Official Luthiers Forum!

So I get home and once again I realize that I forgot to take the humidifier
reservior back down to the shop this morning before I left for work. The
RH dropped to 24% during the day and naturally the three test tops
(joined but not braced) I had in there are cupped like potato chips. Thats
shoots my nights plans on working in there, but got me to thinking.
What determines which direction the top will curl? I assume it has to do
with the quatering of the top. When deciding which side is the inside and
which is the outside, does this come into play? Is it better for the wood
to want to flatted out the top, or bubble it up more? I assume the bracing
grain also plays into this.

This may have been a dumb question, but maybe not. All tops curled up
and were flat on the work bench with nothing under them. It may be the
top surface is exposed to the dry air and contracted first, but why
wouldnt it even out once it curls up and the bottom is exposed?????

 I know it does have to do with the direction of the grain , and one exposed side   could be affected more, initialy, during changes in humidity .  I think the problem with your  train of thought though is with quartered wood , that is split and opened like the pages of a book ,I believe you are  "reversing" the grain . that is to say if the tendency for the unsplit top would be for it to raise up in the middle ,due to grain orientation .when you "flip" them open , the top  now becomes the botton of the other half and it would tend to have a reverse cup .. anyways  thats my understanding of  it .. Jody

zach, you hit the reason on the head ; the exposed side gives off moisture and shrinks, thus the upward curl.

flip the top over and you can watch it go back to flat, then curl upwards again.