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Drying Parts

You'll need to dry your bent wood before you can glue it up and finish it.

Drying Cold-Bend™ hardwood

Compressed Wood is shipped to you at about 20% to 25% moisture content. You dry it on the form after it is bent. You can substitute light clamps during drying because the wood has already taken a "set" or memory of its new shape after only an hour or two on a bending form. For sculptural parts, no drying form is necessary. But for precision parts, use the bending form during drying and keep it fully supported for both inside and outside curves.

Cold-Bend™ hardwood responds well to all drying procedures. The progress of the drying should be monitored with a moisture meter on scrap pieces to confirm the desired result. But you can also just overdry it, and then let it equilibrate with its environment after drying and before gluing up into an assembly like a chair. Lignomat, Wagner, Delmhorst and others make good pinned moisture meters that sell for about $200 to $500. My favorite is the Wagner Orion 930 dual depth pinless moisture meter. It sells for a little under $500 in 2024.

Wood for interior use is usually kiln dried to about 6-8% and it will eventually get there in a room at 35% RH - the typical RH of a home environment. Compressed wood will be quite stable in the 5 to 10% MC range, and less stable over 15%. If the wood is not adequately dried, it can relax its shape, shrink, and affect joint integrity in the same way as using wood that hasn't been properly acclimated or equalized to its environment.

Compressed Wood, just like any wood, shrinks a bit when it dries. Expect about 1% shrinkage in your dried parts (less if it is well secured on a form). This is insignificant for most applications in the width of the part, but usually needs some consideration for the length. If the part is fixed while drying (on the bending form), it will shrink less, and will put pressure on the drying fixture, which should be strong enough to resist this. Sideways shrinkage varies, but you might get ~1/16" shrinkage over a 3" wide piece drying it from 20% to 7%. There will be slightly more shrinkage on the flat sawn surface, and less shrinkage on the quarter sawn face. This is the same for non-compressed lumber too

Air Drying

Compressed wood will air dry eventually, but the time depends on the humidity of the environment it is in and the thickness of the plank. This may not be a problem if the part is going to be fixed to prevent movement, but subsequent drying after assembly may shrink enough to break a joint. A simple curved chair back that is fixed between two upright parts will not need to be dried as hard as another part where the bend is free to move or relax, but joints will be stressed if all parts are not at equilibrium moisture content (usually 7%MC). Therefore we do not recommend air drying Compressed Wood. It is too tempting to use the part before it is actually dry.

In boat restoration, wet compressed wood boards can be fixed to a hull or as a sister rib and allowed to dry in place. But it takes heat and low humidity to dry it out properly. And shrinkage after placing the part may be counter productive. The same approach can be taken with many projects, such as millwork, but keep in mind that a little length will be lost (1%) during drying so millwork projects should not be attempted on site, or callbacks can be expected to deal with joints pulling apart as the wood dries. Millwork projects and boat projects should ideally have their parts dried and surfaced before installation.

Drying problems can be somewhat overcome by pre-bending, then partially air drying the wood before assembly to get it closer to equilibrium moisture content. For example, for a boat rib, the part can be roughly pre-bent to an approximate shape, then pre-dried to about 12% MC (usually a week in a normal heated room will get it there). There will still be a little flexibility to make the final bend adjustments and fasten it in place, and little further shrinking would be expected.

Moisture content may be controlled by controlling the Relative Humidity where it is dried. Here is the relationship between RH and wood MC...

58-64 RH - 11% MC

52-58 RH - 10%MC

46-52 RH - 9% MC

39 -46 RH - 8% MC

32 -39 RH - 7% MC

25 -32 RH - 6% MC

19-25 RH - 5% MC

Adding heat drives the RH down and speeds drying (next section). In most indoor, climate controlled locations, wood will be exposed to an average of 35% RH, and will eventually equilibrate to 7% MC. Outdoor projects will equilibrate to 12% MC because average outdoor humidities will be about 65%. We always over-dry for projects that go to dry areas like Southern California and Arizona. Being located in Washington state, we would have a hard time getting our wood to 5% to 6% MC without kiln heat.

