Bent Wood and Nails

Why bends break

When a stick bends, the inside of the curve gets shorter and the outside gets longer. Wood is very uneven about this. Softened by steam or boiling water, it can be compressed 25 to 30% along the grain, but stretched only 1 to 2% ¹. So a sharp bend has to happen almost entirely by squashing the inside face. When the outside is asked to stretch further than it can, it splinters.

How much a bend asks is simple geometry. For a stick of thickness t bent around an inside radius R, the outside face must be about t/R longer than the inside. The Wood Handbook’s rule for steam bending: if the outer and inner faces differ by much more than 3%, use a metal strap with end stops on the outside, so the strap takes the tension and the whole piece is pushed into compression ¹.

• 3/4-in. rod around a 24-in. radius: about 3%.
• 1-in. rod around a 12-in. radius: about 8%.
• 1-in. rod around a 6-in. radius: about 17%.

Two more rules from the same chapter: choose stock without cross grain, knots, or checks in the bend, and hardwoods generally bend better than softwoods; oak, elm, hickory, ash, birch, and maple are the usual bending woods ¹.

Bending green saplings

Rustic bends are gentler than a steam-bent chair back, and green willow forgives a lot. It bends easily, stays where it’s put, and keeps its bark as it dries ². The technique:

1. Use it fresh. Cut willow loses its flexibility as it dries; build soon after cutting ².
2. Pre-bend the thick end by hand or over a bench edge. Otherwise the thin tip takes all the curve and the arch comes out lopsided ².
3. Bend slowly, working along the rod with both hands, a little at a time.
4. Fasten as you go: nail one end, ease the curve over, and nail the other.

For tighter bends in stiffer wood, do what Old Hickory did: soften the saplings in boiling water, clamp them to a pattern, and let them dry; dried on the form, they keep the shape ³. The Wood Handbook gives steaming or boiling times of about 1-1/4 hours per inch of thickness for dry stock, and about half that for wood at 20-25% moisture ¹.

Let it set. A bend must cool and dry while held in shape. Taken off the form too early it springs back, and even a set bend loosens a little if it gets wet again ¹. In a nailed willow chair, the frame itself holds the bends while they dry.

Nailing green stock

Nails are the joinery of bent willow work, and they work because the frame is triangulated and the bends are held by many fastenings.

• Galvanized nails resist rust in outdoor furniture and green, wet wood. Musgrove Willows uses framing nails of 2-1/2 to 4 in. for the frame and finishing nails of 1 and 1-5/8 in. for arms, seat, and back ⁴.
• Pre-drill for every nail near an end or in small stock, slightly smaller than the nail ². Rustic Carpentry made the same point with a bradawl ⁵.
• Clinch nails that come through: bend the point over across the grain and set it back into the wood.
• Angle the nails slightly toward each other at a joint so the pieces can’t pull straight off.

Try it

Cut a dozen 3/4-in. willow rods 5 ft long. Bend two around a 12-in. radius (a bucket works as a form) without pre-bending, and two after pre-bending the butts. Nail each pair to a scrap board as a hoop and compare the curves after a week.

What to take from this

Bent wood compresses easily on the inside and barely stretches on the outside, so a bend’s severity is about t/R; above roughly 3%, steam benders strap the outside. Use clear, straight-grained stock. Bend willow green, pre-bend the thick end, go slowly, and nail as you go. For stiffer wood, boil and clamp to a form, and let the bend dry in place. Use galvanized nails, pre-drill, clinch, and angle them.