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Steam Bending Wood

Steam bending wood is a technique used for centuries, back in the days it was commonly used to bend hull ribs for ships, different hand tools and weaponry. As time passed it became more popular under the furniture makers and they started using this technique for making chairs, baskets, tables and even musical instruments like violins and guitars. These days steam bending has become less popular in the industrial section but still widely practices by craftsmen and even thought at craftsmen schools.

The ability to bend solid wood can give your projects both a structural advantage and an amazing visual boost. For example, a sweeping curve on the back leg of a chair can be very weak if it is cut from a solid board. There will always be a small part of the leg with short grain which will break if it is subject to stress. If you use the steam bending technique to make this leg, It will keep almost all of the strength of the original straight piece of wood. The grain will also follow the curve and visually show the shape you have created.

The principle

Wood cells are held together by a substance called lignin. Imagine the wood to be a pack of straws, when looking from the top the space in between those straws are filled with lignin. by steaming the wood you decrease the strength of the lignin bond between those straws. Steaming the wood on 100º for about 1 hour for every 3cm in thickness (width is irrelevant) will soften the lignin enough for you to bend the wood in the required shape.

The steam box

Building a steam box for bending wood only requires exterior-grade plywood, waterproof glue and an electric tea kettle.the box should be as small as possible depending on your project, a common used size is around 1.5m x 15cm x 15cm (LxWxH) you can make it longer but you might have to add a steam kettle or find a better way to produce enough steam. on the bottom of the box drill a hole and use iron pipe to spread the steam coming from the kettle.

Use Tongue and groove joints help seal the box’s corners, you can consider to silicon them to make sure no steam will be lost, but if you glue your joints right it should not be necessary. add screws every 12cm along the corners, the box also needs a vew drainage holes in the bottom, imagine you glue your steambox so perfect. it might explode due to the pressure rising inside. Also the steam should be able to circulate around the wood and through the box. therefor it is advised to raise your wood by adding some horizontal dowels. make a hinged door at one or both ends, use a rubber strip to seal them. to be able to monitor the heat inside of the box you can add a hole on top and put a thermometer through.

Choosing your wood

Species Smallest Radius
Oak (red/white) 5cm
Hickory 5cm
Elm 5cm
Walnut 8cm
Ash 12cm
Cherry 15cm
Maple 20cm

There are 2 simple rules when choosing wood:
- Exotic woods do not bend well.
- Softwoods do not bend well and should be avoided.

Not all species bend well and not all species can bend as much as others. It is also important to find the perfect board for your steam bend, take a good look at the grain, it should be as straight as possible and not run out to much, especially where the bend will take place. It should be knot free or any sign of knots, damage in the grain or weird patterns should be avoided.

The moisture content of your board is also important. Freshly sawn wood usually has a moisture content of about 70%. Wood bends best between 20% and 30%. It is better to catch this “on the way down” rather then to remoisturize the wood. The proper moisture content is important if you are bending to small radius- and less important when bending shallow curves. Once the wood has air dried to 6% or 8% it may not be possible to bend it to small radius curves. The lignin bond is only partially reversible at this level, especially if the wood has been sitting for a year or more in this dry state, it will not be possible to gain all the elasticity back by remoisturizing the wood, even by steaming or soaking in water for a longer period of time (days or weeks).

Preparing the bending mold(form)

Before you start steaming your wood, you should have the mold ready to rumble! Basically a mold is the shape you want your wood to bend in, it should be fixated to the table or something similar and very sturdy. Bending the wood will take allot of power so if the mold is not fixated well enough you will not be able to bend the wood around it fast enough. a vew examples of molds below. when making your bending mold, take in consideration your wood species, minimum radius etc.

 Steaming and Bending

Now the Steam box and bending shape are ready to go you can start with steaming the wood, Remember the temperature inside the Steam box should be around 100º then steam your board for about one hour per 3cm of thickness (regardless of width). Wood at 30% will require a little less time, wood at 15% a little more. Oversteaming is not advised, as it may cause compression wrinkles to develop as the bend progresses around the form. when your board had enough sauna its time to bend! get your clamps ready and make sure no objects are in between the steam box and bending mold. Once you open up the steam box and take the wood out the lignin starts to harden. so its all about speed, Bend the wood as soon as possible around your bending mold and clamp them. depending on the size of your bend you might need to ask for some help. the faster you bend and clamp it around the mold the better. Once it is clamped in place, leave it there to dry for a vew days. usually after 5 to 7 it will be fully cooled. (or under 10%)

 Springback and  Close in

Springback can occur when the curve is so shallow so that the lignin does not shift enough to hold the new shape, the part is too dry (moisture content too low before steaming or insufficient steaming time), the clamping pressure was to less, the part was not left on the bending form until fully cooled, or the part didn’t dry thoroughly (less than 10% M.C.).

Close-in can occur iIf the moisture content of the wood is too high when bending (above 30%), the displaced and compressed wood fibers continue to contract as the bent wood dries out or If the end pressure is too high during the bending process, the overcompressed inner face of the blank will contract as the wood dries out. To avoid Springback or close in you can fixate the endings with a rule or connect them to another part of the end product fixating them in place. if you are bending legs for a table or chair for example, Experience is all that will help you to avoid both these problems.

Below some nice examples of steam bent furniture.

Wanna see some Steam bending Action? This 6minute video will show you some amazing Steam Bending!

 

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