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Drying and Finishing

Finishing with oils, lacquers, paint, etc

Finishing techniques are not altered at all by the wood compression process. Be sure that your parts are dry before you finish them, and any finish technique you are familiar with can be used.

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. For sculptural parts, no drying form is necessary. For precission 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 pinned moisture meter on scrap pieces to confirm the desired result. But you can also just overdry it, and then let it equilabrate 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 $150 to $300. A hammer electrode is most useful for dry parts and thick planks.

Wood for interior use is usually kiln dried to about 7% 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 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 the length of 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. Sidways shrinkage varies, but you might get ~1/8" shrinkage over a 3" wide piece drying it from 20% to 7%.

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% to 2%) 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 appoximate 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%..

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 appart 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 it's 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 usefull 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. Home dehumidifiers like the Sears models can sub in for small or temporary projects but they don't last long with repeated use like this. Monitor the drying progress with a pinned 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 its 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.

Gluing Cold-Bend™ hardwood

You can use any glue you like to fasten dried compressed wood. If you need to glue the wood before it is fully dry, let it air dry for a while so that the wood surface is somewhat dry (below 15% MC).

For projects that require rapid assembly, or a quick dry glue joint instead of clamps, the thick CA - cyanoacrylates work very well. Krazy glue and Super glue are CA's, but choose thick viscosity versions. The woodworkers supply web sites like Woodcraft and Rockler have them. If you need 8 oz or more, do a Google search to find reasonable prices. Fastcap and Flashglue have a good selection of the larger sizes and high viscosities. Buy the accelerator too when you buy CA's. CA joints are a little brittle. CA glue is something like glass when it is dry. It is very strong, but not very flexible.

Titebond glues will bond the dry wood very well, but these glues exhibit "creep". Creep is an issue for laminated wood, but Cold-Bend™ hardwood does not spring back after drying, and does not contain any stress from the bendng, so creep is not an issue. Titebond 1 should not be used in areas where it can be re-wetted. Titebond 2 is good for most applications and has water resistance. If you need to work in cooler than normal temperatures, Titebond 3 has a lower "chalk point" (about 47 F) and has a darker glue line - good for walnut, cherry, and even white oak.

Epoxies are excellent with Cold-Bend™ hardwoods, but brittle. Subsequent solid wood movement can cause epoxy joints to pop. Epoxies should not be clamped very strongly as epoxy wants some gap in the joint, which happens to make it a good gap filler too.

Moisture cured polyurethane glues like Gorilla glue are useless on the moist compressed wood. There is too much moisture for the adhesive, and it foams up. This creates a weak joint. Normally we avoid these glues also because they require a lot of clean up - not easy on bent parts.

Ureaformaldehyde adhesives like Polybond 800 are excellent with dried Cold-Bend™ hardwood. They do not exhibit "creep" in laminations, and like to be clamped very hard, which creates tight, almost invisible joints. They offer slight gap filling properties, and you can order various color hardners to match your wood species.

Screwing and Nailing Cold-Bend™ hardwood

There is really no difference to working with Compressed Wood compared with solid hardwoods when the Cold-Bend™ hardwood is dry. Screws and thicker nails should have pilot holes to avoid splitting the grain and turning off screw heads, just like with other solid hardwoods. But moist Cold-Bend™ hardwood doesn't need pilot holes for screwing and nailing. It acts like a softwood when moist. 

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