On 2/22/2012 1:09 PM, Michael Joel wrote:
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
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.
Did you bother to even read the supporting evidence I provided
disproving your unequivocal contention, first above?
I will explain it as one of the books due (please recall I said this was
according to the books - not neccessarily the truth
It is indeed "not necessarily the truth", as I clearly outlined above,
complete with a cite as to the an explanation for why your statement is
not necessarily the case.
By all means, offer a cite that supports your contention to the contrary.
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).
Yes, and as a result, that attitude arguably insures that your knowledge
in this regard will remain limited.
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..
The above is either so poorly worded as to be useless in defining your
contention, or the result of a total misunderstanding of the effects of
moisture and humidity on the dimensional instability of wood.
It's really impossible to tell which?
I can never recall seeing a board that was cupped in any direction
except on the side effected by changes in moisture.
Again, too loosely worded to be of much use in defining the problem ...
and your admission that you have "never recalled" seeing that, is
certainly no proof of your argument.
Yes, it is indeed this _differential_ in moisture content between
opposite faces that causes the phenomenon of cupping, but not
necessarily for the reason you unequivocally stated first above.
Simply put, cupping is most often the result of the opposite (convex)
side being moist, and the cupped (concave) side being drier ... the
exact opposite of your unequivocal statement.
IOW, your understanding is mostly backwards of the actual effect of
moisture on wood, with the exceptions I noted above. You will certainly
want to do your homework, and provide some supporting evidence to the
contrary, if you want to continue a reasonable discussion of the issue.
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