Brittle Strings

I recently tuned a small Ampico grand, from the late 30's or early 40's I would guess.  It had those brittle strings which throw off a lot of high frequencies, making it difficult to tune.  I've noticed this before on old Ampico grands, and old Sohmer grands, too.  Jewetts are famous for this problem - my heart sinks when I open up a Jewett upright and see all those broken strings in the extreme treble.

The high frequencies tend to cause me to tune the strings a little sharp.  Then I struggle to get them back down, a struggle exacerbated by the inevitable rust on the strings and corrosion at the agraffes.  When I was done, though, the tuning was fine, but the piano sounded a little harpsichord-ish, except for the tubby bass, all of which is typical.

I've tuned old Chickerings that were older than a century, and I've not had quite the same trouble, even though the strings were plenty brittle.  The difference seems to be that on the Chickering, the string tension is much lower.  I've also tuned pianos as old as the Ampico but whose strings were not nearly as brittle.  The strings that tend toward premature brittleness must be of a different composition.  Even the rust on them looks a little different.

Increased brittleness in strings is a natural consequence of age.  The strings, which are under constant tension for decades, actually change their molecular structure over time.  The atoms creep and shift, relieving some of the stress.  This is the string "stretching out," something it does for its whole existence.  The string becomes less springy, more brittle, more liable to break, more likely to throw off false beats.  The fundamental frequency weakens, making the string sound tinny, and the string's overtones probably become more inharmonic because of the increased stiffness.  I think that's what fools my ear when I tune.

A few things to bear in mind when tuning a piano like the little Ampico.  Be very careful about raising pitch - you don't need to overshoot A440 much because there's not much stretch left in the strings, and I don't recommend raising more than 100 cents without letting the strings rest.  Expect false beats - don't mess too much with unisons, it'll be easier to touch them up later after the strings have rested a little.  Finally, you'll notice that the brittleness increases as you move up the piano, perhaps because the strings are thinnner.  That's why in a Jewett you'll see all those broken and replaced strings up there.

Related websites:
There does seem to be controversy and confusion about exactly what happens to piano wire under tension over time.  There are several ways that strings "stretch out," but the mechanism I'm referring to is creep embrittlement.
See Wikipedia on:
Deformation of materials
Material strength and toughness (including steel)
Dislocation within crystal structures