Any piece of wood bent or bowed within the limits of its normal stress range, i.e. where the neutral axis between the stretched convex face and the compressed concave face is roughly equal (316:1) will revert to its normal shape immediately the strain is released, owing to the natural elasticity of its fibres. If the limiting stress according to the wood species is exceeded (316:2) some permanent deformation will occur, but there is always a danger that the fibres will be disrupted (316:3). If, however, the wood is rendered semi-plastic either by heating it with live steam or immersing it in boiling water or heated wet sand, its compressibility ratio will be greatly increased, and provided it is locked in position until the fibres have cooled and dried it will set rigid to the desired curve. There will be some recovery as the created internal stresses equalize themselves, but while this slight tendency to straighten out cannot be precisely calculated, a test-piece in the same species and of the same dimension will usually show the extent of the movement.
In practice prolonged immersion in boiling water or heated wet sand is never as efficient as steam bending, and the latter is to be preferred. The technique is straightforward and all that is required is a sufficient head of steam at atmospheric pressure to heat the wood thoroughly to 212° F (100° C), and to maintain that temperature for at least 45 minutes for every inch of thickness, i.e. a 11/4 in (32 mm) section will require approximately one hour's steaming. A simple apparatus using a large kettle or oil-drum, a source of heat (Primus stove, coke fire, etc.), a length of rubber hosepipe and a wooden box or metal or earthenware pipe sealed at each end with wooden bungs is shown in 316:4, and is quite sufficient for the occasional bend. Production in quantity would, of course, call for more sophisticated methods if consistent quality with low waste factors are to be achieved.
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