Moisture Grain in Wood
Wood is hygroscopic, which means it will try to equalize to whatever the conditions where the wood is stored. This is where the term EMC comes from. Equalized Moisture Content. That means that if wood is stored in a room with higher humidity than that in which it was dried, it will absorb moisture until it reaches equalization at that given humidity. Remember that at 35 percent relative humidity, "RH", most wood will reach an EMC of 6 percent. At 50 percent RH, wood will reach about 10 percent EMC. At 75 percent RH, wood will reach 15 percent EMC. That’s why it is so important to store the wood at the same EMC as it was dried. And when you buy wood, you should know what moisture content it is. You should also keep it in an area where the air is conditioned to keep it from reabsorbing moisture.
One final note about air-drying lumber and the lack of conditioning drying lumber this way receives:
1. Air-dried lumber will only reach an EMC of 15 percent stored outside in a barn or in your garage. So if you bring it into a conditioned area that is heated, it will equalize to the new RH. This takes a long time. It will finally get there because it is now in a conditioned area, and is not called air-dried at this point. This is fine if you don’t plan on ripping boards into smaller pieces or milling one side off more than the other. Do you find that boards pinch when you rip them in half? That’s because they have what is called "tension set", or what they used to refer to as "case hardening".
2. Proper conditioning is important to take the tension set out of the shell of a board. Any way you dry wood, be it air-dried, shed dried or kiln dried, it will have tension set. Without getting into the four steps on how wood dries, just think of a board that dries this way: the outside of the board dries first before the center of the board. As the outside of the board dries, it shrinks. Can’t stop wood from shrinking as it dries. But as it is shrinking, the center of the board isn’t shrinking because it hasn’t lost any moisture yet. So the outside cells of the board get stretched out and dry bigger than they were before. The outside of the board has now dried in an enlarged condition. It has taken what they call a "set". Now the center of the board starts to dry. As the wood cells in the center of the board dry, they too want to shrink, but can’t shrink as much as they would normally, because the outside of the board dried already. Remember that dry wood is much stronger than wet wood. So now that the center cells can’t shrink as much as they would normally, they exert a force in tension that tries to pull in on the outside cells of the board. This tension is built in and won’t relieve itself over time.
The only way to properly relieve this tension is by adding moisture very rapidly to the outside cells "shell" of the board. This can only be properly done in a kiln and this process needs heat and live steam to do a complete job. This process rapidly expands the outer cells of the board and they try to expand and relieve the tension set. The high heat aids in softening the cells also. This is the same process used in steam bending of wood.