Allowing lumber to sit outdoors or inside to reach the desired moisture content before working with it may be the easiest, least expensive way to season wood, but it’s also the slowest. And drying times can differ significantly depending on wood types, the primary moisture level in the green wood, its thickness and density, drying environments and processing methods.
On the other hand, all the above drying conditions can by controlled when wood is kiln dried. Is it worth it to you to invest in building or purchasing a kiln for your woodworking shop? Opinions vary as to the values of both methods of drying wood, but the decision is ultimately up to you.
Air vs. Kiln Drying Times
For centuries, the rule of thumb for air-drying wood is to allow one year of drying time for every inch of lumber thickness. This saying obviously only takes the thickness of the lumber into account, not climate or seasonal variables. Of course, properly setting up the lumber for air drying commonly entails a large investment in time and other factors, including stacking techniques and adequate protection from the elements.
While there are various styles, sizes and types of kilns used to dry lumber, they all balance humidity, airflow and temperature to reduce the MC in wood. Because various species demand various times to dry, even in a kiln, there is no hard and fast rule for how long the process can take.
Air drying is a way to dry wood but it will never get down to where furniture grade lumber needs to be. Air dried will only get as low as the outside humidity which isn't low enough. When kiln dried, the cells in the wood shrink and will never retain the same shape as before, air dried will not shrink the cells as small as kiln drying, so it can make the wood a little unstable and allow it to move.
The main advantage of using a kiln is that using such a controlled environment dries the wood faster and more evenly, lessening any moisture deviation between the external shell and the inner core. The uniformity of the process often eliminates insects or larvae hiding within the wood, reduces the chance of mold-related stains and hardens the pitch in softer woods such as pine.
Kiln Drying Defects
Kiln drying can also introduce drying defects into wood, especially if the process is rushed and no corrective steps are taken. For example, case hardening occurs when the outer shell of the wood dries faster than the inner core. The wet core prevents the shell from shrinking, which can cause the shell to dry in a stretched-out condition. Then, as the core finally dries, the shell stops it from shrinking completely, which can split the core, causing a condition known as honeycombing.
Mixing different species of wood in a single batch can also cause good boards go bad by creating cups, checks, warpage, bows and twists in the lumber.
Over-drying wood to below a 6-percent MC puts nearly incurable stresses on the wood, leaving it brittle, hard, prone to splitting and very hard to work by hand or machine.
Even when properly processed, kiln dried wood can lose up to 20-percent of the subtle colors and grains running through the lumber.
Benefits of Air Drying
When wood is air dried, the slower seasoning process doesn’t cause the stresses that can result in splits, warpage, checks and other kiln-dried defects because the wood is not “baked”. However, air dried wood will naturally contain a higher MC level, typically between 12- and 20-percent. The only way to be sure air-dried stock is ready to use is to invest in a dependable moisture meter and test it on a regular basis.
Many experienced woodworkers prefer air-dried lumber, but the truth is that kiln drying can also produce excellent results. The trick is to season the wood properly and store it so it successfully maintains a satisfactory moisture content. Properly kiln-dried lumber is not only a pleasure to work, but can often prove to be more consistent than air-dried wood.