Kiln vs. Air Drying; by Aker Woods
For many applications, it's important to make sure your wood is dry. Logs and lumber shrink in diameter, height, and width as they dry, but not in length. Kiln drying adds an extra cost, and we want to make sure you only spend what you need to. To help you make a decision, we've listed some of the benefits of each method:
Benefits of Kiln Drying:
- Kiln drying is faster. For large sawmills, this is one of the most important benefits. Moving the lumber quickly reduces inventory costs and saves space in the millyard.
- Kiln drying kills insects and eggs. For sawmills using salvaged timber and for sawmills in very humid areas, this can be an important consideration. We use only live timber free of insects, so this isn't a factor for us.
- Kilns "set the pitch". Some species, like Spruce, have pockets of pitch, even when
thoroughly dried. When the wood reaches a new high in temperature, the pitch can
liquefy enough to drip down the side of a board. Kiln-dried Spruce won't do this in normal environments because it will never get warmer than it did in the kiln. (If you notice a bead of pitch creeping down a timber or board, just wait for it to cool and
crystallize again. You can remove it with your fingers.) Note: even in Spruce, our species most prone to have pitch pockets, external pitch runs occur on less than 1% of the surface area.
Myths About Kiln Drying:
- Kiln drying produces a lower moisture content. True, but only for a couple of weeks. All woods have an equilibrium moisture content. It varies with the wood species and the relative humidity where the wood is installed or stored. For Ponderosa Pine, equilibrium moisture content is 12-19%. You can kiln dry Ponderosa to 8% for example, but it would quickly absorb enough moisture to get to equilibrium. For most projects, it's unwise to use wood with very low moisture contents. That's why professional flooring installers like to let the flooring "breathe" for awhile before installing it. If they installed wood that wasn't at equilibrium, the floor might buckle as it absorbed moisture.
- Kiln drying prevents bows and twists. Have you ever noticed that in a lumberyard, there are more bowed and twisted studs at the bottom of a unit of lumber than on the top of a freshly un-banded unit? Part of the reason is that customers have sorted through the studs, leaving more bowed and twisted ones behind. But another reason is that the lumber was kiln dried, planed, and banded in quick succession. The bows and twists don't have a chance to develop until the bands are off and the lumber has a chance to reach equilibrium moisture content. Air drying, on the other hand, lets all the lumber that's prone to bow and twist do so on its own schedule. We can then cull these out of your order for some other use, maybe just firewood. The best way to minimize bows and twists is to start with good, straight timber with small growth rings and knots.
- Kiln drying prevents checking (cracks in the wood). Not true. Woods check in relation to their species characteristics, original moisture content (timber cut in the spring of a period of wet years has a much higher moisture content than wood cut during the winter in a drought period), thickness of the lumber or log, and how fast they dry, not the drying method. For this reason, kilns dry woods on schedules of hotter and cooler temperatures and add moisture to the wood during drying to avoid more checking than would occur with slow air drying. For large beams and logs, it's possible to control checking by cutting a thin slot in the sides of the beam where you want the checking to occur. This works for air or kiln drying, and should be done right after milling.
If you're still doubtful about whether kiln drying produces better wood, go to an antique dealer. None of the wood there was kiln dried.