Steam bending the world’s first mass-produced chair
- Harsh Bhundiya
- Jan 3, 2023
- 4 min read
Updated: Feb 5, 2023
Did you know you can bend wood by heating it with steam? And that this technique led to the first ever mass-produced furniture: the wooden chair?
When you think of wood, you normally don’t associate it with a plastic, bendable material. Most wood is indeed quite hard and difficult to break, hence its use in furniture and as a construction material for homes. This strength is derived from the hierarchical, composite structure of wood, which consists of tiny cellulose fibers surrounded in a lignin matrix. This remarkably efficient structure allows trees to grow up to 115 m tall and live for thousands of years, while supporting their self-weight and other natural loads like wind and snow. However, when it comes to using wood as a structural material, there is actually a technique which has been used for hundreds of years to bend even the strongest woods into beautiful, curved shapes like below [1].
How? With steam. The concept is simple: use the combination of heat and moisture from steam to soften cellulose fibers until the wood can be bent to the desired shape, clamp it over a mold, and let it cool and dry. Although unclear when this technique was first invented, it has been used since the 18th century to make everything from wooden chairs to violins to ship hulls. Today I want to review how this process works, its complications, and explain how it led to the first ever mass-produced furniture: the wooden chair.
Steam bending is a brilliant, direct use of beam bending to make some truly beautiful wooden pieces. The key complication, of course, is to not break the wood while bending it. One way to quantify is this to ensure the maximum strain during bending stays below the failure strain of the wood:
Here the maximum bending strain can be approximated with the desired radius of curvature and thickness of the specimen:
However, one challenge with this approach is that the failure strain of the wood is difficult to measure, since it varies with the amount of steam absorbed by the fibers. Generally the more steam absorbed, the higher the failure strain and the easier to bend (but only up to the point of saturation). Measuring this failure strain as a function of steam absorption is a challenge, so steam bending often relies on simple rules of thumb. For example, one common strategy is to leave the wood in steam for one hour per inch of thickness [2]. Another is to measure the bendability of wood by the number of unbroken samples and the limiting radii of curvature [2]:
Tables like these give craftsman an idea of how much a certain wood could be bent. Some species of wood like oak, walnut, ash, hickory, pecan, and beech bend well while others like cherry, teak, and mahogany are much harder to bend.
Another interesting complication with steam bending is that generally wood fibers have lower strength in tension than in compression. Therefore when bent, the wood often fails first on the outer surface of the bend which is in tension [2]:
To get around this, people have come up with a clever solution: clamp a steel strap to the outer surface of the wood and attach end blocks at the cross sections. Since the steel strap does not stretch during bending (relative to the wood), this strap prevents the fibers on the outer surface from stretching by applying a compressive force into the cross section of the wood. From the perspective of beam theory, this compressive force at the ends essentially moves the neutral axis of the beam towards the outer surface and minimized the area under tension. With less of the material under tension, the wood can be stretched further to a smaller radius of curvature. Here is a picture of this in action [3]:
Another complication of steam bending is springback, which refers to how much the wood “springs” back to its original shape after bending. This is affected by a number of factors such as the geometry of the curve, thickness of the wood, and the type of wood. Generally, this spring back is accounted for with trial and error.
Despite these complications, for centuries steam bending has been used to make bent wood for many applications, from violins to ship hulls. Out of these, my favorite is the wooden chair, which was mass-produced primarily because of advancements in steam bending. In particular, in 1859, the cabinetmaker Michael Thonet made a chair he called “No 14” by steam bending six pieces of beechwood:
He realized the process he used was simple, fast, and particularly suited to industrial production, so he set up a factory to make the chair. In doing so he had started a new revolution, that of mass-produced furniture. The simplicity of the manufacture process allowed the chair to be disassembled into only a few components and be easily exported: 36 disassembled chairs could fit in a one cubic meter box [4]. Soon millions were being shipped globally and the No 14 became known as the “chair of chairs”. In fact, the No 14 is still in production today, with 50 million chairs made and counting. Also this chair was likely the inspiration for IKEA, the biggest manufacturer of ready-to-assemble furniture today [5].
To me, what’s remarkable is that the simple manufacturing process of steam bending could be at the heart of this global shift to mass-produced furniture. And of course, at the heart of steam bending is the classic theory that we learned in school: beam theory.
References/further reading:
[2] Fine Woodworking on Bending Wood, 1985
Comments