"James Toomey" wrote in message
oups.com...
When I hang an old metal clock on the wall, the metal acts almost like
a pencil and leaves gray lines or scuff marks all over the paint.
These
lines won't easily clean off, either. What in the world causes this?
Even if I put Scotch tape over the metal that contacts the wall it
still does this. I've noticed that other metals will do this to paint,
like coins. Is the metal shedding a layer onto the paint? Or is the
paint reacting somehow to the metal, which occurs even through a layer
of Scotch tape? I've searched all over the internet but couldn't find
a
discussion of this phenomenon.

White paint ?
I've only seen this on white
the primary white pigment in paints was white lead
I can't remember if it was the white lead or one of the replacement
pigments
that caused this maybe Titanium Dioxide pigment with aluminum oxides

Spud