JIMRPT@aol.com wrote: > In a message dated 7/28/98 10:56:52 PM, baldyam@sk.sympatico.ca wrote: > > <<Hi Jim, > In the extreme dry zones this is what we find. Many of the pianos > arrive with clear evidence of pressure ridges. (Almost unheard of for new > pianos shipped to this region.) Once the piano dries down the pressure > ridges produce some wonderful cracks. So there you have it pressure ridges > and cracks. > Regards Roger>> > > Roger; > Your comment is reasonably logical of course but .....not logically > reasonable :-) > (sorry da debil made me say dat) > > In my experience with these thingees, pressure ridges have been almost non > existent, but soundboards, being the perverse things they are, can do some > strange things. Perhaps my comment was not clear. What I meant to convey was > that the same ambient humidity condition cannot 'produce' cracks 'and' > pressure ridges. To put this another way... cracks cure pressure ridges in the > same way that giving birth cures pregnancy, i.e. by removing the cause of the > condition and in the process creating a whole new set of problems :-) > Jim Bryant (FL) -------------------------------------------- Jim, Roger, et al - - - "Cracks cure pressure ridges . . ." Well, not really. Even though this sounds logically reasonable it doesn't quite match with reality. I certainly agree that the same ambient humidity condition cannot produce both cracks and pressure ridges at the same time, but then this is not how these things develop. And cracks do not "cure" anything. They are simply a visible manifestation of an even greater problem. The pressure, or "compression," ridge is the visible result of wood fiber damage that has already occurred in a wood panel that has been subjected to more compression stress than it was able to withstand. Typically, compression damage begins to occur when the internal compression exceeds approximately 1%. This compression can come from some outside force -- put a piece of wood in a vise and squeeze -- or it can be created internally by raising the moisture content of a very dry piece of wood that is locked in some structure that will not allow it to freely expand. In the case of certain piano soundboard designs, the wood panel starts out very dry and the wood panel is not allowed to expand -- at least not much -- hence the swelling wood fibers create the damaging levels of internal compression all by themselves. If the wood panel was dry enough to start with and the added moisture great enough, the resulting swelling will create enough compression to cause the wood fibers to self destruct. Compression ridges are created in the process of this destruction. In fact, they are the visible sign that this destruction has already taken place. Once the compression ridges has formed and is visible, the damage has already been done. Removing, or reducing, the moisture will relieve the internal compression -- it may even reduce the visible size of the compression ridge -- but it will not heal the damaged wood fibers. The crack(s) that open up in these areas are only symptoms. The crack(s) appear because the wood fiber has been crushed to the extent that it has changed its dimensional shape. As the wood fibers now lose moisture in Roger's drier climate they actually come under tension. (Remember that even up where Roger lives these boards are probably not going to get as dry as they were when they were installed in the piano.) They are no longer resilient enough to hold together and the crack appears. The wood fiber damage that was caused by the internal compression has not been "cured." The affected fibers have still crushed and have been permanently weakened. Even repairing the crack will not "cure" the compression ridge. It will only put a Band-Aid on the problem. Any board that has developed significant, i.e., visible, compression ridges has already undergone irreparable changes. For a variety of reasons these changes do not always result in catastrophic acoustical problems -- in some extreme cases the tone of an affected piano might actually improve! -- but the changes in the wood structure will remain with that soundboard permanently. If anyone is interested in more on this subject, please refer to my 3-part series on the subject published in the PT's Journal a few months back. Regards, Del
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