Removing Mildew Stains from Marble

CSI Division
Division 4- Masonry
Section
Marble
Last Modified

Technical Procedures Disclaimer

Prior to inclusion in GSA’s library of procedures, documents are reviewed by one or more qualified preservation specialists for general consistency with the Secretary of Interior Standards for rehabilitating historic buildings as understood at the time the procedure is added to the library. All specifications require project-specific editing and professional judgement regarding the applicability of a procedure to a particular building, project or location. References to products and suppliers are to serve as a general guideline and do not constitute a federal endorsement or determination that a product or method is the best or most current alternative, remains available, or is compliant with current environmental regulations and safety standards. The library of procedures is intended to serve as a resource, not a substitute, for specification development by a qualified preservation professional.

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We’ve reviewed these procedures for general consistency with federal standards for rehabilitating historic buildings and provide them only as a reference. Specifications should only be applied under the guidance of a qualified preservation professional who can assess the applicability of a procedure to a particular building, project or location. References to products and suppliers serve as general guidelines and do not constitute a federal endorsement nor a determination that a product or method is the best alternative or compliant with current environmental regulations and safety standards.

PREFACE: The cleaning or removal of stains from stone may involve the use of liquids, detergents or solvents which may run off on adjacent material, discolor the stone or drive the stains deeper into porous stones. Use the products and techniques described here only for the combinations of dirt/stain and stone specified.

PART 1–GENERAL

1.01 SUMMARY

  1. This procedure includes guidance on removing mildew from marble surfaces. Mildew stains from fungus, algae or other living plants are typically black, green, blue, orange or blotchy white in color and tend to disappear when the offending object has been removed. This procedure may be used to remove those stains which remain.
  2. Biological growths such as lichens, algae, moss and fungi growing on stone masonry walls is usually an indication that there is excess moisture in or around the stone masonry. These growths should be removed, as they attract moisture to the masonry surface and hold it there, which can lead to more serious problems. Lichens and mosses in particular, produce oxalic acid which can damage certain types of historic masonry.
  3. See “General Project Guidelines” for general project guidelines to be reviewed along with this procedure. These guidelines cover the following sections:
    1. Safety Precautions
    2. Historic Structures Precautions
    3. Submittals
    4. Quality Assurance
    5. Delivery, Storage and Handling
    6. Project/Site Conditions
    7. Sequencing and Scheduling
    8. General Protection (Surface and Surrounding)
      These guidelines should be reviewed prior to performing this procedure and should be followed, when applicable, along with recommendations from the Regional Historic Preservation Officer (RHPO).
  4. For information on removing other organic stains from marble, see “Removing Organic Stains From Marble”.
  5. For general information on the characteristics, uses and problems associated with marble, see “Marble: Characteristics, Uses and Problems”.

PART 2–PRODUCTS

2.01 MATERIALS

NOTE: Chemical products are sometimes sold under a common name. This usually means that the substance is not as pure as the same chemical sold under its chemical name. The grade of purity of common name substances, however, is usually adequate for stain removal work, and these products should be purchased when available, as they tend to be less expensive. Common names are indicated below by an asterisk (*).

  1. Household Bleach:
    1. Other chemical or common names include Bleaching solution*; Laundry bleach*; Sodium Hypochlorite (NaOCl); Solution of chlorinated soda*.
    2. Potential Hazards: CAUSTIC TO FLESH.
    3. Available from chemical supply house, grocery store or supermarket, hardware store or janitorial supply distributor.
  2. Mild dishwashing detergent such as “Ivory Liquid”, “Joy”, or approved equal.
  3. Clean, potable water
  4. Clean dry towels for blotting the area after treatment

2.02 EQUIPMENT

  1. Plastic spray bottle

PART 3–EXECUTION

3.01 EXAMINATION

  1. Verification of Conditions: Examine the marble surface CAREFULLY to determine the cause of staining before proceeding with any cleaning operation.
  2. Test cleaning procedure in a small inconspicuous area to determine the effectiveness of the treatment. If the stain/mildew is not removed, contact a stone specialist or consult RHPO. Some biological stains are very difficult to identify and require laboratory analysis.

3.02 PREPARATION

  1. Protection: Provide adequate wash solutions (i.e. water, soap and towels) before starting the job.

3.03 ERECTION, INSTALLATION, APPLICATION

NOTE: DO NOT USE BLEACH ON DARK COLORED STONES AS THIS WILL CAUSE THE STONE TO LIGHTEN.

  1. Thoroughly rinse the area to be treated with clean, clear water.
  2. Mix a solution of 3 parts household bleach with 1 part water and a dash of dishwashing detergent in a spray bottle.
  3. Thoroughly moisten the stained surface with this liquid by misting the surface using the spray bottle.
  4. Mist the surface continuously until the stain disappears.
  5. Rinse the surface with clean, clear water and allow to dry.
  6. Once the surface has dried completely, check for remaining residue and repeat the treatment if necessary.
  7. For alternative guidance in removing biological growth from masonry, see “Removing Biological Growth From Exterior Masonry and Stucco”.

 

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Last updated: Jul 17, 2024
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