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Air drying and pre drying are the easiest and least expensive method used to remove water from fresh sawn lumber. A 2,000 pound fresh cut log may actually contain 50% weight water, or 1,000 pounds, that must be removed before it would be considered dry. Water weighs roughly 8.3 pounds per gallon, so this means that approximately 120 gallons of water must be removed from the single log. That’s a lot of water!
Given enough time, an air drying stack of lumber will dry to the maximum extent that the outside environment will allow, typically about 14% EMC (Equilibrium Moisture Content) and is mainly affected by the humidity of the outside air. The rule of thumb is that it will take a year per inch thickness of lumber to air dry, but that is not always the case; it depends on many variables, such as relative humidity and the season of the year. Lumber dries much quicker in the summer than it does in the winter. Using accelerated air drying techniques, it’s possible to get some species of lumber air dry in a matter of weeks.
The problem with air dried lumber is that most houses with modern air conditioning are very dry, and cause wood used inside to dry down to an EMC of 6%. So this means that no matter how long wood has been air dried, it is physically impossible for it to reach the extreme level of dryness required for lumber used inside a house. When air dried lumber at 15% EMC is brought into a house at 6% EMC, it will be forced to lose its moisture too quickly, and will usually crack and split.
Here is a good article on open air wood drying.