On September 10-11, 1997 a discussion on the merits of air-dried and kiln-dried wood was started when someone posted the following question on the forum.
Are there significant useability differences between air-dried and kiln-dried woods?
What follows are three of the posted answers.
from Wayne Miller:
The term air-dried can often be misleading. People use it to mean many different things. It should mean that the wood was allowed to dry outdoors and reach an equilibrium moisture content with its outdoor environment, wherever that may be. This might be in the range of from 10% to 15% moisture content.
That range isn't suitable to work with, at least in cabinetmaking. The wood must be brought indoors to finally establish equilibrium with the environment indoors. Depending upon the time of year and where you live, this can take from one to two weeks or longer.
There are dangers in air-drying wood as well. If allowed to dry out too quickly, case-hardening, honeycombing, etc., will ruin the wood. Properly done, air-drying yields perfectly acceptable lumber; in fact, I prefer the look of air-dried walnut over its kiln-dried version.
Kiln drying can be thought of as controlled air drying. Temperature and humidity can be carefully controlled to avoid the problems often associated with poorly done air-dried stock. Kiln drying often means a more uniform product, but there are plenty of mills which know how to properly air dry stock and if you find one, you will be saving some money in the bargain.
Just remember that the air-dried stock must be brought into the working environment for a couple of weeks before use. If you run into oak that twists into all sorts of distorted shapes after being cut or that is riddled with splits along the grain line, then you know what the dangers of air-drying are. Find another supplier and try again. Oak is the worst culprit I've run into, but all wood can be case-hardened by improper drying.
from Dave Warren:
There have been lots of discussions about this issue. It's been my experience that dry is dry, it doesn't matter how it got that way. If the wood is truely dry and you let it acclimate to your shop environment for a while properly stacked and stickered you won't be able to tell the difference. A relatively inexpensive moisture meter will tell you if it's dry. 8-10% is usually where my wood is when I'm working it. Kiln dried stuff sometimes "gains" moisture while it's acclimating.
and from Bob Sabourin:
I live in eastern Ontario in Canada, about 50 miles from New York state. I generally buy kiln dried lumber but if I find a good deal, I'll also buy air dried.
For example, last week I bought some air dried oak for 75 cents a board foot. It's got a few knots but certainly usable. This wood has been air drying for 6 years at a very small local saw mill. When I got it home, I measured the moisture content at 15%. I moved it inside, stacked and stickered it and when the heating season starts, it should get below 10% fairly quickly.
Three years ago, I bought a couple hundred board feet of butternut air dried for two years at 50 cents a board foot. Again, a few knots but plenty of good sections. The moisture content of that was again around 15%. Once stored in the workshop, it reached 8% within two months.
My point is that it doesn't seem to matter how long it's left outside to dry beyond a couple of years, in this area, it never gets better than 15% MC but will then quickly become usable when brought into the workshop for a coule of months.
I've never had problems with anything I've built with air dried lumber. It is important however to monitor its moisture content. Some professional woodworkers never use anythimg else but air dried.