Plywood shrinkage??? Laminated soundboards???

[Sarah Fox]

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Re: Plywood shrinkage??? Laminated soundboards???Hi Ron, Del,

You're correct that the drawer bottom panels were not coated in any way, =
and I suspect the original quality of the plywood was not as good as =
what one has available today.  (It's a bit hard to tell, considering the =
age and deterioration of the plywood.)  I'd say that the grain =
orientation, at least on the outer laminates, is about the equivalent of =
cheap hardware store variety material, on average.

I'm just curious about the moisture blocking properties of the SB =
finish:  How much does the finish alter the rate of MC change in a SB?  =
I presume it depends on the type and thickness of finish used.  Has =
anyone done any side-by-side measurements?

Back to the plywood...

Regarding quality:  Although the drawer bottoms may  be constructed of =
relatively poor quality material, I would strongly suspect only the best =
materials are used in boat construction.  In fact knowing some boating =
enthusiasts, who pour unbelievable amounts of money into their surrogate =
babies, I'd suspect the quality of these materials would rival what you =
builders/rebuilders use!  Furthermore, the reference to boat deck =
plywood shrinkage is most likely with regard to modern-day materials.

I suspect shrinkage may be the fate under extreme conditions.  The boat =
deck plywood obviously gets baked in the sun, doused with water, and so =
forth.  Drawer bottoms can frequently be wiped out with wet rags -- =
which while not as extreme, still creates a rapid and drastic changes in =
MC.  Perhaps more importantly, another dynamic is going on in both these =
cases.  Deposition of water is never uniform, whether through leakage =
past finish defects (boat) or through differential deposition of water =
with a wet rag.  This can cause a lot of internal stress and strain in =
the panel.  For instance, if a small spot in the center of a panel is =
soaked with water,and the surrounding board remains dry, the swollen =
center spot will be compressed within the "frame" of the surrounding =
wood.  This is actually how modern "door ding" removal is done in cars, =
except with heat expansion, not moisture expansion.  In the case of =
metal door panels, it is a way to shrink the previously expanded metal =
back to its original size.  In the case of the plywood, the compressed =
spot would dry out under tension, and when other parts of the panel =
would later expend, the spot would gladly receed.  I suspect this may be =
the dynamic that results in the progressive shrinkage of these panels.  =
The difference may not be quality of plywood, so much as evenness of =
moisture distribution.

It would be very interesting to model this phenomenon with sheet metal =
and a blow torch, producing overall shrinkage in the metal panel with =
repeated application of heat in isolated spots throughout the panel.  =
Theoretically, would the panel eventually (after bizillions of years of =
this sort of treatment) end up in a sphere?

This is a very interesting phenomenon, to say the least!

Peace,
Sarah

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