Swingman wrote:
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



Sorry but I believe what I said to still be right.

While grain of course will have effects the cupping will take place
towards the moist side (or away from the compression pressure might also
be a way to say it). Take a look at porches or decks, do you seen any
cupped down? Fave you ever found a table top that cupped down?

Your wood floor - is it cupping because it is moist underneath or
because of the mopping and drying taking place above? as for the board
on the lawn - don't know, would have to try it.

Your quote - Is it indeed the sun causing the surface to dry faster or
is it in fact that the problem is not visual until the surface dries?

I will explain it as one of the books due (please recall I said this was
according to the books - not neccessarily the truth - I believe much of
the things "known" are not the truth of what is happening, but only what
we as humans can guess at/or comprehend with our limited knowledge).

Wood cells normally want to swell and then shrink back to their normal
shape and size. When something restricts this the swelling continues on
uncompressed sides (i.e. the bottom of the board, or anywhere else it is
not wet/or drier). This forces the cells into more of an oval shape
instead of their original shape. When they shrink from drying this shape
is not shanged so the compressed sides of the cell (we will call it the
width) ends up smaller than it used to be. As this is repeated we end up
with cupping/warping/checks/etc..

I can never recall seeing a board that was cupped in any direction
except on the side effected by changes in moisture.

Mike