Trisodium Phosphate, also known as TSP, is a powerful chemical cleaner that can be found in most hardware stores. As your fireplace burns, it emits soot and dust particles that are transferred to the stones of your fireplace via the air that circulates throughout the room. TSP is very effective in cleaning this grime that has accumulated on the fireplace stones over a long winter's use.
Wear heavy duty rubber gloves and a pair of goggles. The goggles will protect your eyes from the chemical used to clean you stone fireplace, and the rubber gloves will protect the skin of your hands, as well. TSP is a caustic chemical and can cause burns if it comes in contact with the skin or eyes.
Open up all of the windows in the room that has the stone fireplace that needs cleaning. The fumes from the TSP can be quit overpowering, so it is a good idea to allow fresh air to circulate throughout the room during its use. It is also suggested that pets and children be removed from the room before the cleaning begins.
Mix the TSP in a large bucket with warm water. Pour 1/2 cup of TSP into the bucket and add 3 quarts of warm water. If the fireplace stones are not in need of a deep cleaning, you can further dilute the product by adding more water to the bucket, otherwise the original 3 quarts of water should suffice.
Dip the scrub brush into the bucket of TSP cleaning solution and apply the cleaner to the stones of the fireplace. Scrub the stones until they are free of all dirt, dust and soot. Alternating between dipping the scrub brush into the cleaner and scrubbing the stones should be effective in removing any buildup that has accumulated on the stones during the fireplace's use.
Wipe down the stones with a soft cloth and a second bucket filled with warm water. Add no soap or detergents to the water, since this bucket will strictly be used for rinsing.
Dry the fireplace stones with a second soft cloth. The stones should be dried thoroughly to prevent mold and mildew from building up in between where the mortar lies. Continue this process as often as necessary.