I have noticed recently that any exposed copper in my shop has a black,
burnt look to it.

What is causing this? I first noticed it in my electrical panel, and
thought I must have really overheated that circuit. Then I noticed it
on the exposed ends of some scrap wiring I had around, so I concluded
that wasnt' the case.

This is more than the usual oxidation that occurs over time on copper.
Peeling back the insulation on said scraps of solid wiring, the first
inch will have the discoloration, and the rest will still be the nice
shiny wire.

Explanations? I assume a result of some cleaner or something I used,
but wondering what.....

Thanks

JW