Marble: Characteristics, Uses And Problems
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- Outdoor Sculpture Manual - Center For Public Buildings
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This standard includes general information on the characteristics and common uses of marble and identifies typical problems associated with the material. See 04400-01-S for guidance on inspecting stone masonry failures.
Marble is an extremely hard, metamorphic stone composed of calcite (CaCO3). It is formed as a result of the recrystallization of limestone under the intense pressure and heat of geologic processes. The effect of this process is the creation of a stone with a very tight crystalline structure and small but definite porosity. Because of its structure, marble can take a very high polish and is a very popular decorative stone for architectural and sculptural uses. The limited porosity of marble, especially polished marble, makes it less vulnerable to the leaching effects of water. Calcium carbonate, however, of which marble is composed, is highly susceptible to attack by acidic agents. Marble is readily dissolved by acids, even very dilute acids, however the actual results of acidic exposure will vary with the nature of the acid. Chlorides, nitrates, sulfates and other chemical compounds react differently with marble and produce various by-products, which have a wide range of solubility and impact on the durability of marble. For this reason, it is always important to determine the exact type of pollutants causing marble deterioration.
Marble itself can be of two types, one composed of calcite and the other of dolomite. Dolomitic marble is much more resistant to acid attack than calcite marble. The color of marble ranges from the brilliant white of calcite to black, including blue-gray, red, yellow and green, depending upon the mineral composition.
Marble has many decorative and structural uses. It is used for outdoor sculpture as well as for sculpture bases; in architecture it is used in exterior walls and veneers, flooring, decorative features, stairways and walkways. The way in which the stone is used may be a factor in limiting or controlling the severity of exposure. The use or function of the marble may also affect the feasibility of applying certain treatments, but type of use is not the primary factor in the major types of deterioration and damage to which marble is susceptible.
Problems and Deterioration
The natural forces and agents of weather may have a degrading effect on the appearance and structural soundness of marble. These agents include rain, snow, temperature, wind and atmospheric pollutants. Weathering agents almost never work singly or in isolation, they always act in combination with one another or with other agents of deterioration.
Rain water, especially in combination with atmospheric gases, can result in dissolution of the marble, creating higher levels of salt movement within the micro-structure. Temperature can effect rates of deterioration and (in larger stones) movement of the pieces, as well as patterns of salt migration within the stone. Higher temperatures often increase the rate of chemical changes; low temperatures can create the risk of "freeze-thaw" problems, and rapid changes of temperature can produce stresses in the material due to differential expansion. Most of the natural or inherent problems which can occur with marble require some degree of moisture to occur, however other problems such as wind erosion and vandalism may occur independently.
Natural or Inherent Marble Problems
Marble subjected to exterior exposures deteriorates due to weathering or the natural effects of wind, rain, and thermal change. Marble is extremely durable and because of its limited porosity does not absorb large amounts of water. It does, however, absorb some water and, since it is highly reactive when exposed to acids or even mildly acidic rain water, it can suffer substantial deterioration. In short, while the porosity of marble is low, the shape of the pores (elliptical) allows greater dissolution than in typical round pores, and this fact coupled with marble's inherent solubility in acids can result in two major problems:
- Loss of polish
- Loss of detailing
The most common symptoms of weathering are a loss of the highly polished surface (where it exists) and loss of crisp edge details in decorative carved areas of the stone.
Little can be done to restore edge detailing short of re-carving the stone which is usually infeasible. The gloss or polish can be maintained and, to some degree, restored on the surface of marble by using marble polishing powder and soft buffing pads. Obviously the level of effort or labor required to repolish stone will depend upon the degree of deterioration of the surface, or loss of polish. New pieces, restored pieces or well-maintained surfaces can be retained in a highly polished state with much less effort than surfaces which have been exposed to weathering for long periods of time with little or no maintenance. For this reason, polished marble should receive regular preventive maintenance by polishing.
Erosion can be the result of general weathering described above, or it can be a more localized phenomenon based upon handling or exposure. Wind driven, airborne abrasives such as dirt, grit, and other "particles" may selectively wear away detailing. The effects will depend largely upon the direction of prevailing winds. One of the few effective ways to address this problem is by landscaping where plantings and/or landscape grade can deflect the wind. Such landscaping and/or grading may range from the simple and inexpensive to a major and expensive intervention. It would have to be consistent with the policy for the management of cultural landscapes. It may, however, be cost effective when considering
the extended life of the building.
The symptoms of erosion can be as simple as the general loss of polish and edge sharpness as described above, or it can be very localized, specific wear due to contact with landscaping and mowing equipment. Localized damage due to contact by mowing or other maintenance equipment is preventable. Where there is evidence of recurrent physical damage, steps should be taken to protect the
Discoloration of the marble, whether general or localized, is staining. Staining may be the result of exposure to a variety of exterior substances, or to internal occlusions in the stone or structural elements.
Some of the most common types of staining and the causative agents are:
Oil/grease stains: These stains are usually the result of vandalism or handling. A variety of organic or inorganic oils may be absorbed into the stone upon contact. The depth of penetration will depend upon the viscosity of the oil/grease, temperature, stone porosity, finish and dryness.
