Stone

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Copyright: 2011. The Objects Group Wiki pages are a publication of the Objects Specialty Group of the American Institute for Conservation of Historic and Artistic Works. The Objects Group Wiki pages are published for the members of the Objects Specialty Group. Publication does not endorse or recommend any treatments, methods, or techniques described herein.

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Jadeite pectoral from the Mayan Classic period

Stone (or rock) is a naturally-occurring solid aggregate of minerals. Stone is an extremely diverse material whose chief characteristics depend on the geological formation and mineral constituents of the source rock. For a general reference on the science of geology, see the Wikipedia entry Geology.

The Great Sphinx of Giza, carved from the surrounding limestone seabed

Materials and technology[edit | edit source]

History[edit | edit source]

Devatas, Angkor Wat

Stone has been used by humans for millions of years as a material for toolmaking, architecture, sculpting and adornment. Some of the world’s oldest artifacts are made of stone. Stone tools were discovered at Olduvai Gorge, Tanzania that are believed to be 1.8 – 2.0 million years old (MacGregor 2010, 9). Examples of stone architecture include the Great Pyramid of Giza, the Parthenon of Athens, and the temple of Angkor Wat in Cambodia. The statue of David by Michelangelo, one of the most famous sculptures in the world, is constructed of marble from the Carrara quarries in Tuscany (Attanasio, Planiata, and Rocchi 2005). Precious rocks and minerals have been used to embellish decorative objects and clothing, from medieval reliquaries to the crown jewels of England.

Materials[edit | edit source]

The principal rock categories that can be used to characterize stone types are sedimentary, igneous, and metamorphic. The rock cycle explains how these classes are geologically inter-related.

Although stone is a diverse material, it does share certain characteristics across rock types: compositionally, an abundance of silicate minerals; grains, which can be interlocked or held together by a mineral matrix; and, sometimes, contaminants like salts and clays. The geological conditions under which a rock is formed shapes the physical and chemical properties of stone, including its color(s), hardness, porosity, grain size, shape and sorting, cementation, mineral types present, and solubility. Gemstones are specific minerals that occur in association with other rocks.

Sedimentary rocks[edit | edit source]

The following types of sedimentary rocks are typically encountered by objects conservators: alabaster, limestone, and sandstone.

Sedimentary rocks form from the physical and chemical weathering of existing rocks, which break down into sediment. Deposition of sediment can produce clastic (cemented together) or chemical (precipitated) sedimentary rocks. Mineral compositions vary, as do grain size and shape, depending on the energy and distance of sediment transport. Grains and cementing material can be readily distinguished in thin- or cross-section. Some sedimentary rocks can contain fossils. Sedimentary rocks tend to be less strong and more porous than igneous and metamorphic rock (Wolbers 2010, Marshak G-3,4).

Igneous rocks[edit | edit source]

Porphyry sculpture of the Tetrarchs, Saint Mark's Square, Venice

The following types of igneous rocks are typically encountered by objects conservators: basalt, diorite, granite, and porphyry.

Formation processes that are typical for this type of rock include slow or rapid cooling and crystallization of magma above (extrusive) or below (intrusive) the earth’s surface. Mineral compositions vary, but igneous rocks can contain mixtures of grains of varying shape and size, from large phenocysts to fine grains. Igneous rocks tend to be stronger, harder, and less porous than sedimentary rocks (Wolbers 2010, Marshak G-11).

Marble Jar from Pergamon, Hagia Sofia, Istanbul

Metamorphic rocks[edit | edit source]

The following types of metamorphic rocks are typically encountered by objects conservators: gneiss, marble, quartzite, and slate.

Formation occurs as igneous and/or sedimentary rocks are transformed by high pressure and temperature inside the earth’s crust. Mineral compositions reflect those found in igneous or sedimentary rock, but the way these minerals are assembled are altered during the formation process. Metamorphic rocks tend to be stronger than sedimentary rocks, but can still be relatively soft – for example marble formed by metamorphosed limestone tends to be soft and sensitive to low pH solutions (Wolbers 2010, Marshak G-13).

Gemstones[edit | edit source]

Gemstones encountered by conservators include: diamond, chalcedony, emerald, garnet, jadeite, nephrite, and quartz.

Technology[edit | edit source]

Various tools and techniques have been used to work stone to its desired shape and degree of detail and finish. Techniques for stone extraction include quarrying. Stone can be shaped (cut, hewn, carved, drilled, etc.) by tools such as point, claw, and flat chisels in conjunction with hammer and mallets. Structural elements in buildings are sculpture are brought together by joining, which can involve the use of pins, mortars, and adhesives. Surface finishing techniques include grinding and polishing using rasps and files, and various abrasives (Beecroft 1976). Like wood, bone, ivory and shell, stone is often used for decorative inlay. Gemstones can be affixed or mounted to other objects. Polychromy and gilding have been used to embellish architectural, sculptural, and decorative stone surfaces (Verri, Opper, and Deviese 2010).

See The Art of Making in Antiquity - Stoneworking in the Roman World website for information on ancient stoneworking technology.

Traces of polychromy on an ancient Greek column capital

Identification[edit | edit source]

Testing and measurement techniques, microscopic examination, laboratory testing and instrumental analysis have been used to characterize the physical and chemical properties of stone objects and structures, identify factors in their deterioration, and direct preventive and interventive conservation decisions.

