So, Why Bother?....

[oakiemac]

JimF or others,

If a piece of wood is kiln dried to 7% then stored and it goes up to say 12%, is that wood any better then the same wood air dried to 12%?
I have heard that once kiln dried the trapped moisture is removed from the cells themselves. If it then regains moisture it is free water and is easyier to remove and causes less movement of the board. Any truth to the above?

[Texas Ranger]

From Lumber, by Brown and Bethel, John Wiley & Sons.

"A thorough understanding of the process of lumber drying requires knowledge of the ways in which mosture occurs in wood and the mechanisms by which it leaves the wood.  Some of the water in green wood is contained in the cell cavities.  This water is usually referred to as free water.  The cell simply serves as a container for this water, in much the same way as a cup, glass or bucket may serves as a container."

"In addition to the free water in the cell cavity, green wood also contains water in the cell wall.  This water surrounds the particles and strands of cell wall substance and fills the very minute openings in the cell wall is referred to as bound water.  The bound water is held by the wood in a much more intimate relationship than is the free water.  Some of the bound water is linked to the molecules of cellulose by secondary valence bonds.  Other bound water is held in the fine openings in the cell walll betwen the cellulose molecules throught the forces of capillarity.  The bound water is very important in any consideration of wood utilization.  Because of the magnitude of the physical foreces which hold the bound water, it is much more difficult to separate wood from its bound water than it is to separate it from its free water.  For this reason, the free water always leaves the wood before the bound water does.  In fact, the bound water cannot be removed from a cell until the free water in the cell cavity is gone."

"The condition which exists in a cell when the cell cavity contains no free water and the cell wall is satureated with bound waters is known as fiber saturation point.  Changes in the moisture content of wood above the fiber saturation point have no effect on its strength.  Ther is, however, a direct relation between moisture content and strength below the fiber saturation point.  Above the fiber saturation point, changes in moisture conten do not affect the dimensions of the piece of wood.  Below fiber saturation, a change in the moisture content will result in a change in dimensions.  Most of the physical and mechanical properites of wood are influenced  by the bound water, whereas the free water has no important effect on wood properties, with the single exception of collapse."

In other words, the strength of wood is more or less dependent on moisture content.

[jimF]

oakiemac,
No, it is no easier to remove a given amount of moisture from dried wood than from green wood.  When the wood regains moisture it gains it first as bound water as described by Brown and Bethel inTexas Ranger's note.  As mentioned earlier and if you are talking about EMC not MC,  The wood brought down to 7% and then exposed to 12% EMC will have a lower MC than the one Exposed to 12%- because of hysteresis.  i know I am mixing terms and throwwing in complications, but so everyone is clear on how wood behaves.
  One addition to that quote - the acoustical properties are also effected by moisture content.

While in general practice Don is correct in that generally pitch is better "set" in dried wood than green wood.  However, pitch can still run if the board is dried.  Depending on how well it was set, it can vary from piece to piece, given that the moisture content is the same in both pieces.
  Also, using a hammer as an example, if the handle is very dry, say 5% and then swells, say 12%, it will become very loose when it redries.  Whereas, if it originally was somewhat high MC, say 12% and dried slightly, say 10% it will be tighter than the fisrt situation.

[beenthere]

....  Also, using a hammer as an example, if the handle is very dry, say 5% and then swells, say 12%, it will become very loose when it redries.  Whereas, if it originally was somewhat high MC, say 12% and dried slightly, say 10% it will be tighter than the fisrt situation.

And it was 'looser' in the 'first situation' because the fibers of the wood 'crushed' within the confines of the steel hammer head, likely having exceeded the crushing strength of the wood.   

But, there are lots of variables to take into consideration, even in the analogies offerred.