Steaming is an effective method of softening or plasticizing wood so that it may be molded for craftsman and industrial applications, such as the manufacture of furniture and boat parts. Experts approach this method differently, depending on the type of wood grain and the quantity of the material involved in the project. The timed process involves placing wood scraps in a box, which is supplied with heat via a steam generator. Once the wood pieces are steamed, they are pliable and immediately ready to be shaped during the second part of the steam bending process, with the aid of a jig and other shaping equipment.
Wood Steaming Applications:
Wood Selection and Pre-treatment
Generally, wood should be moist before initiating the steaming process, as damp wood is most effective with heat transfer. Some woods already have high moisture content while others will need to be pre-soaked. As a general rule, hardwoods are well-suited for steaming and bending than softwoods. Experts note that cross-grained wood is more susceptible to cracks and overall bending failure than straight-grained wood, which should be noted before processing the wood. Greenwood, which has a moist composition, is the standard recommendation for steaming and bending, though different applications require hardwood selection. Bending qualities of various hardwoods were tested by the U.S. Forest Service and the results show that white oak, hackberry and red oak were ranked as the best woods, while soft maple, yellow-poplar and hard maple were ranked as the least compatible with the steam bending process.
The Steaming Process: Tools and Timing
The integral tools used for the steaming process include: a steam box, a steam or heat generator, a thermometer and a hose or other device that will connect the generator to the box.
Steam boxes may be constructed from wood or from PVC pipe and must completely encase the wood scraps so that the steam may saturate the entire length of the lumber. Some professionals prefer to construct racks within the box for the individual pieces of wood to rest on, though this is not mandatory. While the box should be tight enough to hold steam, it should also include an opening, like a drilled hole, so that the steam may flow around the wood. Openings are also crucial so that the steam does not become volatile. Additionally, many steam boxes feature a door so that pieces may be added and removed when necessary. Some professionals opt to use a soft material like a rag, which acts as a door during the process.
Steam generators can be a commercial appliance like a teakettle, pressure cooker or metal gas can. The generator, which can be found in most hardware stores, should be unused and clean. This tool must be able to link with a hose that will connect to a hole in the steam box. Do-it-yourself artisans suggest cutting a hole at the top of the generator (of one doesn’t already exist) and fastening the hose with large nuts. The hose may be attached to the mouth of the tea kettle, but must be fitted tightly and properly. Keep in mind that the wood should only be placed in the box when there is a sufficient amount of water in the steam box. Interrupted flow of the steam will greatly affect the heating process.
The general time guideline for steaming wood—advised by professionals— is one hour of steaming for every inch of wood that is processed. According to the Department of Agriculture, 212 degrees Fahrenheit is the sufficient temperature to saturate the wood, and this can be monitored by placing a thermometer at the opening of the box. The heating/steaming process should be executed without interruption as wood may actually be under steamed, causing breakage. If the wood has been steamed for the allotted amount of time and cracks when it is bent, then it has been under steamed. To avoid this, professionals recommend inserting a few test pieces in the steam box.
Place the moist scraps of wood in the steam box. Some professionals advise soaking the lumber at least a day before heating.
The Bending Process
As a general rule, the steamed wood should be placed on the bending clamp immediately after it is removed from the steam box to prevent fractures. Bending forces the flexible outside fibers of the wood to stretch while it compresses the inside surface. The wood is then clamped (secured by straps) to the form which it will be molded to. According to experts, the bending process should begin at one end and slowly progress to the other side. Typically the cooling and setting period will take 24 hours.
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