Kiln dried, air dried, green, seasoned: all words used to describe the condition of wood. But, what do they mean? Which is the best for me?
Kiln dried: Heat and air flow is in an enclosed space is used to dry out all the water content very quickly. This also has the side effect of removing various oils and resins. While this may be OK for construction grade materials, fine woodworking materials will suffer from the process.
Air dried: As the name implies, this is wood that is dried by air alone. No artificial heat source is used in the process.
Green: Freshly cut wood. Opposite of seasoned.
Seasoned: Wood that has been dried while retaining various oils and resins. These oils and resins undergo a chemical transformation where they harden the wood fibers, producing a stronger piece of wood.
In both kiln dried and air dried wood, you'll see shrinkage of the wood fibers, resulting in warpage of the wood. Proper stacking and stickering helps to avoid this warpage. However, with air dried wood, the warpage is generally less severe.
Kiln dried wood has another distinct disadvantage: chemicals removed in the rapid drying process are oils that allow wood movement. Now, generally, woodworkers don't like wood movement in their projects. However, this is movement of a different nature. The movement I'm talking about here is the kind that allows the wood fibers to slip past each other and relieve stress in the wood. The majority of us have heard war stories of case hardened pieces of wood being ripped on the tablesaw, warping without warning, and then causing a severe kick-back.
Air dried wood, on the other hand, thanks to the seasoning process, is much more stable. It tends to be less brittle, take a better finish, and in general be easier to work with. The downside, of course, is that air drying takes much longer and therefore is much more expensive.
Hope this helps someone.
Regards,