I just pushed this up to youtube. Working with self harvested hardwoods just became a whole lot easier.
http://www.youtube.com/paulwheaton12#p/u/0/yphYtfSK-8c
Wood is always trying to reach what is called equilibrium moisture content, EMC. This is a point where it is neither gaining moisture from, nor releasing moisture to the air at that particular temperature and humidity... this is a constantly moving target unless you are in a constantly controlled environment. It will reach that equilibrium whether it is kiln dried or air dried. The kiln is faster and has better control of the drying conditions.
As one woodworker told me "I don't particularly care if my clothes are dried in a dryer or on the line as long as they are dry when I put them on". The important thing is to know the target moisture content... the EMC of the place my wood is going to live, and get it as close as possible to that moisture content. How I achieve that is of less concern. When I bring wood into a house for finish use I often check it with my moisture meter. In my area I can air dry wood down to 12-14% mc, in the winter my house has an emc of around 7-8%. Air drying alone is not going to get me there. A winter with the wood stickered in the living room and the woodstove running will... that is how all the finish wood in our house was dried, the house was the kiln. I've also used the homeowner's sauna before, and the attic of our barn. Right now emc is probably around 14%... a non air conditioned house, especially in the southeast, goes through quite a range.
Wood swells when it absorbs moisture and shrinks when it loses moisture... moisture change is what makes wood move. Doors swell and bind in the humid summer air, they shrink and let drafts in during winter when the air is colder and drier. These are examples of wood moisture content changing to stay at equilibrium with the changing relative humidity. Paints, stains and varnishes can moderate rate but cannot stop wood from reaching emc.
If wood is dried too fast it checks. The surface dries and shrinks around a wet and swollen core. The thicker a piece is the more likely it is to have a high moisture gradient and thus high stress. As soon as the shrinkage stress exceeds the strength of the wood perpendicular to grain a check forms. Much like a tear in cloth it is relatively hard to initiate but once the rip starts it is very easy to continue very deep. I was in one of our timber frames today. There is a basement post I can stick my hand into. Nothing structurally wrong with it and they were forewarned that many timbers would check heavily at 8" thick dimension...just nature doing it's thing.
In thinner wood we consider a check a defect. Some woods check much more easily than others, they are more "refractive". Oaks are well known for checking heavily if mistreated after sawing. I can saw a board, lay it in the sun and destroy it in an hour or two, it will be honeycombed with checks. White pine under the same conditions will usually just dry. It is recommended to put oak into the kiln "green from the saw". The kiln schedule starts with moderate heat and high humidity to keep the wood from checking and mobilize the water vapor. As the wood loses moisture the drying "steps" become more and more aggressive. The core is shrinking in step with the shell and drying stress is relatively low. another advantage of a kiln is that dry wood is roughly twice as strong a green. If you can get a relatively dry shell quickly... without checking it, the strong shell "locks" the wood in shape and reduces distortions like cupping. A well run kiln can be superior to air drying but is not absolutely required IF you understand how wood behaves.
The most common mistake is to use green lumber, or lumber that is well above emc in a finish application and then have it dry to a lower moisture content and shrink. This causes gaps, or if the wood was firmly attached, splits.
Don't forget solar kilns I helped work on one 30 years ago :) I've got the guts to a small dehumidification kiln waiting on a round tuit.
This is a picture of my moisture meter in some acclimated framing.
(https://i180.photobucket.com/albums/x109/windyhilll/eqmoistcontent.jpg)
There is a thermometer/hygrometer giving readings. If you look up those readings on the chart linked below it will agree with the moisture meter readings... the wood is at emc, it is "dry", "seasoned", "acclimated"...ready
http://www.woodbin.com/ref/wood/emc.htm
This calc shows how much movement to expect on average if the wood is not yet at emc. Remember these are averages...trees don't read charts.
http://www.woodbin.com/calcs/shrinkulator.htm
One trick is to try to work wood a few % drier than target moisture content. It will then swell slightly and tighten your joinery...much like a wine barrel swelling watertight. Don't play that game too hard though. I've seen guys push the walls right off the floor :o.
Another good resource came to mind, this is a link to the US Forest Products Labs' publication "Air Drying of Lumber". Its a free download and a good reference;
http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr117.pdf
More good references from them on drying are "Drying of Eastern Hardwood Lumber"
http://www.fpl.fs.fed.us/documnts/usda/ah528.pdf
and "The Dry Kiln Operators Manual" (even if you aren't operating a kiln ;), better search through the index on their site for sections out of this one, it's large.)
"EMC of Wood in Outdoor Locations in the US and Worldwide", this gives average emc by month and location;
http://www.fpl.fs.fed.us/documnts/fplrn/fplrn268.pdf
VA Tech and Woodweb also have plans for solar kilns on their websites. Tech has a weekend visit and short course on theirs about every year around May.