Since wood can be difficult to cut in exact pieces with the right curves and flowing lines needed to create a smooth veneer, bending the wood through other means without actually cutting it has been a long-established practice in woodworking. Although this process saves all the wood from the strips being used, it also takes time and the right materials. Since every piece of wood is different and has its own characteristics and flaws, the more skill the woodworker has, the more easily the wood can be curved.
Steam Boxes and Steaming
Today, wood is curved using either chemicals, mechanical means or steam. Steam is the cheapest and most readily available method, so it has stayed the most popular. Thin wood strips used in veneer are of course much easier to bend than thick plywood samples, but the process remains very similar. The first step involves a steam box: first the wood is soaked in water, then placed in an specially designed container called a steam box (which may also be a pipe or a can or some other container). These boxes vary greatly depending on the wood and size of the planks being bent, but they have two primary openings. On one end steam is pumped into the box, raising the pressure, heat and moisture inside. The pressure never gets too high, though, because the box is also drilled with a small number of ventilation holes, usually located on the other end, so that the steam can escape.
The time it takes to steam wood is usually determined by its thickness. A rule of thumb used by many woodworkers is one hour per every inch of thickness in the wood. This allows the steam to penetrate deep into the wood and activate many of the flexible cellulose cells toward the center of the timber that have dried out. Active again, these cells provide enough give that the wood can be conformed to other shapes, but time is key. When ready, the wood is immediately moved from the steam box and bent into place.
Bending methods differ from woodworker to woodworker. Many use a bending jig for simple curves, which bends the strip of wood alone a semi-circular wood base and holds it there in the right position with a top section that is carefully bolted down on top of the wood. For more complicated patterns that require waves or multiple curves in one piece of wood, a mold is created. These molds are made out of foam or plaster and have a top and bottom half. In between a section is cut out in the exact shape and pattern that the wood needs to be bent in. The wood is placed in this space, and the two sections of the mold are clamped down tight on it. The wood is kept in this position until dry, then carefully taken out and treated.