Wood Bending

Steamed beech chairs, dating late 1880's.

 

Wood is remarkably elastic. If you bend it round a curve it either breaks or springs back to its original shape. However, wood can be temporarily plasticised so that it can be bent to various shapes.

Wood is comprised of a rigid cellulose polymer in a matrix of lignin and hemicellulose. The lignin is thermoplastic and softens on heating. Alone you would have to heat the wood to about 170ºC, but at that temperature the wood will start to chemically decompose and lose strength. However, heat  with moisture can make wood sufficiently plastic for bending operations. Steaming at atmospheric pressure is fine for most purposes. Microwave heating can also be used on wood at lower moisture contents.

Typically, steaming for 15 minutes per cm of thickness is adequate for wood that is at 25% moisture content or double this for drier timber.  Microwave heating raises the temperature of the core of the wood much more rapidly and can achive very rapid bending processes.

Wood can be plasticized by using chemicals. Water of course can be used but also urea and ammonia although these chemicals are now rarely used.

Bending wood is popular for furniture components and sports goods of course, but has seen recent interest by architects for features.

When wood is bent around a curve, the outside of the bend will be subjected to tension but can only be stretched 1 or 2% before breakage. On the inside of the bend, the wood is subject to compression After boiling wood can be compressed as much as 25 to 30% parallel to the grain, which accommodates most bends. It follows that if wood is pre-compressed parallel to the grain, on bending, the side under tension will just return to its original state. This process has been perfected and is best known by the brand name BENDYWOOD.  This material is dried after compression and remains flexible.

In order to achieve a normal steam bend, the outside of the bend must be protected from stretching. This is achieved by using a metal strap with end fittings that can put the whole piece in compression. The tension is thus transferred to the metal strap.

What can you bend?

In general, hardwoods bend more easily than softwoods. This is strange because hardwoods have less lignin than softwoods, but more hemicelluloses. Most commonly steam bent timbers are the oaks, elm, ash, beech, birch, maple, walnut and mahogany. Douglas fir and Southern yellow pine are exceptions for softwoods and bend well – a useful property for boatbuilders.

All stock for bending should have straight grain and be free of knots and decay.

After the bending operation, usually in a machine, the piece must be held in position while it cools and dries to the final moisture content.

Some companies have developed the bending operation to a very sophisticated level, doing bends in several planes at once.

Very small or thin pieces can be bent when dry by applying local heat with a hot poker or iron. Many instrument makers use this technique.

Steam bending of oak - modern application

 

Bent oak in a modern context.

Above visuals courtesy of Millimetre 

 

Contributor:  Gervais Sawyer FIMMM

 

Interesting Links:    Contemporary chairs (similar to those at the top of this page)
                               manufactured by Thonet  www.thonet.de

                               Modern "bent" wooden furniture www.ercol.co.uk

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