Drying

Understanding the Drying Process

 

Before you can glue up and finish your bent wood projects, proper drying is essential. Compressed wood arrives at approximately 20 to 25% moisture content and must be dried on the form after bending. The wood develops a "set" or memory of its new shape within 30 minutes to an hour on the bending form, allowing you to substitute lighter clamps during the drying process. While sculptural parts may not require a drying form, precision parts should remain on the bending form during drying with full support for both inside and outside curves.

 

Monitoring Moisture Content

 

Cold-Bend™ hardwood responds well to all drying procedures, but progress should be monitored using a moisture meter on scrap pieces to confirm desired results. Alternatively, you can overdry the wood and then allow it to equilibrate with its environment after drying and before assembly. Quality pinned moisture meters from manufacturers like Lignomat, Wagner, and Delmhorst typically cost between $200 and $500. But pinned meters are not easy to use on hardwood. I recommend a pinless meter. The Wagner Orion 930 dual depth pinless moisture meter, priced slightly under $500 in 2024, represents an excellent choice for this application.

 

Wood intended for interior use typically requires kiln drying to about 6 to 8% moisture content, which it will eventually reach in a room at 35% relative humidity, the typical environment of most homes. Compressed wood remains quite stable in the 5 to 10% moisture content range but becomes less stable above 12%. Inadequately dried wood can relax its shape, shrink, and affect joint integrity in the same way as using wood that hasn't been properly acclimated to its environment.

 

Understanding Shrinkage

 

Like all wood, compressed wood shrinks during drying. Expect approximately 1% shrinkage in your dried parts, though this will be less if the wood is well secured on a form. This shrinkage is typically insignificant for most applications regarding the width of the part but usually requires consideration for length. When a part is fixed while drying on the bending form, it will shrink less and put pressure on the drying fixture, which should be strong enough to resist this force.

 

Sideways shrinkage varies, but you might experience approximately 1/16 inch shrinkage over a 6-inch wide piece when drying from 20% to 7% moisture content. Slightly more shrinkage occurs on the flat sawn surface compared to the quarter sawn face, which is consistent with non-compressed lumber behavior as well.

 

Air Drying Limitations

 

While compressed wood will eventually air dry, the time required depends on environmental humidity and plank thickness. This approach may work for parts that will be fixed to prevent movement, but subsequent drying after assembly may cause enough shrinkage to break joints. A simple curved chair back fixed between two upright parts won't need to be dried as thoroughly as a part where the bend is free to move or relax, but joints will be stressed if all parts aren't at equilibrium moisture content, usually around 7%.

 

Air drying compressed wood is generally not recommended because it's too tempting to use the part before it's actually dry. In boat restoration, wet compressed wood boards can be fixed to a hull or installed as sister ribs and allowed to dry in place, but this requires heat and low humidity for proper drying. Shrinkage after installation may be counterproductive, and the same consideration applies to millwork projects. Since length will be lost during drying (less than .25%), millwork projects shouldn't be attempted on site, or callbacks can be expected to deal with joints pulling apart as the wood dries.

 

Partial Drying Solutions

 

Drying problems can be somewhat overcome through pre-bending followed by partial air drying before assembly to bring the wood closer to equilibrium moisture content. For example, a boat rib can be roughly pre-bent to an approximate shape, then pre-dried to about 12% moisture content, which usually takes about a week in a normal heated room. This leaves sufficient flexibility for final bend adjustments while minimizing further shrinkage after installation.

 

Controlling Moisture Through Relative Humidity

 

Moisture content can be controlled by managing the relative humidity where the wood is dried. The relationship between RH and wood moisture content follows predictable patterns:

 

65-68% RH produces 12%MC in the wood (Washington State in the winter)

58-64% RH produces 11% MC,

52-58% RH yields 10% MC,

46-52% RH results in 9% MC,

39-46% RH gives 8% MC,

32-39% RH produces 7% MC, (average in most spaces with HVAC)

25-32% RH yields 6% MC, and

19-25% RH results in 5% MC (Washington State in the summer)

 

Adding heat drives the relative humidity down and speeds drying. In most indoor, climate-controlled locations, wood is exposed to an average of 35% RH and will eventually equilibrate to 7% moisture content. Outdoor projects typically equilibrate to 12% moisture content because average outdoor humidity runs about 65%. For projects destined for dry areas like Southern California and Arizona, overdrying is always recommended. Located in Washington state, achieving 5 to 6% moisture content without kiln heat would be extremely difficult.

