I had to silver-solder two small components together today. To get a proper
solder flow I had to apply heat directly to the part of one component that
will later be visible. It ended up covered with black gunk. Is this what
they call firescale?

I tried removing it with steel wool with only partial success. In the end a
judicial application of 400 grit sandpaper did the trick without
obliterating the pattern underneath.

If this is firescale I understand Sparex (sodium bisulphate) is the thing to
clean it. Am I correct?

--
Michael Koblic,
Campbell River, BC