The appearance of grease/oil stains will usually consist of a darkening of the stone at the area of contact. The edges of the staining will generally be diffused, especially after an extended period. There are standard techniques for removing oil and grease stains and may often be accomplished by maintenance personnel..
For specific guidance on removing oil/grease stains from marble, see 04455-10-R and 04455-11-R.
Dyes and inks: The staining can be any color depending on the type and source of the dye. This type of stain is likely to be extremely localized around the area of contact. The liquid containing the coloration may be absorbed into the stone and during the normal process of evaporation, the coloring pigment is deposited within the stone.
For specific guidance on removing ink and dye stains from marble, see 04455-18-R.
- Organic stains: Organic stains are caused by direct contact with decomposing organic matter, such as leaves, bird or animal droppings, flowers, tea or coffee. Regardless of the source, these stains tend to be a slight reddish-brown in color. They also frequently disappear after the source has been removed. Organic stains may be left to weather and bleach or oxidize out after the removal of the organic source, however a residue may still remain on the stone.
For specific guidance on removing organic stains from marble, see 04455-14-R.
Metallic stains: Two major categories of metallic staining are based on either iron or copper. The source of the staining may be internal structural components or features. A common source is the water wash, or run-off, from adjacent metallic elements, especially bronze.
- Rust stains: These stains are reddish-orange and are caused by the oxidation (rusting) of iron. The source of iron staining is usually the structural or connecting components. These components are usually hidden and protected; however, water penetration from bad joints or cracks can activate or accelerate rusting. The discoloration may be within the stone or it may be a deposit of rust on the surface of the stone. Surface deposits of rust may sometimes be removed by hand rubbing with a clean cloth. The examination of the stain should include such rubbing to determine if it is only a surface deposit.
- For specific guidance on removing rust stains from marble, see 04400-06-R.
Copper stains: Stains from water run-off from bronze can range in color from a light green to a dark brown. The staining results from the dissolved copper salts (from copper or bronze) which wash down onto the stone, then oxidize. The pattern of the staining is likely to be localized, streaked and in the path of the run-off from the metallic source. In some cases, especially plaques on stone, where biological growth or mildew may form on shaded elevations, the copper salts in the water running off the bronze acts as a fungicide, killing the growth and making areas directly below the metal look cleaner than surrounding areas.
For specific guidance on removing copper stains from marble, see 04400-07-R.
General dirt, soot and pollution: Marble can be discolored generally or locally by atmospheric dirt, grime and other airborne particulates which adhere to the material. The visual appearance is usually a dulling or graying effect which mutes or obscures the original color and gloss. Effects of dirt are usually intensified in the protected areas, where the rinsing effect of rain water is diminished.
Dirt can be a complex composition of finely divided solids held together by organic material including soot, siliceous dust, and other airborne material. It may include particles of metals, glass, ceramics, metal oxides and minerals.
Dirt is usually held to the surface by a combination of absorption and static attraction. Dirt can become incorporated into "crusts" which can develop during the deterioration of some stones. Biological agents can also collect on dirty surfaces and the same water used for cleaning can stimulate algal growth. Algae, lichens and moss can produce acid by-products which can damage acid sensitive stone. In addition, some water proof and water repellent coatings used on stone actually increase the static attraction of the surface and result in the stone getting dirty faster.
For specific guidance on removing dirt/pollution from marble, see 04400-01-P, 04400-02-P, 04400-03-P.
This condition is due to a certain brittleness or tendency of the stone to break up or dissolve. It may be caused by an inherent weakness in the stone or gradual breakdown of the binder or crystalline structure, or it may be the result of external factors affecting the strength and durability of the marble.
This condition may be caused by the use of de-icing salts, or any other source of salt migration, such as that which can occur when rising damp is present. There is currently little which can be done to repair the damage once this condition has developed, however the early detection of potential problems and elimination of sources of salts is critical to arresting the process. When this condition is severe and obviously caused by the heavy or inappropriate use of de-icing salts, it is sometimes called "Salt Fretting". Regular preservation maintenance may eliminate the causes promoting crumbling, however, once the condition has occurred, its correction or repair is beyond the level of a maintenance procedure. The Regional Historic Preservation Officer (RHPO) should be contacted for assistance.
The separation of small pieces or larger fragments from a masonry unit, frequently at the corners, edges or mortar joints is known as chipping. These fractures are generally caused by deterioration and repointing, especially due to the use of too hard a pointing mortar, or by accident or vandalism.
Repairs include detachment repairs, patching and splicing. Repair of chipped stone requires a skilled mason and is not a maintenance procedure. If chipping is due to occasional impact from mowing or other landscape maintenance, steps should be taken to prevent future damage
For specific guidance on repairing chips in marble, see 04455-03-R.