Kiln Drying with Hot Air (vented drying)

This is the drying method we recommend for Compressed Wood. Simple kilns (drying boxes) may be built from 4' x 8' sheets of rigid foam insulation, a space heater, and a small fan. You can build a simple drying box as big as you like. A really easy one to build is to use 4 sheets of 2" foil backed, rigid foam sheets to make ~ a 4'x4'x8' box (no bottom layer, and cut one sheet in 1/2 for the two ends). Just stack it together so that you can take it apart again to remove heavy forms. You don't seal it as you need gaps to vent the moisture off. Temperatures of 90 - 120 degrees F will be sufficient to dry the part fairly quickly. Monitor the temperature with a portable weather station or suitable thermometers. Inexpensive frothing thermometers used in espresso brewing can be inserted through the walls and work great. Watch for and eliminate hot spots in the kiln with venting or fans. Aim an extra fan right at the heater to disperse the heat better and to keep the heater itself from getting too hot. The round radiant dish heaters work well. If they get too hot, they'll quit heating. With a fan pointed directly at them to disperse the heat and cool the heater, mine will stop heating at about 125 F, hotter than I like to run the portable kilns anyway. I use a low to medium setting on the heater. Start low, with lots of venting, then close up and/or increase the heat gradually until you are familiar with what conditions give you a good steady temperature. The small ceramic heaters that are 1500 watts are too hot, unless they also have a 750 watt setting. But we don't use them because they burn out too easily and often trip out at only 90F.

Keep the part on the form for drying and well supported on both sides, if you need a precise part. It is still flexible if it isn't dry, but it might not be fully dry either when it seems rigid.

A 1/2" board should kiln dry in about 3 days at 110F. 1" boards will take about a week and 2" boards about 3 weeks. It is possible to over dry the part with this approach. Over dried parts tend to "over bend" a little on their fixture. This is a very unusual characteristic of this wood and the opposite of the "spring back" suffered by steam bent wood. However, it will move towards its intended shape if it is allowed to gain ambient moisture after it is removed from the fixture. This can take several days and depends on humidity conditions. This is called an equilibration process and is useful for all parts to be kept together before assembly at the same RH, for a few days to a few weeks.

Kiln Drying by Dehumidification (drying without venting)

Commercial drying operations often employ dehumidification to dry lumber, but it is not necessary for small projects that respond faster to small inputs of heat (see above). DH may be attempted on a smaller scale by wrapping the part with a sheet of plastic with a dehumidifier inside. Boat interiors could be treated in this way. Commercial dehumidifiers are made to operate 24/7 in harsh environments and run at low relative humidity. Home dehumidifiers don't drop the humidity very low, and don't last long with repeated use. Monitor the drying progress with a moisture meter on scrap parts. It helps to maintain a little heat - about 80 to 90 F even with DH, but you can easily drive the RH down so far with this heat that the DH is not doing anything, and it's just the heat doing the drying. The dehumidifier itself will probably generate this much heat anyway. For bigger projects, add heaters, and fans. Unlike drying with heat, DH drying should not be vented. If it is, you will have to also dry all the air infiltration as well as the kiln area.

For slow DH drying, target a relative humidity of 35% and your parts will equilibrate to 7% moisture content eventually - an ideal target for indoor projects and a good way to equilibrate parts prior to assembly.

100 bd ft of lumber at 25% moisture content (roughly what you will receive Cold-Bend™ hardwood at) will have about 125 lbs of water, or about 15 gallons of water in it. To bring it down to 7% MC, you need to remove about 2/3 of that water, or about 10 gallons. Scale this math up or down for your project. Keep in mind that if you have wood forms, you will be drying them too. You have to add their board footage, and moisture content.

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