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.
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.
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.
A. This procedure includes guidance on removing efflorescence from marble surfaces. The variety of efflorescence or salt stains include among others:
- sulfates of sodium, potassium, magnesium, or calcium;
- nitrates of sodium, potassium, and calcium;
- calcium carbonate;
- sodium chloride;
B. Efflorescence is a condition where white (salt) deposits form on the surface of the marble. The formation of salts is usually a sign of excessive amounts of moisture in the stone masonry. Salt deposits on the stone surface may develop from:
- Soluble compounds within the marble or in the soil. In the presence of water, these compounds gradually migrate to the wall surface, where they remain when the water evaporates.
- Improper or insufficient rinsing of stone masonry after chemical cleaning or repointing.
- The penetration of rain into the stone masonry through deteriorated mortar joints.
- Exposure to air pollution, which can result in the formation of thick sulfate (salt) crusts on the underside of moldings and eaves, areas not regularly washed by rainfall.
- Capillary movement of moisture through stone >masonry, the drying out of walls associated with a damp proofing treatment or the elimination of a ground water source may increase the amount of salt at or near the wall surface.
C. These deposits are generally not harmful to the building, just unattractive. But, the mitgration of dissolved salts into the mortar joints could result in damage with salt crystals forming and expanding. The source of the water that carries the dissolved salts should be determined and abated before attempting to remove the staining.
D. See 01100-07-S for general project guidelines to be reviewed along with this procedure. These guidelines cover the following sections:
- Safety Precautions
- Historic Structures Precautions
- Quality Assurance
- Delivery, Storage and Handling
- Project/Site Conditions
- Sequencing and Scheduling
- 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).
E. For general information on the characteristics, uses and problems associated with marble, see 04455-01-S.
A. Dry, white, cotton cloths
B. Stainless steel wool (000)
A. Wood or plastic spatula
B. Stiff, fiber bristle brush
A. Examine the marble surface CAREFULLY to determine the cause of staining before proceeding with any cleaning operation. Look for potential sources of moisture and make repairs as required:
- Determine the age of the structure: Efflorescence on older buildings is typically caused by the presence of soluble salts in the construction combined with moisture.
- Determine the location of the efflorescence: Examination may show where the water is entering.
a. Are the salt crystals accumulating on the joints or on the units?
b. Can any changes in the wall composition or in the adjacent surroundings be recognized that might show the source of the problem?
- Examine the condition of the stone masonry:
a. CAREFULLY EXAMINE the wall for open gaps or cracks in joints and around openings that could allow water to enter the building.
1) Are joints properly caulked or sealed?
2) Are flashings and drips in good condition?
3) Are there open or eroded mortar joints in copings or in sills?
b. Carefully note the condition and profile of the mortar joints.
c. Repair cracks in masonry and/or repoint as necessary before proceeding with the cleaning operations.
- Examine wall sections and details of construction: Carefully examine roof and wall junctures and flashing details for possible sources of moisture entry.
- Examine laboratory test reports on the materials: The problem may stem from the composition or misuse of the material.
3.02 ERECTION, INSTALLATION, APPLICATION
A. Carefully remove any surface deposits using a soft cloth first.
B. If necessary, gently buff the surface using a stiff fiber bristle brush or 000 steel wool.
C. Remove sulfate crusts using a heavy wooden scraper.
D. Wipe the surface again with a clean, soft cloth.
E. If the efflorescence reappears, repeat the process. It may take several months to eliminate the problem once the source of excess moisture has been controlled.
C. If efflorescence is a persistent problem, it may be necessary to reduce the level of soluble salts present within the masonry. Two methods of masonry desalination are described in 04500-03-R. Refer to this procedure for guidance.