On 2/22/2012 3:36 PM, dpb wrote:

The short story is---wood shrinks as it dries and it does so
non-uniformly. The tangential/radial shrinkage ratio from green to
oven-dry has been measured for some hundreds of species and averages
about 2 (roughly an average of 8%/4% T/R) but shows a significant
variation between species of from just barely above 1:1 to highs
approaching 3:1. The higher the ratio the more the particular would will
distort as it is dried as the relative shrinkage in the two directions
competes at a different rate.

The difference between tangential and radial shrinkage isn't random nor
magic; it's caused by the anatomical structure, principally the effect
of wood rays whose lengthwise axes are oriented radially outward.
Species w/ more predominant rays are more stable.

Over the range of moisture content shrinkage is roughly proportional to
moisture loss. This doesn't matter too much as raw lumber is dried; the
roughsawn stock is oversize to begin with and if dried uniformly will be
stable after it is milled given a uniform environment. This is why it is
so important to finish both sides of a furniture panel, say--if one side
is finished and the other not, differential moisture absorption is
highly likely to cause movement.

Cupping after the fact is owing to one of two causes--either the piece
wasn't at equilibrium to begin with and dried after milling with the
resultant change in dimension as determined by the species' particular
T/R ratio and the percentage change in moisture.

The second is that the piece has subsequently absorbed moisture and
therefore grown. The relative amount in the direction is also dependent
on T/R and how uniform (or un-uniform) the moisture absorption is.

Cupping in flatsawn boards results in concavity away from the pith, the
result of greater tangential than radial shrinkage. The magnitude is
greater as the location of the board from the original trunk is closer
to the pith on a surface. This face is completely radial while the
opposite is tangential in the portion across from the location of the
pith and will therefore shrink at twice the rate. Woodworkers tend to
say the "rings flatten out" which an easy way to remember the direction
the board will cup but the reason for the cup has virtually nothing to
do w/ the growth rings themselves.

Cup is reversible on swelling which is why the idea of wetting is given
as a cure. Of course, the end then has to be to get the whole board in
equilibrium again at that point which goes back to point a) above--if
the piece was milled in inequilibrium, it's an insoluble problem w/o
mechanical repair.


Very well put and, as a result, that much more informative.

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