Steam Bending Wood

by Norm Kidder (September 1, 2001)



If you want a real challenge, walk through the woods and find a straight stick. Even if you think you have succeeded, leave it in a corner for a few weeks and check it again. Trees are living things, and they respond to changing conditions and stresses, especially moisture and wind. The life history of each tree is reflected in the wood grain. As they dry, they shrink unevenly - resulting in warping and cracking. Today kiln drying, milling and laminating are all used to deal with these tendencies, but in ancient times one became intimate with the art of steam bending.

Wood is made up of long tube-like cells of cellulose connected end to end forming long fibers running the length of the tree. Additional fibers run across the grain, tying everything together. Except for the outermost layer (cambium), wood is dead, serving the plant for structural support and water storage and transport. Each year a new layer of fibers is added under the bark, its thickness determined by growing conditions and stress - more moisture = a thicker ring, and more stress = a thicker ring on the stressed side. Over time, the inner layers may fill with resins, forming heartwood, which is usually denser, and more rigid. The newer, outer layers, sapwood, are still relatively flexible and wet. (Excellent bows are made with the compression resistant heartwood on the face and flexible sapwood on the back.) Heating the wet wood turns the water to steam which dissolves some of the bonds between fibers allowing them to realign, reforming the bonds when they cool. So, steam bending is the process of weakening, stretching and reforming wood fibers to the desired shape. Rawhide acts in a similar and more dramatic way when it is wet, and then dried to shape.

There are many applications for steam bending in primitive technology. Straightening shafts for arrows, spears and fire drills is probably the most common. Others uses include straightening blowguns, recurving and reflexing bows, bending basket rim sticks, net hoops and looped stirring sticks, bending curfed wooden boxes, and shaping dugout canoes. A complete list wood (oops) would quickly become encyclopedic.

There are three basic ways of softening the wood fibers. The first is to heat moisture already in the wood. This means using already moist, green wood, or soaking dry wood to replace the necessary moisture, then using a fire, or other heat source to turn the water to steams. The second method is to create steam first, and then force the steam into the wood. The final method is to use boiling water to penetrate the wood fibers. [There are also chemicals that will dissolve the wood bonds - definitely not primitive.]

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. Small green sticks of solid wood for arrow shafts are easy to do over the coals of the fire or a heated soapstone arrow straightener. The only problem that may arise comes from over heating the stick, drying it out and making it brittle. Thicker pieces of wood, such as bow staves and boxes are more difficult, as the outside may dry out before the inside becomes soft enough to bend. For these, wet heat - boiling or steaming - is necessary. Dry wood may be soaked and heated if it is thin enough (this is how the sides of violins and guitars are made). When steaming or boiling is required, the method will be determined by what you can practically do with the available material.
The process most used in a primitive camp is direct heating of a still green stick over the coals, and then bending it to shape. The stick may be solid wood, such as chokecherry, or hollow, like rivercane, or have a pithy core as with elderberry. If your goal is to straighten out gentle curves, heat the inside of the curve over the fire (a good even bed of coals is preferable) as you are going to stretch this side. Using gloved hands, bend the stick past where you want it to end up, so that it will spring back to the desired point. You know to apply this pressure when you feel the wood lose most of its springiness. Sight down the length of the stick after each bending operation to check on your progress and to make sure you didn't over-bend it. Sharp bends can sometimes be worked out using a soapstone arrow straightener or an arrow wrench (see illustrations). Some compound bends can be handled by a push/pull maneuver. . If you heat both sides of the stick, you run the risk of the inside collapsing while you are stretching the outside. This is especially true of elderberry and other soft woods. Rivercane and phragmites both have hollow spaces between sealed nodes, so expanding air may cause these sealed containers to explode, or burst out the side if overheated.
If the stick you start with is fresh cut, there is a very good chance it will warp as it dries. If cut during peak growth times - late spring and summer, the large amount of water in the stick may actually make it more brittle. Dormant season wood, with less moisture may be more flexible. This is one reason to start with a dry stick and rewet it. It is even better to let the stick dry to a point where it has just enough moisture left to produce steam, but the process of bending brings it to full dryness. Knowing just when this happens takes experience, but it will keep you from having to re-straighten your drills and arrows repeatedly as they dry. Changes in air moisture will always cause wood to warp, so some re-straightening will be necessary for arrows and darts to fly perfectly straight. Cold straightening will sometimes hold long enough for you to shoot, or start your fire before the shaft warps back (since you won't be able to heat bend it until you have a fire started).
Bending a green stick into a circle for a basket rim, or a net hoop can be done with no heat if the bend is not too sharp for the diameter of the stick. The hoop must be tied until the wood has dried completely for it to hold its shape. If the curves are too sharp, then heat must be used. Tying off is still necessary if the wood is still wet. Bending compound curves, as in a looped stirring stick, or simple rims when there is a weak spot are aided by the use of forms, which can be as simple as a chunk of wood and some cordage (see illustration). Even with a form, the hot wood should be massaged gently over the form, and clamped or held to avoid split outs (delaminating) on the outside of the bend or compression failures on the inside. Once the stick is cooled, it should hold its shape.
To make and use steam to bend already dry and thick (over about an inch thick) wood, you'll need to construct a device of some kind. BPT # 9 (or Primitive Technology - a Book of Earth Skills) has a couple of set ups for steaming boxes within the article Bent Corner Box Making, by Greg Blomberg (page 47 in BPT #9). For small stuff, I simply boil water on my stove, put aluminum foil over the top and slide in my stick. Bigger stuff requires building a box or pipe to hold the wood, and then channeling steam into it from a boiler. None of these methods is of much use out in the field. One quick field method used along the coast was to take a piece of bull kelp, which is hollow at its upper end, put the stick inside and heat it under a fire. Where bull kelp doesn't grow (in other words most everywhere), steaming can be done in a long pit oven with lots of green plant material to provide the moisture. [Dig a pit, line it with rocks, heat the rocks, rake out the fire, fill the pit with wet grass or leaves, lay in the wood, add green stuff, cover with dirt, wait a few hours, remove and bend the wood].
I've used boiling to recurve the ends of a bow stave. I put the wood into the boiling water for about 20 minutes, which softened it enough to do the job. I bent the wood by pushing down against a solid surface, and had to hold it or tie it in place until it cooled. Over boiling could soften the wood too much and allow compression failure. Wooden hayforks with steam bent tines still made today are scorched along the outside of the bend in each tine. I assume this acts to harden the wood against future compression failure (the tines tend to break above this scorched bend). I haven't seen any bow staves treated this way, but heat drying the curve should help it hold its shape. An application of sinew on the inside of a curve will also hold the curve for the long run, or even increase the curve as it dries.
Another, rather dramatic use of boiling is found in the creation of dugout canoes. Once the digging out is completed, the canoe is filled with water and red-hot rocks added until it is boiling. The water is kept hot until the sides of the vessel are softened at which time a thwart (crosspiece) is hammered into place, causing the sides to bend out and the keel to round up a bit.
The ability to shape wood with simple tools and methods such as steam bending served our ancestors for the millennia it took to create more sophisticated systems. This technology belongs with fire making, cordage spinning, knapping, twinning and tanning in the skills collection of all primitive technologists.


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