Stone work on a fireplace adds a distinctive, attractive look to any room. Unfortunately, in wood burning fireplaces, the stone can quickly become black with soot, especially if the fireplace is not maintained and run properly. Sealed stone is easier to clean, but all stone work on fireplaces can be cleaned.
Although you can simply wash the stone work with warm, soapy water, soot stains need to be scrubbed off with strong chemicals. You need a bucket of warm, soapy water, a stiff brush, a gallon of hot water, and 1/2 cup of Trisodium Phosphate (TSP). Also, a vacuum with a hose attachment can be very helpful in getting any debris and cobwebs out of cracks in or between the stone. Finally, collect some dust sheets or old sheets.
First, protect the floor around the fireplace with the old sheets or drop clothes. Then, run the hose attachment from a vacuum over the stone to remove loose bits and debris. You can sweep the stone with a broom if you don't have a vacuum. Use the stiff brush and warm, soapy water to clean dust and dirt off the stones. Getting rid of the soot takes a bit more elbow grease. Slowly add 1/2 cup of TSP to a gallon of hot water. The TSP will dissolve in the water. Dip the scrub brush into the solution (do not touch it with bare skin) and scrub vigorously at the soot stains. It may take several applications to rid the stone work of the stains.
Trisodium phosphate is poisonous. Use care when applying it to the fireplace. Wear rubber gloves and open windows to provide ventilation, and do not let children anywhere near you when you are cleaning the fireplace. TSP will burn bare skin, sting the eyes and can be fatal if swallowed.
If the stone work around your fireplace has not been sealed, now is a good time to do it. Once you have finished cleaning and the stone is dry, brush on a layer of sealant made for fireplaces. This will not only repel soot, but protect the stones from heat damage as well as nicks and scratches. Some tints are glossy or tinted, so be sure to check the label before purchasing. Let the sealant dry thoroughly before using the fireplace.