry.
Steam bending has a long tradition in the history of woodworking. It is believed that Vikings were the first to use this method. Steam bending was an ideal technique for producing the curved wood needed for their boat construction.