What should I use to clean black fire stains from concrete?

Question

It's on a patio where someone tried to have a camp fire...

Answer 1

I believe that you would have to mechanically remove the carbon.  Wire brush it, maybe try a pressure washer, and if all else fails, then you could call a repair company and have them scarify the patio.  This would involve removing a very small amount of concrete from the surface.

You may also try muratic acid on it before you start removing concrete.

good luck