I think you folks misunderstood my earlier post about steel and copper.
Perhaps it is because we also made bass strings and the terminology was
common to us.  I should have gone into more detail.

There are two types of bass strings in a piano: single wound and double
wound.  All have a steel wire core that takes the tension and wraps
around the tuning pin, etc.  A single wound string has one layer of
copper [wire] on the steel wire.  A double wound string has two layers:
an under or inner layer and a top or outer layer.

My post referenced to American strings having the under/inner layer
being wound in steel or iron of some kind, followed by an outer layer
of copper.  European strings had copper as inner and outer layers.
I've had no luck rejuvenating strings with the steel inner layer by
cleaning them.

Regards,
Bernt Damm
Sydney