THE SCIENCE

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The hollow, porous straw-like plant cell walls that wood is made from are composed of about 50% Cellulose and 30% Lignin.  When the wood is heated the lignin softens to allow the material to be bent/squashed/stretched/twisted into new forms.

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Cellulose (what paper is made from), is a polymer; a chain of ring-like Glucose molecules which the plant miraculously makes from nothing but water, CO2 and sunlight during photosynthesis.  The thin Cellulose molecules tend to group together as bundles to make flexible fibres which are very strong in tension. 

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Lignin (a natural thermo-plastic) is composed of molecules that have many arms that bond onto themselves and the adjacent Cellulose bundles.  When the wood is heated a profound change occurs to the Lignin at a very tiny scale weakening or breaking bonds which enable the cellulose bundles to slide past each other when we bend the wood.  When it cools the Lignin bonds re-establish and the bend become permanent (unless we re-heat the wood and bend it again!)

 

Water in the porous cell walls (from steam or may already be present in unseasoned wood) makes the material softer and more flexible so helps with the bending process too.  Proportionally tighter bends are possible with steam/wet bending than dry heat bending. 

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Wood heated for long enough comes out of the steam chamber around 20-25% moisture content, so for green wood you are starting to season it as well as getting it ready for bending (which is magic coup in the world of woodwork).  You should expect any radius to increase by about 30% after it cools but has not dried (for outside work), and if you also dry the wood below about 12% moisture content you can expect the radius to increase (spring back) by between 1% and 4% depending on the exact characteristics of the wood.  Specifically, the early/spring wood of each growth ring is very low density and doesn’t shrink much, and the denser late/summer wood, which will vary in thickness from year to year, shrinks a lot more; as no piece of wood is completely uniform some variation of spring back should always be expected.  It is possible to ‘set the bend’ afterwards in several ways for very accurate bends if needed.

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All wood is made from Lignin and Cellulose so all species of wood can be steam-bent to some extent.  Generally ‘Temperate Ring-Porous Hardwoods’ are best if you want to do tight bends but there are a lot of exceptions, and gentle bends can still be achieved with most other woods.    Extreme bends with a thickness:radius ratio of 1:2 can be achieved with very straight grained Ash or Oak using a well designed compression strap, however typically 1:5 is maximum we recommend as wood is rarely perfectly straight grain.  Without a compression strap bends of 1:7 and tighter can often be done with carefully selected straight grain Ash or Oak, however, we recommend 1:10 or less typically again because wood is rarely as uniform as you may want.

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‘Jigs’ are tools you make to help you make the work.  Fundamentally steam-bending is the process of compressing and/or stretching the material when it is hot; the straw-like hollow wood cells will compress, crinkle and ‘concertina’  along the length on the inside of a bend or the core of a twist, and they will flatten and stretch alittle on the outside of a bend or the outer part of a twist.  The hollow cells can squash a lot more than they can stretch and your jigs, bending forms, compression straps etc… all function to limit and/or regulate how much stretching or squashing happens, ideally within the limitations of your particular peice of wood.  It’s a fascinating and endless creative process that offers the opportunity to gain myriad insights into wood and trees in no other way; part science and part art.