How To Remove Mold and Mildew from Natural Stone
Borax is effective against mold and mildew, helping to remove existing deposits and inhibiting new growth. It is basic/alkaline, which makes is a good option for application to natural stone surfaces which tend to be more sensitive to acids.
If the natural stone surface that you need to remove mold and mildew from is in the bathroom, run the shower at a hot temperature with the door closed to create a steam to loosen the mold/mildew residue.
In a clean bucket or large basin, mix 1/2 cup of borax well with water. Diluting the borax helps ensure that it is not too strongly alkaline and will help prevent damage to the natural stone surface. Use a clean soft sponge to apply the borax mixture to the stained areas using a rubbing motion. Work a section at a time, rinsing with plain water as you go. Refresh the borax/water solution as needed to maintain cleanliness.
Once the removal of the mold and mildew is completed to your satisfaction, follow with a light overall cleaning. In a separate plastic basin, mix a lightly sudsy solution with water and a mild pH neutral detergent like Ivory Liquid detergent. Apply the solution to the stone surface with a clean, soft sponge. Rinse well with water and use a squeegee to wick away extra moisture and avoid over-wetting and pooling that may damage your natural stone surfaces and encourage buildup.
Caution: Never mix cleaning agents or chemicals, the result can be dangerous or deadly. Before cleaning, always test the agent on an inconspicuous location to determine its suitability and to make certain it does not damage the material. Wear appropriate clothing such as gloves and protective eyewear, and work in a well-ventilated area.