Equilibrium Moisture Content

For some reason, I couldn't add text, so here it is.  The table you see is an Equilibrium Moisture Content chart.  What this charts does it show you, given the temperature and relative humidity ( RH), where the piece of wood's moisture content, given a sufficient amount of time, will be at those environmental conditions.  For example, if the temp. is 120 and the RH is 60, the EMC ( moisture content) will be, eventually, 9.7.  I should say here that I  used to run dry kiln for ten years, drying mostly white pine and hard maple.  While the methods and terminology I used in that industry are a little different, the principles are the same.  The only thing different about a kiln is that higher temps are used to dry the wood faster, but the wood undergoes essentially the same process, free and then bound water are driven off as the MC drops, and eventually the MC falls into the target zone, 6-8 % for hardwoods and 8-10 % for the pine.  As the woods dries, it shrinks and stiffens, and in the case of pine, some of the turpens are driven off, leaving the pitch hard and set.  Think pitch bleed.  The same process takes place when wood is air dried, it just takes a lot longer because the environment can't be controlled.  You're at the mercy of the weather, and if it's raining, you may actually be adding moisture to the wood, even if the wood is in a shed out of the rain.  You must remember that wood is hygroscopic, it always gains and loses moisture to try to equalize with the environment.  A finish can slow the process, but not stop it.  That's why a tight door in August swings freely in February, at least here in Maine.
One other thing to remember, and where I'm going with this,  is that green wood, MC above 70 %, say, must be slowly dried at first,  or else you'll get cracks, see the top pic.  The gauge below shows why the piece cracked, too dry, waaaay too dry.  I turned it dead green, and left it in the basement at the conditions on the gauge, which would put the EMC at 6.1, which is way too fast right off the bat.  When I used to run kilns, I would start the maple at 100 F and RH about 90%, EMC about 20 %, not 6.  So, the drying must be slowed down, what to do.  Well, just put the turning in a paper bag for a day or so, this will slow the drying enough to prevent cracking.  In case your wondering why wood cracks when it dries, the wood on the outside of the piece dries first.  And as the wood dries, it shrinks and becomes stiff.  So the outside is dry and stiff, but the inside is still wet.  So as the inside dries, it too shrinks, but the outside can't move as readily, and at a certain point the shrink forces become greater than the strength of the wood and CRACK! the outside gives way.   So choose paper instead of plastic when you stock up for the games on Sunday ( Go Packers and Jets!)  Wicked cold tonite, and the next few days, at least I have a use for the cracked leg, it'll get really dry in the fireplace!