Many tuners are fearful that if they tune an old piano up to pitch,
they are going to start breaking strings. There are several reasons for
this, the least of which is not rotten strings or an asymetrically
relaxed soundboard. But they might also profit from knowing something
else about tension and tone before they decide that a 20 cent (which
just means a 20%) pitch raise could break strings.

 [ Editor's Note:
 [
 [ In this article when % (per cent) is in the context of pitch,
 [ it means "percent of one half step".

Piano wire (old) is liable to break anytime you exceed its elastic
limit, which is still only 70% of its breaking strength. No piano is
strung beyond 80-90% of the elastic limit, anyway, so when you tune a
piano, you are (usually) between 40 and 60% of the breaking strength.
At  A 440, if the tension at tune is about 170lbs (big piano), then the
tension  to pull that same string up one note (A# 466.164) is 190 lbs.
That means 100 cents sharp here will raise an A to an A# with only
11.7% increase in tension! So you can see that pitch raising is not
damaging to a piano or a string at all, by itself, particularly when we
are usually talking a maximum of 20%, not 100%-- one fifth that amount.
A 20% pitch raise would increase the tension of that string therefore,
only 2.34%! Not hardly what you call dangerous!

If the strings are so rusted and dinged up that they start to break
when you tune them and they don't break as long as you tune flat (in
place), then they are just going to nickel-dime you to death anyway.
They are going to be breaking a few at a time, each time you tune the
piano. Why not get it over with and restring it?

There is also such a thing as "rotten" string. Nobody knows exactly
what happens, but the string apparently crystallizes and snaps, usually
in sections of the piano, so that's why they believe it involves
improper annealing or something not quite right in the steel when it
was drawn. But in far and away most cases, any old piano can be brought
right up to pitch. If the difference is 2% in the tension, as I have
shown, then no way will that break a string by itself. And you should
also keep in mind that when a hammer hits a string it exceeds that
tension momentarily by several times in shock-wave fashion, which is
even more painful to an overstretched string.

By the way, the difference between an A435 tuning and an A440 tuning is
less than 5% overall, from bass to treble. So anyone who has been
afraid to tune to standard pitch doesn't have to be. However, the 435
pitch has a flavor of its own that is original and a bit warmer, too. I
suggest that you leave the pitch as originally designed if you want it
to sound perfectly authentic. But if you are also playing the
instrument, you can raise it to 440 without any fears at all.

Craig B.