Anne Field
If fireplace is to be used for family entertainment such as popcorn popping, or
marshmallow toasting, etc., it would be wise to have brick or stone fireplace front surface sealed so as to resist absorption of grease or oils, and smoky soot.
Smoke and soot may be removed from stone fireplaces by the TSP method. Be sure to wear rubber gloves to protect hands from strong alkalis. Dissolve 8 teaspoons (1/2 cup) Trisodium Phosphate (TSP) in 1 gallon of hot water. With stiff scrub brush, scrub stone surface. Rinse with plenty of warm water. Repeat if soot or greasy stain not removed. More TSP may be added if necessary, up to 1 cup per gallon. This is a very strong solution; avoid getting on skin, carpet, or fabrics.
Source:findstone.com