In my personal experience, I found that American piano strings did
not use copper for the under-layer of the coil, but steel or some
other material that seems to corrode.  European strings had mostly
copper for both layers.

I have removed and cleaned a lot of strings and found that those with
steel as under-layer did not always sound much better afterwards.

I do also wonder whether the all-copper replacement strings are
causing that super, deep full sound that others have mentioned earlier.
I certainly think that this may not be original perhaps?

Bernt Damm
Sydney