Wood is a hygroscopic material. Always containing water, it constantly exchanges water vapor with the air After a tree is cut, water continues to move through wood, moving again from wet to dry until the wood reaches equilibrium with its environment, that is, no water moves in or out. In a conditioned home, at perhaps 75 degrees and 55% relative humidity, wood reaches equilibrium at about 8% moisture content

Moisture in wood exists in two forms:
1. Free water, liquid filling the wood cell cavities
2. Bound water, liquid or vapor chemically bound by hydrogen bonding to the cellulose of the wood cell walls
As wood dries, the free water in the cell cavities is drawn away first. Once the free water is removed, the bound water is gradually released from the cell walls.
Molecules of water are constantly leaving and returning to the wood surface. If the same number of molecules of water return to as leave from the surface an equilibrium condition exists. Since the wood is neither gaining or losing water it is said to have reached an equilibrium moisture content, or EMC.
If, on the other hand, more molecules of water leave than return to the wood surface, evaporation or drying takes place.
If more molecules of water return to than leave the wood surface, wetting takes place and the wood increases in moisture content
For strength testing purposes wood is conditioned to an EMC of 13% at a relative humidity of 60%
Wood located outdoors, but which is protected from rainwater, commonly attains an EMC of 14-18%. The EMC will be lower during prolonged dry periods.
The fact that moisture moves ten times as fast lengthwise as crosswise is why a green log left in a dry place develops so many cracks (checking) at the ends - the ends dry faster, so shrink faster, than the middle of the log. To prevent this, coat the end grain with something that stops evaporation, such as thinned parafin wax or commercial sealers.