Steam bending goes back a very long way, in a crude form it predates conventional cabinet making and is particularly well-suited to UK indigenous hardwoods such as ash and oak.
The technique takes advantage of the core nature of wood as a composite material, like carbon fibre or reinforced concrete; it also recognizes and makes use of the structural change that takes place between it being a fibrous growing plant and being a relatively stable, tough and exceptionally versatile structural material.
Wood consists of fibres, which are stuck together with gluey stuff called lignum. The fibres give tensile strength and the lignum as well as sticking the fibres together gives strength in compression. Carbon fibre and epoxy resin, as used in Olympic bicycles, boats and tennis racquets, work in exactly the same way, and not supprisingly, until recently many bits of sports equipment like skis and racquets were made from steam bent ash, because of its formidable strength-to-weight ratio.
Remarkably, the faster ash is grown, the stronger it is. This fast-grown material was formerly called Sports Ash. At this point I can't resist adding that this material absorbs more atmospheric carbons than any other tree.
Conveniently lignum softens at 100°C, the temperature of steam. Having softened the lignum, the fibres can be slid over each other so lengthening or shortening a section of the timber, which is what is required to achieve a structural curve; as it cools, the lignum slowly sets hard again.
There are variables but the recipe is something like this:
Cut a piece of unseasoned wood about 5% bigger than the required section and a little over-length. The added width is to allow for shrinkage as the wood becomes dry (seasoned). The extra length allows for some sacrificial material. End grain which is subjected to heat and compression and can get discoloured.
Put it in steam for 1 hour per 25mm of thickness.
Wood can be simply bent over a mould or former and held in place to cool and dry if the desired inside radius is at least 18 times the thickness of the section to be bent. At 25mm thick the inside radius should be 450mm or more.
To achieve a tighter radius, the outside of the bend will need to be supported with a strap with blocks at each end to stop the outside surface from stretching and failing in tension. Using this method, an inside radius of 8 times the materials thickness is achievable, so a 25mm thick piece of wood can be bent to an inside radius of 200mm. In practice these ratios are not absolut - quality of material, quality of moulds, temperature of steam and wood, ambient temperature and acceptable failure rate will all have an effect.
The bent wood should then be dried. Having been taken up to 100°C before bending, it can be dried very much more quickly than would otherwise be possible; this is because immersion in steam has resulted in the outside being damper than the inside, and also because the wood's cells have been made porous by the heating and will release moisture more readily as a result.
The recipe according to HMSO publication ‘Bending Solid Timber’ is to dry it for 8 hours at 50°C. Again there are variable -, how unseasoned was the wood in the first place, how exposed is the wood to air circulation, and critically, what production rate is required. I think it best if the bent wood can be removed from its mould and strap when cool and held in shape whilst it is dried. In practice in production the drying time is over night, giving some 18 hours, and I find it best to use a means of holding the bent bits with maximum exposure to circulating dry air, thus freeing the moulds and straps for repeated use.
The higher the moisture content of the timber, the better it will bend, but the more it will shrink and distort. In my own work I am increasingly using it in its least processed form, thus shortening the supply chain.
Wood, and particularly ash, is a phenomenally good structural material but its down-side is its jointing. Traditional joints require removing half of both bits at exactly the point where most strength is required. We developed Tubular Rivets, (a registered design) to get round this and liberate its full potential, allowing much more ambitious uses. This is particularly relevant to chairs, where light weight and strength are an obvious advantage. The fact that they look so good is a bonus.
O Range takes a radical departure from usual woodworking. By using a new generation of band saw blades, the first cut is the only one. This way an energy-intensive and noisy process of planing is removed, together with its significant dust extraction, which is also energy intensive and noisy. Removing this process also reduces the volume of wood used. The texture produced is very well suited to our soap finish too.