The decision to clean should be based on a genuine necessity, as all masonry cleaning techniques subject the stone to potential hazards.

Cleaning Marble Tablets & Sculpture

A monument which is darkened with soiling, biological growth and metallic staining, is not only disfigured, but also is susceptible to masonry deterioration and, therefore, requires cleaning. A lightly soiled monument with legible details, however, does not require a major cleaning. All cleaning methods must be tested in a discreet location for each monument before full-scale treatment begins and all but the simplest methods should be left to the professional. The gentlest method should be tested first to avoid unnecessary damage. Fragile, bowed tablets should not be cleaned prior to stabilization.

Water washing is the gentlest, safest, and least expensive method for cleaning masonry and may be performed by the nonprofessional provided the tomb or stonework is sound. Most general surface soiling and some biological growth are easily removed with water. All open joints must be repaired first, to prevent penetration of large quantities of water into the masonry. The water should have a low metals content to avoid staining. Usually, potable water is adequate. Water can be applied at low pressure with a garden hose spray and may be supplemented by gentle scrubbing with nonmetallic soft bristle brushes and household detergent.

Much of the black staining occurring on tomb marble and stucco is not atmospheric soiling, but fungal growth. This can be most effectively removed after wet brushing by applying a 2-5% solution of calcium hypochlorite as found in commercial pool chlorine (2-5 parts dry powder to 100 parts water by volume) mixed with an inert clay such as talc or kaolin or paper pulp as a poultice. Once dry this can be removed by brushing and the surfaces well rinsed with a hose or pressure washer. Proper safety precautions must be taken as this material is a strong oxidizing agent. Eye and skin protection is required.

Since black gypsum crusts, resulting from the interaction of the marble or limestone with acidic atmospheric pollution, are water soluble, they may be removed with a slow water soak. For this method it is most important that all joints and seams are watertight to prevent the introduction of water to the tomb interior. Necessary drainage is critical to avoid water collection. As slight brown or yellow oxide stains can develop on certain stones possessing iron impurities, tests should always be done first.

Many commercial chemical products are currently available for cleaning, based on acidic and alkaline compounds and detergents. If used improperly, these can cause etching of stone, insoluble residues, and can introduce harmful salts which can further stone decay. Use of such cleaning systems is best left to experienced professionals.

Abrasive cleaning involving any grit or aggregate applied under pressure should not be used on stucco, brick, or friable stone. The technique is too aggressive and can cause irreversible damage to historic fabric. Abrasive cleaning can lead to accelerated weathering by pitting the surface, thus opening the masonry to increased moisture penetration, atmospheric reactivity, and subsequent deterioration. Basically, any method which removes stone should be avoided. Only an experienced conservator should perform such specialized cleaning methods and only after individual tests have been performed.

Text adapted from St. Louis Cemetery No. 1 Guidelines for Preservation and Restoration. The full document of guidelines can be downloaded as a pdf file. Go There.