I found this one:
This is a situation that does require the use of a chemical. All that remains is to choose the safest method. Oven cleaner will take off that old baked on soot (the usual method is the use of harsh acids). Spray the brick (or stone) and leave for 5 to 10 min. Test a spot to see if its ready to be wiped off. Leave a bit longer if not. Wipe it off, then wash down with hot sudsy water and allow to dry.
On the other hand, although I've never had a chance to try it, Dishwasher Glisten (the product mentioned under Big Jobs above) could possibly do the job too.
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I dimly remember the "how clean is your house" ladies on the BBC America had a tip, but I can't remember what it is.