All the other 4 conductors are bright clean copper when stripped.
Could anyone tell me why some insulated copper conductors turn black in
this way and whether there's a better way of cleaning it off.
Water inside the insulation plus sulfur and/or irons that formed copper
sulfide or covellite.
I've been successful using phosphoric acid. It is sold as a clear liquid
wire or mag wheel cleaner around here. You'll know it by how red and painful
it turns your hands.
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