How did the 18th century furniture makers build such beautiful furniture if they only had access to air dried lumber? Unless I'm wrong, and please tell me if I am, I don't believe they had kilns to dry their lumber to a 6%-8% MC. Didn't they use the same 10%-14% air dried lumber that us do-it-yourselfers use today?
If not, what did they do differently that we can't do today? And are we really going to encounter problems with air dried lumber as many people indicate we will? Maybe the old timers air dried their lumber first and then brought it into their working environment for a sufficient period of time to acclimate the wood before using it. Maybe this made the difference. Anyone have any historical information on this?
I've read that in the old days wood craftsmen would air dry their lumber in open sided barns for many months just as we do now days in air drying. Once the wood had dried out enough they would bring it inside their workshops or homes and keep it close to a hearth or stove when in use to draw out the rest of the moisture (many more months). They say that you could tell when it was ready by the sound made when you knocked on it.
I'm by no means a carpenter, but I couldn't see myself putting all this time and craftsmanship into a piece of furniture only to watch it go unsquare and develop cracks in the joints.
Air exchange would be responsible for greater swings in moisture content especially season to season, as well as a higher MC in winter months because of moisture being able to enter the living space more readily. Because of this, their designs had to be capable of adjusting to a greater range of humidities. A more recent example of this would be wooden single pane windows. They were difficult to operate in the summer because of swelling due to higher humidities and drafty and loose fitting in the winter due to reduced humidities because of heating.
Winter typically sets the low MC requirement because air that is cold is denser (let's say compressed) and when heated it expands (decompressing), creating a greater volume: the water molecules in the air are now spread out over a greater volume and thus the amount of water present in a given volume of air is less than it was before it was heated, hence the term relative humidity. Also, there is greater area between the air molecules allowing more water to be held.
The real key to remember in drying is balance. The moisture content in the wood must balance with that of the air, neither gaining nor losing any significant amount of moisture. Fortunately, the wood will reach that balance for us. Unfortunately, its time table doesn't usually fit into ours, thus requiring kilns to create artificial conditions that don't normally exist year round (extremely high temperatures with extremely low humidities), allowing us not only to dry wood faster, but to be able to acheive MC levels comparable to those only found in winter months.
If you lived in a greenhouse "dry" would be quite different compared to the desert Southwest. All that is required is balance, not a magical number or percentage, to have functional wood.
You can reach the same MCs as kiln dried wood by air drying--it is merely a function of time and conditions (and sweat).
Comment from contributor J:
There are several other factors:
Wood was winter cut. Wood was stored for long periods of time, up to 1 year for 2X material. The wood industry was geared for long air drying so, logging, storage, and useage were complimentary. Primary cut timber and second growth timber have vast amounts of moisture difference.