By Garrett Hack
I’m often asked what the difference between air-dried and kiln-dried lumber is. Since most wood sold today is kiln-dried, that’s what we are familiar with. Seek out a source of air-dried wood or dry some of your own, and you’ll discover some pleasing differences.
The differences all start in the kiln. In careful kilning, the wood is subjected to periods of heat, steam, and ventilation to pull moisture from it gradually. Most of the time, this yields good lumber, but when things aren’t so gentle, the outside of the boards can dry too quickly, hardening the cells while the interior is still moist. In the worst case, the cells collapse, and interior voids develop, known as honeycombing. You’ll know something is not right when you start cutting parts, and they start warping almost immediately.
Kilning leaves the wood more brittle too, and steam used during the kilning process also leaches color from the heartwood. In a wood such as walnut, the rich shades of reds, purples, browns, and into the very white sapwood can turn a gray-brown. Subtle color contrasts, depth, and even brilliance are lost. In very few woods, like pear, the steam enhances the color.
While air drying takes a lot longer, it is far more gentle. The result is wood that feels more alive, elastic, and friendly. I really notice how it cuts more sweetly with hand tools; my shavings are continuous and fluffy. It’s this elasticity that makes air-dried wood able to steam bend more consistently and make tighter bends than kiln-dried. Chopping the end grain is where I really notice its elasticity, which cuts cleanly. Kiln-dried end grain is often more ragged as the brittle cells tear out.
Kiln drying does have its advantages, or it wouldn’t be done. It speeds up drying and the production of usable lumber. The high temperatures kill fungus and other organisms in the wood and set the pitch in softwoods such as white pine. Some claim kiln drying’s main advantage is that it produces lumber more stable by drying the cells so fully they can no longer readily reabsorb moisture. Air-dried, with all of its other advantages, is no less stable if you dry it adequately and cut your parts with forethought. And if you’ve chosen a species known to be stable, so much the better.
Allowing time for acclimation is key to getting stable air-dried wood. It needs time to acclimate to shop conditions and a workable moisture content (MC). Even after many years stored in a barn, my wood is around 12% moisture content (MC) — too high for interior furniture. Once in my shop for a few weeks, especially in winter, it falls to 8 – 9%, or even lower. Softwood acclimates even faster. Even kiln-dried wood needs acclimation if it’s stored outside or in an unheated space, as it will reabsorb moisture.
Don’t rush acclimation is step one of producing stable air-dried wood. I never had a moisture meter for a long time and just felt the wood to see when it was ready to work. It should feel warm to the touch, and the shavings shouldn’t clump together when squeezed.
The next step to getting stable parts is to plan out the cuts. I aim to use each board’s best parts for the most important parts — well-behaved and attractive quarter-sawn grain for legs or door stiles and rails and less consistent grain for secondary parts. Since I’m drying my own lumber and it isn’t costing me much, I can be critical at this stage and use the best of the best. Sighting down the length of a board tells me a lot about how stable parts cut from it will be — the straighter and less warped, the better. I use a lumber crayon to mark out the cuts, note what each part is, and check them off a cutlist. Naturally, there can be some surprises when I start actual cutting, but I can rearrange some cuts if necessary, and I always have some extra material for backup.
Over 40 years of cutting and drying my lumber have only increased my appreciation for air-dried wood — to say nothing of the cost savings and having a whole tree of boards to work with. It’s not hard to dry some of your own if you have space, time, and inclination. Just don’t rush the final acclimation and cut your parts smartly; you’ll find air-dried wood a joy to work in every way.
DRY YOUR OWN
You will be successful air drying wood if you follow some basic guidelines:
~ maximize the amount of air flowing around each board or plank
~ use a roof to keep the rain off the top of the pile and as much as practical from blowing into the sides (snow is less of a problem)
~ slope your pile slightly to drain off any water that does get into it
~ align your stickers vertically between layers and space them somewhat evenly
~ use dry stickers just a little longer than the width of the pile; if they stick out, rain can wick into the pile
~ dry the wood for at least a year per inch of thickness
The ideal location to build a stack is out in the open, where breezes can blow through the pile. Some shade is okay as long as the drying is not so slow that moisture lingers and molds start growing. The common sense tradition of milling winter-felled trees in the spring takes advantage of the ideal drying months of April and May — breezy and cool. Cut your wood in the summer, and mold has a greater potential.
Build a stable foundation for your pile with timber bunks (8X8’s) up on cement blocks for good air circulation under the pile. Most important is that the bunks are not twisted in relation to one another, or one is low, or you will dry a twist and sag into each board. Sight across them to see. I build piles about 36” wide for good airflow with bunks about 3’ apart.
Dry hardwood stickers 1” X 1” are best. Stack the same thickness in each layer and longer boards towards the bottom. I often start with the thickest material to add solidity to the foundation. For 8’ long boards, 3 or 4 stickers are enough. My best boards get stacked in the middle of the pile. I don’t seal the ends of boards with old paint to prevent end-checking. Some degrade is to be expected, and seeing where any cracks occur gives me insight into weaknesses in the board.
A roof is very important. It needs to be weather tight or nearly so. Old roofing tin is ideal. It’s easy to place on the stack or remove yet strong enough to support a foot or more of overhang. I weigh the roof down with cement blocks, old timbers, or whatever is heavy and handy to keep the roof from blowing off. A tarp can work, but be careful that air can still circulate easily. After a year or more, you can stack your lumber in a shed or barn without stickers and eventually build something nice with it.