Steam bending of wood; embellishments to an ancient technique

Abstract

Bending wood dates back to antiquity in the form of baskets from willow branches. Fresh growth willow twigs are readily bent into practically any shape. When wood has been separated from the tree and dried it is more rigid, difficult to bend, and breakable. Bending drier wood with the help of heat and water is centuries old. Fishing hooks, barrel staves, and planks turned into boat hulls are examples. Steamed wood is less rigid, since adding moisture and heat to wood results in plasticization. Steaming at atmospheric pressure is common, wherein diffusion prevails as the predominant mechanism governing moisture movement. Applications using conventional atmospheric steaming can be time consuming, non-uniform, and can result in failures. Vacuum steam technology offers a promising method that utilizes pressure differentials to accelerate the addition of steam to wood due to water vapor bulk flow and subsequently an accelerated temperature rise. More uniform plasticization results in less breakage of the wood.