Why does a string go out of tune

[James Ellis]

David asked about the demonstration I used.  It was a monochord
instrumented to show minute movement of the tuning pin, both at top and
botton, how it twists in the pinblock, etc.  Also, I demonstrated the
effect of sudden temperature changes, and pitch change of speaking length
required to move the string at the bridge measured by monitoring the pitch
of the tail stecion - i.e., to indicate when the string moved at the bridge.

Don asked me to mention the things that cause instability.  They are many.
We all know about loose pins, and all that.  That's not what I was talking
about.  The piano has many wood to metal interfaces, all with different
coefficients of expansion.  If we look at the bridge, for example, on a
microscopic scale, things are moving all over the place every time the
temperature or humidity change, in addition to the well known rise and fall
of the soundboard crown.  If we could look at these parts with a microscope
over a period of time (which is not easy to do) we would see all kinds of
movement, shifting, swelling, shrinking, squirming, etc.  Exactly why some
pianos are more stable than others, I wish I knew.  If I ever find out, you
fellows will be the first to know.  The point I was trying to make is:
Pounding the daylights out of the piano won't correct the problem.  It will
settle a string that's about to move anyway, but it won't completely
equalize the tension across the bridge.

I'm not a pounder when I tune, but I don't use timid blows either.  I just
use firm blows and an occasional test blow.  When tuning for a concert, I
go back over the piano several times, tweaking octaves and unisons until I
get everything settled to my satisfaction.  Most of the time, the piano
stays where I put it throughout the concert.  If the artist pounds hard
enough to break strings, no amount of pounding by the tuner will make it
stay in tune.  But you guys know this already.

Sincerely, Jim Ellis