This condition is manifested by the appearance of narrow fissures ranging from less than 1/16 to 1/2 inch wide or more in the stone. It results from a variety of causes, such as structural overloading due to settlement, the use of too hard a mortar mix or a flaw in the material. Minor cracking may be no problem, in and of itself, but it can be an indication of structural problems and the cracks can be a source of entry of water into the interior of the stone, promoting salt migration. Marble is a relatively homogenous material since it is crystallized under intense heat and pressure. It has, however, evolved from sedimentary stones and may have structural/physical planes of different density and strength. Cracking, which allows water or salts to enter the stone, increases the possibility of failure along the planes with subsequent spalling. Repairs include patching and replacement.
For specific guidance on repairing cracks in marble, see 04455-03-R.
This is not a failure of the marble, but rather a failure of the construction system, i.e. the connectors and/or joints. The definition implies that the failed component survives intact and may be re-installed using appropriate mechanical techniques. Detachment cannot occur with a monolithic piece. Visually, detached pieces may be separated from surrounding ones.
The failure of anchors or metal connectors which lead to detachment may be caused and/or accelerated by the penetration of water into the structure behind the stone, causing rust and corrosion. Adequate pointing and caulking will prevent leakage and penetration of water into the system.
For specific guidance on re-securing detached marble, see 04455-21- R.
The appearance of a whitish deposit locally or uniformly over the surface may be efflorescence, the surface deposition of soluble salts. There are numerous sources for the soluble salts which create the hazy appearance; salts can come from mortar, improper cleaning agents, rising damp, de-icing salts, chemical landscaping treatments and air pollution. Whitish deposits of salts may be much less obvious on white marble than on darker stones and brick. On polished surfaces, deposits will reduce the gloss and present a diffuse or hazy area.
Efflorescence can be a salt residue resulting from improper chemical cleaning, i.e. too strong a chemical cleaner or inadequate rinsing. It can also be an indication of water problems. Salt migration and/or sub-florescence and efflorescence should be considered a symptom which should be investigated to identify the
source of the soluble salts and/or the source of moisture. Corrective action should then be taken to eliminate the source of the problem once it is identified.
Some efflorescence may occur naturally with new stones, mortar and installation materials. Normally, this efflorescence will be removed by natural rain and weathering processes and/or by regular washing. The new or continued appearance of efflorescence is a stronger indicator of problems like rising damp, inappropriate cleaning methods, all of which should be referred to the RHPO.
For specific guidance on removing efflorescence from marble, see 04455-25-R.
Erosion is the wearing away of the material surface by the natural action of wind, windblown particles and water. It can occur with marble as well as any exposed materials. Inspections should include examination for any apparent loss of detail and edge sharpness which could be due to erosion.
Erosion may be less of a problem on rock-faced or quarry-faced marble, but may be a more serious problem on stone with more precise detail. Little can be done to correct this problem once it occurs, other than to protect the surface from further exposure to stop or retard the process.
This is an early stage of peeling, exfoliation, delamination or spalling evidenced by the detachment of small flat thin pieces of the outer layers of stone from a larger piece of stone. Flaking is usually caused by capillary moisture or freeze-thaw cycles which occur within the masonry.
The problem can also occur due to sub-florescence, so that if flaking occurs, the area should be examined to determine if salt crystallization is occurring in the flaked areas.
Peeling is flaking away of the surface from the substrate in strips or layers. It may result from the improper application of masonry coatings which result in failure of the coating and/or stone surface. It may also result from a defect in the stone, or from weathering.
Encrustation of the surface caused by chemical reactions with environmental elements may also peel or flake along the bedding plane.
Rising damp is the suction of ground water into the base of masonry through capillary action. Moisture is drawn up into the marble and may rise and fall due to conditions of temperature; humidity; site grading; absence or failure of damp courses, and/or treatments to the masonry surfaces which affect evaporation.
During active wet periods, rising damp may be visible as a darkening of the marble along the base at ground level. Due to the continuous changing of the moisture level due to varying exposure conditions, staining or efflorescence may be visible at a range of several feet up from the ground. Continuation of the problem can lead to more severe problems of flaking, peeling and/or spalling, but the correction of the problem requires the elimination of the source of water or the interruption of its path into the stone by physical or chemical damp-proofing.
Spalling is the separation and breaking away of pieces of stone due to sub-florescence, freeze-thaw, improper repointing with too hard a mortar mix or portland cement, or structural overloading of the stone.
Spalling is less frequent with marble than with sedimentary stones which are also less dense. Marble is dense enough to resist internal forces which would cause spalling in other natural stones or fabricated masonry.
For specific guidance on repairing spalling marble, see 04400-03-R and 04455-03-R.
This is a potentially harmful internal accumulation of soluble salts deposited under or just beneath the masonry surface as moisture in the wall evaporates.
The build-up of salts and their crystallization can create substantial pressures within the marble, causing pieces to break off along the planes of deposition. Efflorescence at the surface is an indication that sub-florescence is possible. Techniques for mitigating the problem include poulticing, removal of identified salt sources, elimination of moisture in the stone and damp-proofing.
This is a gradual disintegration of the surface of the marble, possibly caused by salt migration and exposure to moisture. Excessive moisture may have the effect of dissolving the binder.
Carbonate stones, especially fine grained marbles, are particularly susceptible to this form of deterioration. The surface takes on a rough granular, crystalline or sometimes powdery appearance.
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