On 2/22/2012 12:05 AM, Michael Joel wrote:
Paul Conners wrote:

Just a note - cupping takes place towards the moisture not away from it.


Just to clarify: the concave side is moist, and the convex side is dry?

Yes.


Not necessarily ... and AAMOF, and depending upon the grain and the way
the wood was cut off the tree, it more likely to be just the opposite of
what you stated ... the concave side being the dry side, and the convex
the moist.

There are indeed some exception with certain grain patterns due to the
way the wood was cut from the log (plain sawn, rift sawn, etc.), or if
laminated, but mostly wood cups and checks parallel to the grain and to
the drier side.

Excellent example is the cupping of hardwood floors, which "cup"
(concave side up) when wet from the underneath ... dry on the top, wet
on bottom ... one of the main reasons why a moisture barrier is always
used _beneath_ a hardwood floor.

Another hole in your balloon ... leaving a board to dry out in the sun,
The concave side is drier from exposure to the sun and heat, and, once
again, the board generally cups toward the dry side.

Try this by laying a board on wet grass before the morning sun hits on
sunny day, check it out by noon.

Here is indeed, "according to the books":
http://www.fpl.fs.fed.us/documnts/fp...chapter_16.pdf 16-6:

quote
Wood warmed by the sun experiences a virtual RH far below the ambient
RH. The surface dries faster than the rest of the lumber. This is why
cupping and checking often occur on decking boards; the top surface is
much drier than the rest of the board. Shrinkage of the top surface
commensurate with this dryness causes cupping and checking parallel to
the grain
/quote


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