Tom; I'm going to take selective excerpts from your post and ask questions about them rather than give my opinion...OK? (tension = stretching........compression = compacting, truisms both....except when piano techs are talking and then we can mean either :) <<"I've seen how drying a board makes the cracks open up, and returning the board to normal humidity closes the cracks again, more or less.">> What is causing the cracks to appear and disappear? Is it change in tension? Compression? <<" But I don't see how increasing the humidity would increase tension and cause cracks.">> This question still stands, i.e., <<<<"Does a board that has been 'dried' exert enough 'tension so that even a dried shim will have a tendency to split/crack along the glue line when it (the board) regains its moisture equilibrium?">>>> Think about a ballon with a line drawn on it while it is deflated ...now inflate the ballon ...what happens to the line? Does the same thing happen to the shim/glue line as the board returns to its normal moisture content? This is kinda out of context since we are speaking of 'shims' installed after a board is in place but still it reflects the same general principle.... <<"The rehumidifying of the dried out panel glued to the ribs generates a compressive force which, constrained by the ribs glued on the underside, can only widen on the side away from the ribs (the top), and therefore a curved surface develops. So even the top surface is under compression, ....">> If the "top surface is under compression" where do cracks come from? >From compression failed wood cells? Is this the cause of the cracks or is this the fault that allows the crack to show? If the top surface of the panel is "under compression" is it possible for a crack to show? <<"That's why I have this fantasy that drying the board and shims, before routing the board and gluing the shims, would tend to support the crown,">> Well I am not 'sure' that it is a "fantasy" but...........is a shim, or even a small panel replacement, on a compression crowned board going to exert enough force to compete against the rest of the preformed panels? Will the curve of the crown still be intact while you are replacing this shim/panel?...if so what is going to happen to that shim/panel when you allow the board to regain moisture? In a compression crowned board is the top surface under compression? If so what allows the board to curve/develop crown? Is it the difference between the ability of the ribs to hold the board flat versus the ability of the board to curve the ribs when both are held captive by the rim?...or is it the ribs curving and forcing the board to crown?...or a combination of the two? If the entire board is under compression why would it curve/crown at all? Why is the measurement across the top of the crown greater in 'normal' state of equilibrium than when it is at a 'dry' state? Where does that extra measurement come from?....... because the board is under compression?...or because.................................................? Nuff questions......... IMO a shim fitted while in a dried condition to a board in a dried condition is gonna fail...don't matter what kind of glue you use...'if' fitted/glued to an unsupported board. But then what does I know? Mr. Nossaman ain't tole me what I think yet! :-) Jim Bryant (FL)
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