WHAT IS STEAM BENDING?
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.
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.
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.
Almost everything you need apart from the wood can be bought at a builders yard or DIY store.
You will need:
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.
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.
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.
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.
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.
What types of wood can you bend?All wood can be steam-bent usefully to some extent. For extreme or tight bends you want straight grain, unseasoned ‘Ring Porous Temperate Hardwoods’ with a medium-high density such as Ash, Oak, Chestnut, Walnut and Elm, and also Yew is fantastic. Beech, Sycamore, Maple, Cherry and many others you'll know are also good, and woods with low or very high density, or short grai can also be bent but not as tight, such as softwoods, tropical woods, fruit woods etc...
How tight can you bend the wood?With very straight grained Ash using a compression strap you can achived bends with a thickness:radius ratio of 1:2 or even tighter, and a thickness:radius ratio of 1:7 without a compression strap. This does however rely in having perfectly straight grained wood (which is rare), and for designing production work with a compression strap it is best to use a maximum ratio of 1:5 for a good success rate, and the more gentle you can make the bend the higher the yeild will be. It's worth noting that many of the best steam-bent works we know use relatively easy to achive gentle bends.
How thick can you go?There is no limit, we bend 160mm thick as a standand bend - however we have found the forces involved rise exponentually. For example you need something like four-times as much force to bend a piece of wood twice as thick. As bends get bigger it becomes essential that forms and jigs are appropiately engineered and strong enough. It would be possible bend an entire tree with the correct jig and design.
Why did my wood break?Wood not supported as it was bent. As the wood is bent it will break at the weaker places where the grain leaves the surface of the wood. For most beginner-bends you will be bending over a form - it is essential to ensure the wood cannot 'kink' during bending onto the form as this will be at a weaker place and any kinks will almost always result in some cracking. The best way to avoid this is to keep the distance between the jig and where you are pushing the wood at a minimum and add clamps regularly onto the form. With experience this will very rarely be a problem. The wood was too cold. The wood really does need to be hot while you are bending, especially for thinner bends or not using a compression strap. Make sure the wood really is hot through-out, and if it is taking too long do get some help or re-configure how you are doing things. Wood was too dry. A common problem, it needs to be over about 25% humidity before steaming ideally, if in doubt or if the wood is kilned, soak in a bath of water for 24hours/15mm thickness. The bend was too tight for the grain of the wood If you have successfully bent the wood smoothly onto your form and it was hot, and it still breaks, the bend might just be too tight for the grain of the wood. Look very carefully at the grain of the wood, if there are knots or the run-out of the grain is more frequent than every 100mm (for example) that might be the problem. When you cut the boards you could try to follow the direction of the grain if you can. You could try some other more gentle radius's to discover what your stock can do, or find some cleaner straighter wood or a different species. Compression Strap was not tight enough. If you are using a compression strap, it really does need to be tight to work. A common problem is for the wood to try to 'bow out' of a compression strap during bending – if this is happening you need to clamp or tie it onto the wood as you bend. Another problem can be a tendency to loosen a compression strap too much when trying to release pressure on a long bend – if this is it you could try either a much bigger lever to put a lot more compression into the wood, or a disposable softwood insert next to the end-stop can be crushed during the bend to relieve some of the pressure. For larger bends a hydralic end-stop can also offer constant pressure throughtout a bend.