Graham has it, Formica, assuming you can get a bit!
It does melt though, I suspect that the black line has as much to do with melted Formica as to the charring effect of the wire, It is also great on bowls and platters where you cannot get a wire on to.
I expect Andy Coates to chip in now and tell me where I got the idea from, well it was from one of his demo's
Ron