Getting soot off fireplace bricks is not an easy job, especially if it has cooked in over a long period of time. We tested: vinegar & water ; baking soda & water ; bubbly soda water ; and muriatic acid.
The Vinegar did all right, the baking soda better, and the regular bubbly soda water did the best job. Muriatic acid did quite a good job as well and would probably do a better job than the soda water on stains that are older than those on the bricks in our set-up. Of course, when using muriatic acid, if you start with a concentrate be sure to add the acid to water, not the other way around. This makes sure it is diluted to the maximum during mixing. Wear gloves and goggles. Try the soda water first, as it is much easier to work with.