Fireplaces offer cool, dark and sometimes damp conditions. An infrequently used or decorative fireplace will usually have poor ventilation on top of that, and these conditions are perfect for breeding mold. Mold infestations in your fireplace may be out of sight, but even so, they can spread to other areas of the house and therefore must be removed. Thankfully, the brick and stone that fireplaces are made of are tough materials, so you can employ the strongest cleansers to remove any moldy infestations from the dark corners of your fireplace.
Mix a solution of 1/8 to 1/4 cup of household bleach per qt. of water in a pail. Do not exceed more than 1/4 cup. Fill a spray bottle with this solution, using a funnel.
Crouch in your fireplace and saturate the mold-infested area with your cleansing solution. Allow this to stand for three to five minutes for stonework and a full five minutes for more porous brickwork.
Scrub the infested area with a stiff-bristle plastic brush. A long-handled brush may be necessary for a hard to reach area inside the chimney.
Rinse the area with a clean rag and a pail of clean water. Dunk and ring the rag regularly since the fireplace will probably be dirty with either soot or dust on top of having a mold problem.
Inspect your handiwork, and if any mold remains repeat the procedure.