Either carbon tetrachloride, or carbon disulphide. Can't remember which, but
it dissolves carbon. It's been about 35 years since I needed this
The fire fighting technique for a fire in a vat of carbon disulphide is
really unique. I'll leave everyone to guess. You can also ask me......
Christopher A. Young
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Carbon is slightly soluble in liquid iron - that's how they make steel -
and, according to my CRC Handbook, virtually nothing else.
Inasmuch as there is no viable chemical solution to your problem, you'll
have to rely on a mechanical solution, i.e., scrubbing or removing the top
layer of the fireplace bricks.
You might consider wallpapering over the ugly-looking black stuff.