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 mould. 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 mouldy 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 mould-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 mould problem.
Inspect your handiwork, and if any mould remains repeat the procedure.
A mould infestation in your fireplace may mean a leak in the chimney, so be on the lookout for signs of dripping water. Inspect the outside of the chimney for damage to the masonry as well. Remedying your mould problem permanently will require changing the conditions in your fireplace. If the fireplace works, the most direct way to prevent mould is to light a fire periodically. If this is not possible or your fireplace is decorative, try improving the ventilation in that part of the room or eliminating sources of moisture.
Wear a respirator mask when working with any solution of bleach in an enclosed area, such as a fireplace.