Anyone sucessfully cleaned a limestone fire place

we had fossilised limestone, it was polished. I would definitely not try anything harsh or acidic, it is a soft carboniferous stone and would slightly dissolve. It will not clean but I would try a tiny bit of bicarb, which is also a carbonate like limestone. Just try a tiny area with something small like a toothbrush and then wipe with water and a white cloth, see if anything comes off onto the cloth. If it does then persist. Slightly more abrasive would be a bit of toothpaste.

If you really cannot cope with it then see about getting it re-ground, however years of black may have gone too deep for that.

Ours was cleaned by a chimney sweep company who also install fireplaces and it came up looking as new .

He painted on a gel type substance which somehow drew out the ‘soot’ , and he returned the following day to peal it off .
The gel looked dreadful as it dried but the result was amazing.