How Long Do You Have to Steam Wood Before You Can Bend It?


Steam bending is an old technique that softens wood to allow it to bend beyond its natural limits. Steamed timber has a high degree of plasticity that allows it to curve and twist as needed to create bespoke products. But how long should you steam the wood to achieve the desired level of plasticity?

You have to steam air-dried wood at 212°F (100°C) for an hour for every inch (2.5 cm) of thickness to make it soft and flexible. The steaming process softens lignin, a heat-sensitive glue that binds the wood’s fibers together. That allows the wood to bend extensively without breaking or snapping.

The rest of the article will explore other topics related to this question, including wood structure, how steaming affects wood’s structure, the steaming process, and steam bending tips.

Wood Structure

It is essential to examine the wood cell structure to understand how steaming makes the wood bend. Like any other plant, trees are made of cells, and each cell comprises three substances:

Cellulose forms the wood’s fibrous bulk, while lignin is the glue that binds the fibers together, giving trees and, by extension, wood its rigidity. The inner structure determines what the wood looks like, behaves, and what it should be used for.

Unique Characteristics of Wood

While dry wood is strong and stiff, it’s lighter and more flexible than metal, but it has some unique properties. 

Metals and plastics are isotropic, i.e., they have a uniform inner structure, and their properties don’t change depending on the orientation. Wood is anisotropic thanks to its annual ring and grain structure – a piece of wood will have different properties depending on its orientation.

That’s because the wood has high compressive strength and will resist squeezing when used in an upright position, which is why wood makes great support beams. However, when placed horizontally, wood tends to bend or snap if it’s not supported from below. Wood has low tensile strength and can’t resist bending or pulling across the grain.

These qualities extend to cutting wood as well. It’s easy to split wood along the grains but much more difficult to cut through the grain.

Lastly, when exposed to water, wood absorbs water and swells up, just like a sponge because it’s hygroscopic. Wood is part of a tree’s vascular system designed to carry water from the roots to the leaves. These structures absorb water and swell but give up the water when the temperature rises and the air dries.

See my article on how to dry wood needs to be for woodworking for extra information[Link Here]

How Does Steaming Alter Wood Structure?

Bending wood exposes it to two forces – tension outside the bend and compression inside the curve.

A dry piece of wood can only handle a 2% to 3% difference between the tension and compression before snapping and breaking. Coupling heat and moisture increase the movement’s range of movement to let the wood bend to a remarkable degree without breaking.

Steaming affects lignin – the glue that holds the fibers together to give wood its rigidity. Lignin is a thermal sensitive glue that softens on heating. To melt lignin, you’d need to heat the wood to about 250°C (482°F), which would destroy the wood.

However, heating the wood in boiling water or steam weakens the lignin bonds at a much lower temperature and allows the fibers to bend. Steaming wood for at least an hour at 100°C (212°F) for every inch (2.54cm) of thickness allows for severe bends.

The heat and moisture soften the fibers and allows them to move and distort, thus permitting the bend. The steam also warms the wood at the boiling point while increasing the moisture content, especially in the drier outer layers.

The Wood Bending Process

Bending a piece of dry wood subjects the convex (outer side) to tensile stress and subjects the concave (inner side) to compressive stress. 

However, wood fibers can accommodate a slight stretch outside the bend. That might explain why a dry twig snaps on the outside while the inside part remains attached.

Steaming wood increases its compression range but not the tension range. The compression range rises to 10 times the dry range, but the tension range remains largely the same. Therefore, the tension on the outside of the bend remains a limiting factor that needs addressing when steam bending wood.

To make severe bends, you must control or reduce the outer radius tension of the wood. Pushing on the ends of the wood generates a compression force capable of offsetting the tension force when bending the wood piece.

Fine Woodworking on Bending Wood advises artisans to use end stops to restrain the wood on both ends and a metal backing strap to support the wood on the convex side to offset the tensile stress. Placing the denser heartwood side of the board inside the curve also helps the bending process.

Such a setup accommodates the resultant changes in the wood dimensions during bending.

Since the outer tension radius cannot stretch more than 3% without failing, you should keep the outer radius at a constant length when bending wood. Placing a metal strap with end fittings such as clamps or end block on the wood’s convex side prevents tensile failures.

The metal strap absorbs tensile stress that the wood would have otherwise absorbed, causing it to elongate more than 3%. Eliminating the tensile stress subjects the entire wood plank to compressive stress to let you bend the wood to small radii.

Bending a 1-inch (2.54cm) thick plank that’s 18 inches (45.72cm) long over a 4-inch(10.16cm) radius shaves off 2 inches (5.08cm) inside the bend. The wooden piece remains 18 inches long on the outside but reduces to 16 inches (40.6cm) on the inside.

Steam bending wood is a slow process because wood takes a while to absorb the bending stress. Therefore, you should make all bends in a slow, deliberate manner.

Tips to Successfully Steam Bend Wood

  • Hardwoods give better results. Hardwoods are easier to bend because they have more hemicelluloses and less lignin compared to softwoods. Hackberry, oaks, ash, elm, beech, maple, birch, mahogany, and walnut are the best steam bending choices. Southern yellow pine, Alaskan cedar, Yew, and Douglas fir are exceptions among the softwoods. Yew is a bit toxic, so be careful working with it. Check out my article on the best wood for woodworking [Link Here]
  • Pick quality stock. Straight grained, knot-free wood with square-cut corners and smooth planed sides makes the best steam bending stock. Typically, avoid any wood with strength reducing defects, including cross-grain, shake, surface checks, pith, and brash wood. Also, avoid a stock with a grain that runs out to the side and is less than 15 inches long.
  • Air-dried wood is preferable. Air-dried timber with a 15% to 25% moisture content gives the best steam bending results. At this moisture level, the lignin will plasticize quickly to allow successful bending.

Agriculture Handbook No. 125  says the kiln-dried wood doesn’t bend well as it has a low moisture content, in the range of 8% to 12%. The hot and dry kilning process also causes the lignin to set, lowering its plasticity ability when steamed—soaking kiln-dried wood in water for a day before steam softens it enough to allow shallow curves.

Conclusion

Heating wood to the same temperature as boiling water lowers its rigidity and allows the fibers to bend. The hot and moist heating process softens lignin, which binds the wood fibers, allowing them to move.

Bending a piece of wood stretches the outer side of the bend while compressing the inner side. Steaming increases the wood compression capacity but doesn’t improve it’s stretching ability.

Hardwoods such as oaks, hackberries, ash, and elm are highly malleable when steam is heated and bent to small radii.

Sources

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