N_Cook wrote:
Perhaps it is chemistry . Bear in mind that the wire is silver and not
copper and the surface area of a squashed wire is more than when it was
round and any corrosion on that surface will have proportionally more effect
on the thin flats than the bulky round.

The voice coil wire looked like copper because it was under a coppery brown
lacquer. But, before it disintegrated, the ribbon section was darker brown
than the round section.
Now if air could get under the lacquer and tarnish the silver to black
copper sulphide, or whatever that blackening is, then that would explain
it.
Unfortunately none of that section remains as it literally turned to dust
after photographing it, you could not pick it up even with fingers, it was
little more than a wraithe.

Are you sure that voice coil isn't aluminium? Some aluminuium v/cs are
copper coated btw.

Ron(UK)