WHAT IS STEAM BENDING?
Steam Bending is a term used to describe heating wood to boiling point to soften it, so you can bend/twist/squash it into new shapes.
The plant-cell-walls in wood are a composite of about 50% Cellulose Fibers (what paper is made from) and 30% Lignin (a natural thermo-plastic). If you heat Lignin to boiling point (100 degrees Centigrade) it softens enough to allow the cellose fibers to move in the wood to enable the whole piece to yield to a new form.
When the wood cools the lignin bonds set again and when the wood dries (below 12% moiture content) the bend becomes permanent.
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.
EXAMPLES OF DIFFERENT BENDS
Steam Bending is a cheap, clean, efficent and eco-friendly process that lets you bend solid wood - no glue, cleaning up, or gluelines and it can be done by anyone.
Steam Bending is great with air-dried or fresh ‘green’ wood. The steaming and drying process will leave you with wood at around 8% moisure - a shortcut to seasoning and makes using a locally sourced wood a viable, economical and desirable option. You can also use kiln dried wood too, however for extreme bends green wood is easier.
With the right wood and a well designed ‘jig‘ or form, Steam Bending will give you simple reliable results every time. The process lends itself to batch production as well as sculptural exploration!
Almost everything you need can be bought at a builders yard or DIY store. We also have a selection of equipment available to buy in our kit store.
You will need:
Any way of boiling water and venting/piping the steam into a steam chamber. A $20 electric 'wall paper stripper' is very convenient but 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 fire can be used too for certain bends.
Really anything to contain the steam which you can get the wood into. Plastic pipes are great (be aware PVC gets soft when hot, HDPE or Polypropylene are better), plywood boxes are good, we make boxes of 50mm foam lined with thick polyethene as they are lightweight and very well insulated. Lids, racking, faceplates, venting pipes etc… can all be added if desired. You need to make sure the wood really is 100 degrees Centigrade for best results. Digital ’meat thermometers‘ are great for checking the temperature inside.
The most important skill in steam-bending is knowing how to look at wood and understand what you are looking at. If you are a beginner buy straight grained Ash or Oak that has not been kiln dried, or for free experiment with fresh branches. Generally 'ring-porous temperate hardwoods' are best, however there are a lot of exceptions. Top five woods for steam bending in our workshop are: Oak, Ash, Elm, Yew and Walnut. All wood is made from Lignin and Cellulose, and so all wood can be steam-bent to some extent, we also love experimenting with Plywood, Softwoods and Reclaimed wood which is much trickier but can give fantastic results.
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 to what you want (a spare wheel, stone pillar, lamppost!) or cut the shape (-15% of the final desired radius) from plywood.
These are normally ‘G-Clamps’. If you are starting out I recommend buying at least one decent square threaded G-clamp such as by Bessy 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.
Only needed for tighter bends or thicker wood. This is normally a peice of >1mm thick steel secured to the ends of the hot wood, on the outside of the bend. As wood will break (snap) in tension before (kinking) in compression, a compression strap which does not allow the wood fibres to stretch at all enables bends to be much tighter.
Types of strap include ‘wedged‘, ‘adjustable’, ‘hydralic‘, ‘friction‘ and ‘sliding‘ (all terms refor to how the wood is secured and managed during the bend) and materials include galvanised or stainless steel, woven stainless steel and composite for the strap itself (don't use ratchet straps as they will stretch). If you are starting out and want to try thicker bends I recommend making a classic ‘wedged compression strap’ and taking from there, or buying the Veritas Adjustable Compression Strap for thick narrow boards.
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
What types of wood can you bend?
All wood can be steam-bent usefully to some extent, and the good woods are ‘Temperate Hardwoods’ (Ash, Oak, Chestnut, Walnut, Cherry, Beech, Sycamore, Elm) and also Yew is fantastic. The best woods are also ‘ring porous’ (Ash, Elm, Oak, Chestnut). On several courses we bend a wide range of woods including kiln-dried tropicals and pine from the hardware store; and they can all be bent to some extent It is worth noting that you can do great designs using easy gentle bends which are possible in a wide range of woods.
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 ratio of 1:5 for a good success rate, unless you know you a source of perfect logs.
How thick can you go?
There is no limit - 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. Charlie Whinney Studio has a design for a jig to be bend an entire tree - with the correct jig there is no reason why not.
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.