A steam box is needed to bend wood to rounded shapes without breaking the fibers of the grain. Under no circumstances should the steam chamber be pressurized or allowed to become pressurized should, for example, the drain holes become clogged. Some put the steam inlet at one end and incline the box up to an exit hole: the steam travels along the box this way, but looking at the majority of steam box’s built most seem to have the inlet in the middle. It will allow the steam pressure inside the steam box to force the water out of the wooden box. The steam should be able to surround your wood-piece and flow through the steam box. Except for the outermost layer (cambium), wood is dead, serving the plant for structural support and water storage and transport.
If the wood has been steamed for the allotted amount of time and cracks when it is bent, then it has been under steamed. The hoop must be tied until the wood has dried completely for it to hold its shape. Some builders use good quality plywood to make the steam box: while the glue may be WBP cheaper plywood may have ‘voids’ of trapped air, which when heated could expand, pressurise then lead to a delaminating box. The other half and I are fiercely debating this one-he is convinced that the wood will warp if we use steam to remove paint and glazing putty.
I may try using a metal strap on the outside of the wood as well, as research shows that it compresses the inside wall of the wood more than expanding the outside edge, thus leading to less breakage. The first three failures can frequently be prevented by bending in a slow, steady motion. I think it would work well for your first attempts, since it would be a little cheaper and easier than building a box out of wood. The choice of steaming method is determined by the size and dryness of the wood to be bent, the method of bending, and the available options.
Susan, sandblasting usually scars the wood surface and I have seen irreparable damage to all kind of wood and brick from sandblasting. Alternatives to steambending – You may not need to use steam to bend ribs or other framing members. You will want your steam box (or pipe) to be slightly tilted up from the opening. The same goes for the Mike Dunbar Ultimate Steam Box” and in the setup shown in the Veritas Tools folder on steaming.
Wood is better at compressing than stretching, so if you keep the outside edge from stretching and tearing, you should be able to apply more bend without cracking. I have seen a steam box made from ABS pipe, the chairmaker that showed it to me claimed it worked well. This means steam (typically rising) is away from my hands when the door is opened: a top opening door will expose the builder’s hands to steam or a bottom opening door will expose hands dripping condensed hot-water.