WHAT IS STEAM BENDING?

THE SCIENCE

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.

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. 

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. 

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.

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.

‘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.

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THE ART

Steam Bending is a process that opens a lot of doors creatively.

 

Knowledge of the process not only allows makers, wood workers and artists to gain a deeper, more accurate insight into the real nature of wood; but also provides the practical tools to fabricate designs that wouldn't otherwise be achievable by any other process. 

 

There is a lifetime of creative opportunity waiting to be explored.

THE BENEFITS

Steam Bending is a cheap, clean, efficient and eco-friendly process that lets you bend solid wood - no glue, cleaning up, or glue lines and it can be done by anyone.

You can ‘season’ the wood as you put it through the process, so fresh or unseasoned wood can be bent and dried to around 8% moisture content in a couple of days.  This hugely increases the range of wood you can use for projects as newly felled or sawn trees can be used almost straight away.  Locally sourced unseasoned timber is one of the most eco-friendly materials available to makers, and if done well will have a lower embodied energy than imported and/or kilned dried wood even after you have steam-bent it, and will save you money too.

All wood can be steam-bent to some extent, no matter what condition or moisture content and many types of plywood can be bent too.  With some experience you’ll learn limitations and opportunities of each species and specification you’re interested in working with and design work for success within these bounds. 

It’s fun and seems like magic!  After many years I’m still amazed every day that it works, and we never stop learning.

EQUIPMENT

 

Almost everything you need apart from the wood can be bought at a builders yard or DIY store.

You will need:

 

  • Steam-maker
    A $30 electric 'wall paper stripper' is very convenient and you will also find many other good solutions, gas is used by most professional makers as you can adjust the output for maximum efficiency.  You can also boil or microwave the wood if possible/practical with great success, and a campfire can also be used too in several ways.

 

  • Steam chamber
    Anything to hand that will efficiently contain the steam, and which you can get the wood into. Reclaimed plastic pipes are great (be aware PVC gets soft when hot, HDPE or Polypropylene are better), double walled stainless steel flue pipe makes a brilliant steam chamber, marine/WPD plywood boxes are good if you need something abit bigger and always insulate the chamber ensuring the insulation stays dry.  We use aluminium and stainless-steel boxes insulated with a minimum of 50mm of insulation for professional daily use.  Normally put your steampipe in the back of the chamber which should be reasonably leak free except a small hole/gap to allow condensed water to dip out, and allow the steam to vent out of a slightly leaky lid at the front.  The whole chamber can be tilted at about 2 degrees so the water only drips out of the back, and use a thermometer located at the bottom of your lid to monitor the temperature inside.

  • Jigs
    A ‘jig’ is a generic term for a tool made for a particular purpose, and for steam-bending these are almost always home-made.  If you are starting out, for a 'bending form' (a jig to bend around) find something solid with a radius simelar or a little smaller than what you want or cut the shape from plywood.  If you google 'steam bending jig' you'll see a wide variety of inventive forms that may help.

 

  • Clamps
    These are normally ‘G-Clamps’ or 'F-Clamps'.  If you are starting out I recommend buying at least one powerful clamp (by Bessy or Urko for example) which you can use to actually help bend the wood, and other cheaper ones for holding wood in place. As you make ‘jigs’ you may start to use wedges to clamp the wood which is very fast, effective and cheap.

 

  • Compression Strap
    Needed for tighter bends, thicker wood, or harder-to-bend or knotty wood.  This is normally a peice of >1mm thick steel secured to  the ends of the hot wood, on the outside of the bend, which forces the whole peice into compression as it is bent.  Stainless steel, galvanised steel, spring steel, pallet banding, builder's straps and many composite material such as reclaimed firehose all make good compression straps, however woven webbing or anything that can stretch in  any way is not suitable.  End stops include ‘wedged‘, ‘adjustable’, ‘hydraulic‘, ‘friction‘ and ‘sliding‘ (all terms refer to how the wood is secured and managed during the bend).   If you are starting out and want to try thicker bends I recommend making the easy ‘wedged compression strap’ we teach on our courses and taking from there, or buying the Veritas Adjustable Compression Strap.

WOOD & STEAM BOOK

A good starting place with the process is our book 'Wood & Steam' published by Kyle Books which covers the most basic aspects and provides starting points for 18 simple projects

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