August 27, 2020
A lovely curved steam bent oak bench. While I love a straight line, often a curve in the right place makes all the difference. However while making things straight is relatively easy (although evidently not so easy for some), getting bends and curves is a bit more involved. In the case of wood you need know how and, importantly, heat. If you can get wood above 90 degrees centigrade then the lignin that glues the cellulose fibres together in the timber starts to soften and eventually will allow those fibres to slide over one another, when it cools the lignin sets and the fibres are once again held.
My method of bending is “steam bending” using unpressurised steam (conveniently 100 degrees centigrade!) in a hot box, this enables me to soften the wood and allows me to create beautiful curves and lovely objects such as benches for the gardens I design.
This steam bent oak bench was for a circular garden placed to view down a main vista and was produced in oiled oak on a grey finish metal frame.
To steam bend you need a heat box, mine is a bit homemade using insulation boards, but it works a treat – and the meat thermometer in the top tells me if the timber is rare or well done.
This is rigged up with a steam generator – a wall paper stripper unit in my case.
The length of time needed in the steamer depends on the section of oak being softened.
When you steam bend oak or any timber, while the softening of the lignin will allow the fibres to slide, there is always some inherent elasticity in the wood, and so when you release it from any clamping arrangement you have devised, it will want to spring back. This can be a bit of trial and error, but it is essential that you overbend to allow it to then relax back to the final curve desired. Those with good eye sight might also spot a steel strap on the timber, this is designed to ensure the outside of the bend is kept in compression, as the fibres in the timber fail in tension before they fail in compression.
I’ve spent some time finding reliable sources of straight-grained oak as if the grain runs out to the edge of the timber then this is a potential point of weakness when steam bending. The rule of thumb is that the grain must not cross the section in anything less than 300mm or 1 foot in old money. The narrower the section the more key this is. Also, if you overcook your timber it’s even more critical to adhere to the above, as the glue that holds the fibres that form the grain is even weaker and will lead to failures while steam bending. I was taught to keep the timber at 100 degrees for an hour per inch section, I’ve found my set up seems to need a little less than this, but you soon get the feel for it.
Well almost, I’m always looking to improve technique and make each piece better than the last – the reality is a craftsperson is not as good as their “last job”, but as good as “all the jobs they have done to date” – including the ones that went wrong from which they learnt what not to do. In other words, it is practice and experience that counts, perhaps with a bit of talent, so I would encourage anyone interested to just get stuck in and give it a go. Charles Whinney up in the Lake District does excellent training courses on the matter, and if you want to trade ideas please just get in touch.
