Re Humidity

[Kenneth Sloane]

                      Subject:                              Time:  6:28 PM
  OFFICE MEMO         Re Humidity                           Date:  2/13/96

---------------ORIGINAL POST----------------

As the relative humidity goes up because of teperature drop, does the
moisture content of the wood also go up?  Seeing that there is the
same amount of moisture, only the air's ability to hold it has
changed, I would think the moisture content would also stay the same,
unless the change in temp. also affects the wood's ability to hold
moisture.  What we are concerned about is, bottom line, the wood's
moisture content, I think.


vince mrykalo  rpt
byu provo utah

-------------------MY REPLY---------------------

The following is taken from almost every woodworker's bible, "Understanding
Wood", by R. Bruce Hoadley.

For wood that has not been dried sufficiently and has water beyond that
absorbed in its fibers (known as "free" water contained within cell enclosures
as opposed to "bound" water absorbed in fibers), the water content is
expressed as moisture content (MC). To determine the MC, weigh a piece of wood
and then dry it out completely in an oven and reweigh it. The wet weight minus
the dried weight divided by the dried weight is the MC, which is the
percentage of water (by weight) that was in the wood before drying. Let's go
on to discuss wood that has been dried for use in pianos, but allow me to
quote Mr. Hoadley.

"Wood always remains hygroscopic -- it responds to changes in atmospheric
humidity and loses bound water as the RH drops, regaining bound water as the
RH increases . For a given RH level, a balance is eventually reached at which
the wood is no longer gaining or losing moisture.........(where the wood has
reached its) equilibrium moisture content (EMC)."

So the answer to Vince's question is that the moisture content (or EMC) of the
wood in the situation he describes would increase.

Mr. Hoadley goes on to say the "relationship between the amount of bound water
in wood and relative humidity is...... the most important item in this book."
He advises everyone to remember that wood at 25% RH has about 5% EMC, 50%RH
about 9% EMC, and 75% RH about14% EMC. 0% RH gives 0% EMC and 100% humidity
gives total fiber saturation, which ranges from about 22% EMC for woods high
in extractive content (redwood for example) to as high as 35% EMC for woods
low in extractives (birch for example).  These figures vary, of course, with
the species and individual trees involved.

Ken Sloane, Oberlin Conservatory