Why and how solid wood can be bent. Which species can be bent most easily

Wood has been used by humans for millennia for many purposes, from building houses to making furniture and art objects. One property that has attracted the interest of engineers, designers and artists alike is its ability to be molded into shapes that to some seem impossible, or images created by artificial intelligence. This is the plasticity of wood, which is the ability of wood to retain the shape it has taken after warping without cracking or splitting. This property is used to produce curved wooden elements used in furniture, musical instruments, decorative objects or artistic works. Why and how solid wood can be bent, and which species are best suited to bending treatments, below.

Wood bending has been known since ancient times

Evidence of the bending of wood goes far back in history, with the ancient Egyptians, Native Americans and Vikings all 'vying' for the top spot in making various objects out of bent wood. Some say the Egyptians were the first to use curved elements to make battle chariots and boats. Others claim that Native Americans used bent wood to make snowshoes or to carry weights. The Vikings are believed to have been the first to bend wood to make their famous ships, believed to have discovered America long before Columbus. Regardless of who was first, all agree that wood could be shaped after being repeatedly soaked in boiling water or even boiled in water if the dimensions allowed.

But it was Michael Thonet who revolutionized the wood bending process. In 1836 he patented the method whereby solid wood elements are kept in special containers in steam for 5 hours, then molded in molds in a very short time and left to cool. This is how the first chairs were made from curved round solid wood elements. Thonet was also the first to introduce the concept of disassembled furniture, which led to lower transportation costs. As a result, the price of the furniture fell and became more affordable. In fact, the best-selling piece of furniture of all time is a Thonet chair - Chair no.14 - published in 1930 and said to have sold over 50 million copies to date.

Elasticity or plasticity?

We are tempted to think that elasticity is the property that makes solid wood bendable. Only an elastic wood can take those round shapes without cracking or splitting. But elasticity is the ability of wood to return to its original shape after the force acting on it ceases. Or curved wood stays that way after the treatment to which it has been subjected ceases.

The property that allows wood to remain curved is its plasticity, which refers to its ability to undergo permanent deformation without breaking or losing strength. Plasticity is the property of materials to maintain their shape and dimensions after the action of external forces has ceased. So it is the inverse of elasticity. Normally, the natural plasticity of wood is reduced and decreases the older the tree gets. With special treatments, however, wood can become much more plastic. As wood is not a uniform material, its plasticity is not and does not vary in the same way. It depends on the species, the age of the tree, the growth defects, the part of the tree from which the wood comes, the felling method, etc.

Getting solid bent wood is actually a combination of its elasticity and its plasticity. It doesn't seem very possible since plasticity is the inverse of elasticity. In this case, however, the properties do not cancel but complement each other. Elasticity is what allows the wood to be deformed as boldly as possible without repercussions on the integrity and strength of the element, while plasticity allows the wood to keep its new shape. However, this is not a natural phenomenon but a whole technological process. It involves a treatment to increase the elasticity of the wood and a mould to keep it immobilized from the end of the treatment until the temperature and humidity return to normal.

Wood bending methods

The oldest method by which wood could be bent was repeated treatment with boiled water. This was how ships were built in the past. If the elements used were smaller, they were boiled directly in water until they were easy to bend. Once elastic, the elements were put in a mould or bent into a desired shape (in the case of the large beams used in ships) and immobilized until completely cold.

Steam is the best known and most widely used method of bending wood. It involves heating the wood with steam so that it becomes easy to shape. The process takes place in an enclosure specially built to keep the wood in an environment of high temperature, steam and humidity. This is because at 99°C and above, lignin (a component of the wood cell along with cellulose and hemicelluloses and responsible for the rigidity of the wood) loses its rigidity and allows the wood to bend without significant resistance. The wood remains in this enclosure for a certain time, depending on the thickness of the piece and the species. Some sources say that one hour is needed for every 25 mm thickness of the element. After removal from the steam enclosure, the piece of wood should immediately be placed in a mold and clamped. After cooling and drying the wood the piece will remain in the shape given by the template.
The method can be used both industrially and in small or DIY projects. The difference is made by the type of enclosure, the molds used and the ease of working. Those who propose a project involving curved elements find here an easy way to steam them.

Hot pressing is another bending method that involves heating wood to high temperatures and shaping under pressure. During the process, the wood is exposed to heat and pressure, which allows it to change shape into a specific matrix or form. The method is flexible and efficient, with pressing being achieved in a relatively short time.

High-frequency current bending (CIF) is also hot-pressing, where the temperature is raised by exposing the wood to a high-frequency electromagnetic field which heats the wood quickly and uniformly. Hot pressing in CIF has the advantage of very short bending time (up to 2 minutes) and uniform heat transmission inside the piece. Both hot-pressing methods are mainly used for the industrial production of moulded tackle.

Wood can also be bent by methods involving various interventions on the wood. Getting laminated beams (glulam) by parallel overlapping and gluing several layers of wood is one of them. Another is partial cutting of the wood by removing parts of it and then gluing the remaining segments so that the face of the modeled object has a continuous grain. But these methods are no longer dependent on the elasticity and plasticity of the wood and can be applied to any species.

Wood species that bend well with temperature and humidity

Not all wood species have the same plastic deformation capacity. Some species are known for their superior flexibility and plasticity. Here are some of them:

Fagul is perhaps the species best known for its ability to bend. Thonet practiced its bending method on such wood. In the past, the bentwood chairs made in Romania in the Comănești factory or the one near Arad were made of beech. Its disadvantage is its very low resistance to humidity and environmental factors, which is why it cannot be used for outdoor projects.

However, it can be used oak tree, another species that bends well and is tough on the outside. Compared to beech it can be slightly more difficult to bend, with a higher rate of splitting. Knots and irregular grain contribute to the risk of cracking.

Frasin is known for its flexibility and ease of bending. That's why it's used to make musical instruments and sports goods (tennis rackets, for example). And ulmul is elastic and bends easily under steam.

Sources say that teak bends easily and was used in the past to make ships. It is a durable and plastic wood and is often used for outdoor furniture.

In general, hardwood bends much better than softwood, younger wood better than older wood, bending in the longitudinal direction infinitely better than in the radial or tangential direction, straight-grained wood better than twisted or irregular-grained wood, wood without knots and defects better than wood with growth defects.

I hope you find the above information useful. As always, additions are welcome. And if you have any questions or queries, please leave them below in the space provided. I will be sure to reply.