Stock Options: Air- or Kiln-Dried?Comments (0)
This article is from Issue 37 of Woodcraft Magazine.
Price is important, but moisture matters most.
Anyone who’s struggled to light a campfire with green logs understands that wood contains water. A freshly cut tree has a moisture content (MC) ranging from 80% to over 100%. At the highest MC levels, the water in the piece of wood weighs more than the wood itself.
Lumber isn’t stable enough for furnituremaking until it’s properly dried, or seasoned, to between 6% and 10% MC. The trick lies in lowering the moisture content fast enough to avoid wood-eating organisms, but slowly enough to minimize stresses that can create unwanted defects. Using a kiln, this seasoning process can take place in a few weeks. Another option is to stack the lumber in the open air and allow it to dry on its own.
You’ll find legions of woodworkers who prefer air-dried lumber, but the truth is that both methods yield excellent results when carried out correctly. The trick is to season the stock properly and store it in a manner that maintains the equilibrium moisture content (EMC) at working levels. Here’s what you need to know before buying kiln-dried or air-dried lumber to steer clear of drying defects and to minimize problems after you bring the stock home.
Moisture Content (MC): the amount of water in a piece of wood, expressed as a ratio of water to the weight of the wood when it’s completely dry.
Equilibrium Moisture Content (EMC): the MC level at which a board neither takes up water, nor gives it off to the surrounding environment. EMC fluctuates with temperature and humidity.
6% -10% MC
• Large quantities and varieties of available stock
• Heat treatment controls insects, staining, and pitch
• Improper drying can introduce stresses in wood
• Higher cost
For good reason, most woodworkers rely on kiln-dried stock. It costs more, but boards are ready to work after a week or so in your shop. By controlling airflow, temperature, and humidity, a kiln condenses several months’ worth of air-drying into a few weeks. Properly kiln-dried lumber is not only a pleasure to work, but it also proves more consistent. In addition to reducing drying time, exposing lumber to higher temperatures kills insects or larvae hiding within, reduces the chance of mold-related staining, and crystallizes, or hardens, the pitch in woods such as pine.
Rushing the kiln-drying process (which can happen when mixing different species of wood in a single batch), can make good boards go bad. Warpage—cups, twists, and bows—may result. While you fix the problems with a jointer and planer or by cutting boards into shorter lengths, more serious defects may still hide within. Drying a board too quickly creates internal stresses referred to as “case hardening.” Rip a case-hardened board at the tablesaw, and you’re liable to see the kerf close up at the back side of the blade, which becomes a safety concern. Checks on the face and honeycombing (splits located deep within the wood) are visible signs of case-hardening, but the best way to check your stock is to cut a slice from the middle of a sample board and bandsaw a kerf, as shown in “Case-Hardening Test,” below. To test stock up to 11⁄2" thick, bandsaw a 1⁄2"-thick notch out of the center to create two prongs as shown. (To test thicker stock, cut an additional kerf down the center.) If the stock has a drying defect, the open ends of the sample will open or close up as soon as it’s cut
It is possible to have too much of a good thing. Wood that has been over-dried (below 6% MC) never really recovers. While it will regain moisture to match ambient conditions, the stresses incurred are there forever—it will always be hard, brittle, more prone to splits, and harder to shape by hand or machine.
Many woodworkers forget that EMC is a constantly moving target. To keep the MC of kiln-dried lumber under 9%, store it indoors, preferably at or near 70°, and with a relative humidity under 50%. Swings in temperature or humidity, such as those that can happen in a damp basement or unheated garage shop, can result in unpredictable MC increases. To avoid furniture failure, check your stock with a moisture meter before you build. Wet wood can be restored to safe working levels using a dehumidification tent, like the one shown on page 69.
A case-hardened board contains stresses created when the exterior is dried more quickly than the interior, shrinking and compressing the wood within. Using case-hardened wood can do more than create project assembly problems. If a kerf closes in mid-cut, it can bind on a blade and kick back.
12% - 18% MC
• Lower cost
• Slower drying process introduces fewer stresses on stock
• Selecting from same tree results in consistent color and grain
• Wood selection limited to local species
• Wet stock susceptible to insect infestation, staining, or decay
• Drying process takes many months
Exposing wood to outdoor temperatures and humidity lets the stock dry at its own pace. Compared to kiln drying, the slower seasoning introduces fewer stresses that can result in checks, splits, and other drying defects. Eventually, the stock will stabilize to a MC level between 12% and 20%.
It’s important to note that the lumber is not ready for furniture straight from the stack. Once the stock achieves EMC, it will need to be restacked indoors and given more time to acclimate to lower moisture levels.
In addition to storage space, using air-dried lumber requires patience. The general rule is that green lumber takes one year of air-drying per inch of lumber thickness, but time varies depending on species and climate. The only way to be certain that your stock is ready to put to use is to invest in a reliable moisture meter and test the stock regularly.
Slower drying eliminates kiln-related problems, but air-drying isn’t defect-free. If not properly stacked and protected from the elements, as shown in “Well-Stacked Stock,” opposite page, freshly sawn wood can become a buffet for insects and microscopic organisms. Because air-dried lumber typically comes from trees not specifically grown for commercial use, it’s also more likely to harbor metal, such as a nails, barbed wire, or bird shot. These foreign invaders can destroy cutters and stain the wood.
Wood to Watch Out For
Sawing logs yourself, or hiring a local sawyer, is a great way to get lumber for a song. But when you calculate the cost of your time, and the replacement cost of a saw blade, it makes sense to be picky. Stay clear of those logs that might not offer high-quality lumber or that threaten to damage your equipment. Here are some defects to avoid, and some advice for safeguarding freshly-sawn stock.
Rot. Finding fungus growth on the bark usually means soft, punky wood within. Stack wood immediately to discourage further growth.
Metal. Protect knives and blades by avoiding boards with black iron stains or shiny glints of freshly sawn metal.
Bugs. Holes in the bark or the log may suggest hidden bug larvae. Use a non-toxic pesticide such as borate to prevent further infestations.
Compared to kiln-dried stock, getting air-dried lumber ready for milling requires several storage stages. First, the freshly sawn boards must be stacked and stickered outside or in an open-air shed to freely release moisture. When lumber reaches 20% MC or less, the boards must be restacked and stickered in a climate-controlled space so that the wood can continue losing moisture.
Depending on the humidity levels and temperature in your shop, it can take between 6 and 10 weeks to reach EMC levels of 8%. Many woodworkers prefer using wood when the moisture is closer to 10% because it's easier to work. Meter your stock every few days so that you’ll know when you achieve EMC.
Basement and garage shops don’t always offer an optimum wood-drying environment. During the spring and summer months, relative humidity can easily rise above 50%. At these higher levels, air-dried wood will stop losing moisture and kiln-dried stock will begin absorbing it.
Alf Sharp, an air-dried lumber dealer in Woodbury, Tennessee, has an easy, practical technique for bringing air- and kiln-dried wood to furniture-friendly MC levels: a dehumidification tent. To make a tent, cut the stock to rough lengths, and then arrange the boards around the dehumidifier. Cover the stack with plastic sheeting, turn on the dehumidifier, and wait. The tent can bring air-dried stock from 12% to 8% in a few days.
Compared to heated kilns, the ad hoc dehumidification kiln works slowly, but Alf finds that the gentler drying cycle does not introduce stresses into the wood.
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