Wood & Steam. Charlie Whinney

       When I started steam bending, the day after a talk by designer David Colwell at my college, I did not fully understand what I was doing. Wood would often break and I did not know why. I was coming to the process as a metalworker and tried to do to wood what I was already doing to metal; heating the material to bend, twist, and squash it into a wide range of shapes. I had a lot of interestingly shaped pieces of wood on my workbench, but also at least as many broken pieces under it. It was only when I took the time to read about wood, to understand in my own terms about how trees grow, what wood is, and what actually happens when we heat the wood and bend it, that things became easier.

      THE STRUCTURE OF WOOD

      Trees grow outwards, adding a thick veneer of wood over the whole surface of the tree every year. If we zoom in to look at the tiny molecules that make up the cell walls of wood, we see that about 50 percent of wood is cellulose, a very regular polymer chain made of hexagons of carbon linked together with hydrogen and oxygen. Filling the voids between the cellulose molecules is lignin, which makes up about 30 percent of wood. Lignin is a hard-to-pin-down molecule; there will be many types existing together and they share a hydra-like structure, with their limbs wrapped around the adjacent cellulose, binding them together with weak bonds. A popular analogy for wood is to think of it as a composite material like fiberglass. The cellulose chains are like the glass fibers and the lignin like the resin that binds it all together. When we heat wood, electrons speed up, everything is energized, and the bonds between the lignin molecules weaken. When it reaches boiling point, the wood is soft and ready to bend.

      If we zoom out now to look at the whole individual cells, the first things you will notice are the shapes. The cells are like long, hollow tubes, all packed together, and they have little holes in the sides that link each cell to the next. Next, you may notice the cells are empty. Living plant cells contain the full complement of organelles for life, but these are long gone here, evacuated only a few weeks after the cell was fully grown, and now even the water and sap that filled the voids prior to heating are mostly gone as the heat has turned it to steam – the heat has actually already partly dried out and seasoned the wood.

      If we zoom out again, we will see how the alignment of the cells parallel to the edge of the tree make up the grain of the wood, and that the cells that grow every spring are huge compared to the rest of the year. As we bend the wood, the softened cells start to completely change shape. The cells on the inside of the bend are squashed, the grain zigzags as the wood crumples and compresses. On the outside of the bend, the cells flatten and stretch. When they start to rupture, we learn that we can squash the wood a lot more than we can stretch it. Keep bending and we can see the weak points are the “run-out,” where grain does not run parallel to the surface of the wood and threatens to peel away as we keep bending – better not bend the wood any more in case it breaks! If we had been using a compression strap, we could have kept going, as the steel band on the outside of the bend would have ensured almost all the wood would have been compressed and crinkled. The wood is now cooling. If we release the wood while it is still hot, we will see it unbend until almost straight again, but if we wait for the wood to fully cool, the lignin will have re-set around the cellulose molecules and the “spring back” will be about 25 percent of the bend we have created. The porous plant cells of the wood will still be slightly wet and soft; we need to let dry air circulate around the surface of the wood to allow the cells to set hard. When the wood is dry (less than about 15 percent moisture content) and we release the clamps, the spring back will be only about 5 percent of the bend, and we can call ourselves steam benders!

      WORKING WITH STEAM

      It typically takes less than a tenth of the energy, fuel, and effort to make something from local green wood compared to using imported kiln-dried wood bought in a lumberyard, and over fifty and often hundreds of times less energy, fuel, and pollution compared to metal and plastics. Even after we count the energy consumed making steam, working unseasoned wood in this way is massively efficient and the objects you make will have a much lower embodied energy, carbon footprint, and pollution quotient, and so on, compared to almost anything you can buy from a shop. A large part of my interest in steam bending is the alternative route to making that the process offers. The starting point can be fresh local wood (sometimes actual branches from your garden, as you will see in this book), and the final results are useful, beautiful curved shapes with a moisture content the same as kiln-dried wood.

      SELECTING & STORING WOOD

      All types of wood, indeed all vascular plants, are made from cells with cell walls of cellulose and lignin, and all wood no matter the species, age, and moisture content can be bent to some extent using a heat/steam process. If you are after extreme results (anything tighter than about a 1:20 thickness to radius ratio), you need to take note of the individual species, consider how dry the wood is, and look carefully at how straight the grain of the wood is. Very generally, ring-porous temperate hardwoods (broad-leafed trees from a temperate climate where you can see the growth rings easily) are normally good for steam bending, but there are plenty of exceptions. Very roughly in order of how “extreme you can go,” my top ten favorites from the woods near my workshop in Cumbria in the UK are ash, oak, elm, yew, walnut, beech, willow, birch, sycamore, and larch. If the wood is fairly low-density, such as pine, larch, and poplar, it tends to crinkle and buckle on the inside of tight bends, but gentle bends are still possible with softer timbers and softwoods. I use almost exclusively local wood, so I am not in a position to comment on every species, but I have had some really interesting and unexpectedly promising results twisting eucalyptus and free-bending softwoods. Rattan is a joy to work with – it’s impossible to break – but it shrinks along the length after bending, which took some getting used to!

      I find my success rate for bends goes up if the wood is fresh or properly air-dried. If the wood has been kiln-dried or is reclaimed, I normally soak it in water first for a day or two. The worst results come when the wood has been stored incorrectly and allowed to rot or “spalt” too much; this is not always obvious until you start to bend, and then find the wood is very brittle. Tiny microorganisms have been feasting on the lignin that holds the wood together.

      THE GRAIN

      The grain is the most important factor in the science of successful steam bending, especially for a maker without access to industrial bending tools. As you will discover, wood is not a consistent material – there will be weaker and