 

Kiln Drying with Hot Air

 

Hot air kiln drying represents the recommended method for drying compressed wood. Simple kilns or drying boxes can be constructed from 4x8 foot sheets of rigid foam insulation, a space heater, and a small fan. You can build a drying box as large as needed. An easy construction approach uses four sheets of 2-inch foil-backed rigid foam to create approximately a 4x4x8 foot box, with no bottom layer and one sheet cut in half for the two ends. Stack the components so they can be disassembled to remove heavy forms, leaving gaps for moisture venting rather than sealing completely.

 

Temperatures of 90 to 120 degrees Fahrenheit will dry parts fairly quickly. Monitor temperature with a portable weather station or suitable thermometers. Inexpensive frothing thermometers used in espresso brewing can be inserted through the walls and work excellently. Watch for and eliminate hot spots through venting or fans. Aim an extra fan directly at the heater to disperse heat better and prevent the heater from overheating.

 

Round radiant dish heaters work well for this application. When they get too hot, they stop heating, but with a fan pointed directly at them to disperse heat and cool the unit, they typically stop heating at about 125°F, which is hotter than ideal for portable kilns anyway. Use low to medium heater settings, starting low with lots of venting, then gradually closing up and increasing heat until you become familiar with conditions that provide steady temperature. Small ceramic heaters rated at 1500 watts run too hot unless they have a 750-watt setting, but they're generally avoided because they burn out easily and often trip out at only 90°F.

Keep parts on the form during drying with good support on both sides for precision work. The wood remains flexible until fully dry, though it may seem rigid before reaching complete dryness. A half-inch board should kiln dry in about three days at 110°F, while 1-inch boards require about a week and 2-inch boards need approximately three weeks.

 

Overdrying Considerations

 

It's possible to overdry parts with hot air kiln drying. Overdried parts tend to "overbend" slightly on their fixture, which is an unusual characteristic of this wood and opposite to the springback experienced with steam-bent wood. However, the wood will move toward its intended shape if allowed to gain ambient moisture after removal from the fixture. This process can take several days and depends on humidity conditions. This equilibration process proves useful for keeping all parts together before assembly at the same relative humidity for several days to weeks.

 

Dehumidification Drying

 

Commercial drying operations often employ dehumidification to dry lumber, though it's not necessary for small projects that respond faster to heat inputs. Dehumidification can be attempted on a smaller scale by wrapping the part with plastic sheeting and placing a dehumidifier inside. Boat interiors could be treated this way. Commercial dehumidifiers are designed to operate continuously in harsh environments at low relative humidity, while home dehumidifiers don't reduce humidity very low and don't last long with repeated use.

Monitor drying progress with a moisture meter on scrap parts. Maintaining some heat around 80 to 90°F helps even with dehumidification, though you can easily drive the relative humidity so low with this heat that the dehumidifier becomes ineffective, leaving heat to do all the drying work. The dehumidifier itself probably generates this much heat anyway. For larger projects, add heaters and fans. Unlike heat drying, dehumidification drying should not be vented, as this would require drying all air infiltration as well as the kiln area.

 

For slow dehumidification drying, target 35% relative humidity and parts will eventually equilibrate to 7% moisture content, an ideal target for indoor projects and an excellent way to equilibrate parts before assembly.

 

Calculating Water Removal

 

One hundred board feet of lumber at 25% moisture content (roughly what you'll receive with Cold-Bend™ hardwood) contains about 125 pounds of water, or approximately 15 gallons. To bring it down to 7% moisture content, you need to remove about two-thirds of that water, or roughly 10 gallons. Scale this calculation up or down for your specific project. Remember that if you're using wood forms, you'll be drying them too, so add their board footage and moisture content to your calculations.