Photomicrograph of a thin section of Gabbro

The following techniques have been used to assess stone characteristics, sources and rates of decay: visual and microscopic examination, including stone cross-section and thin-section examination, as well as backscattered electron imaging with SEM-EDS. Laboratory testing and measurement of stone and treatment materials has been used to quantify inherent properties like hardness and porosity, and to assess the properties of contaminants and treatment materials through salt cycling and consolidant penetration tests. Instrumental analytical techniques for the characterization of mineral content include x-ray diffraction, scanning electron microscopy with energy-dispersive spectroscopy, neutron activation analysis, and Raman spectroscopy. Infrared spectroscopy, x-ray diffraction, and ion chromatography have been employed in the characterization of salts.

Deterioration[edit | edit source]

Saint Stephansom, Vienna, before laser cleaning to remove black crusts

The leading circumstances that influence deterioration include the mineral constituents present, amount of porosity, and climate (Wolbers 2010). Deterioration of objects made from stone comes in many forms; as a result of these factors stone can breaks, fractures, cracks, splits, delamination, spalling, sugaring, powdering, and the formation of black crusts. Deterioration can be caused by inherent, external, and environmental factors. Inherent factors include mineral constituents, pore size distribution, water absorption, and hardness. External factors like soluble salts, biological activity, and metal corrosion products are also linked to stone deterioration. Environmental factors include physical forces (impact, abrasion, wind), thermal effects (wetting-drying, freeze-thaw, and temperature & RH cycles), atmospheric pollutants (hydrogen sulfide, soot), weathering (water infiltration, salt migration and cycling), as well as handling (touching, graffiti) (Wheeler and Riccardeli 2010).

ICOMOS has published a useful illustrated glossary of stone deterioration phenomena, available for free download online: ICOMOS Illustrated glossary on stone deterioration patterns.

Conservation and care[edit | edit source]

This information is intended to be used by conservators, museum professionals, and members of the public for educational purposes only. It is not designed to substitute for the consultation of a trained conservator.

To find a conservator, please visit AIC's Find a Conservator page.

Documentation[edit | edit source]

Guidelines for the photographic documentation of stone and other three-dimensional objects can be found on the AIC Guidelines for Practice.

Three-dimensional documentation and imaging of stone has been performed using Reflectance Transformation Imaging (RTI), as well as various methods of laser scanning (Beaubien, Karas, and Fitzhugh 2007). Monitoring of deterioration of stone surfaces over time include time-lapse imaging and videography (Pinchin et al. 2008). Profileometry is a technique employed to measure surface characteristics like roughness; ultrasonic measurement and transducers have been used to quantify changes in stone structures and surfaces.

Preventive conservation[edit | edit source]

A number of protective measures have been taken to protect stone installed in musuems, such as glass or plexiglass barriers for carved reliefs or architectural elements. Environmental controls have been implemented in display environments has been used to protect salt-laden stone objects from potential salt cycling damage (Nunberg, Heywood, and Wheeler 1996). Stone sculpture in outdoor environments has been protected by seasonal covers such as wrapping and tents (Berry et al. 2005). Protective coatings, both permanent and sacrificial, have been used to protect outdoor stone from vandalism. Site preservation and management have also been used to protect architectural remains and other immoveable stone material at archaeological and historic sites.

Marble Bixi at Harvard University Art Museums
Marble Bixi with protective covering

Interventive treatments[edit | edit source]

Cleaning[edit | edit source]

Marble sculpture, partially cleaned

Cleaning techniques vary from simple mechanical procedures like dusting and loose dirt removal to scalpel removal of burial accretions. Solvent and aqueous cleaning are also used, often in conjunction with a poultice or gel carrier.

Solvents can be useful in removing certain types of surface dirt and grime, especially sooty or greasy types. Aqueous cleaning is also used, with surfactants or detergents, or with added compounds to control pH levels. Some stones – like alabaster – can be easily abraded during cleaning, and are particularly sensitive to aqueous cleaning agents.

Poultices are used for cleaning, stain reduction, and desalination. Materials for poulticing include carriers like paper pulp and other cellulosics, clays, gels and latex, cleaning agents like ion exchange resins, chelators and release agents like sand and other aggregates. A number of agents are used for the reduction of stains in stone, including aqueous agents, chelators, oxidizing and reducing agents. These agents can are often delivered in a poultice to control the direction of stain migration.

Laser ablation has also been used to clean stone surfaces, and has proven to be very effective in the removal of black crusts often seen on calcareous stones cleaning mechanism and safety precautions (Cooper 2005). Bioremoval of these crusts has also been studied (Toniolo et al. 2008).

Stabilization[edit | edit source]

Consolidation materials for stone include synthetic polymers like B-72, which can be delivered with solvent, polymerized in situ (Išik-Yürüksoy and Güven 1997), or combined with other consolidants (Brus and Kotlik 1996). Silane treatments have been widely used in the stabilization of building stones (Wheeler 2005), as have tartrate and oxalate treatments (Doherty et al. 2007). Inorganic consolidants have also been introduced, including calcium hydroxide nanoparticles for carbonate stones (Hansen et al. 2003).

Desalination is an important stabilization method for salt-laden stones. Soluble salts can be removed by immersing the stone in water, or by means of a poultice. Poultice properties, including porosity, thickness, and drying rate can be adjusted to improve the rate of desalination.

Structural treatments[edit | edit source]

Filling of cracks on prehistorical painting rock wall, Serra Capivara National Park

Stone delamination and cracks can be stabilized using bulked adhesives and fill materials like acrylic emulsions, grouts and mortars. Large structural cracks and breaks are often joined with the help of pins, as well as adhesives. The reversal of previous repairs can involve the removal of corroded metal pins, or the reinforcement of old joins or failed adhesives.

Aesthetic reintegration[edit | edit source]

Aesthetic reintegration can consist of carved stone joined or adhered to original material. Fills can be carried out using mortars, grouts, or bulked adhesives like Paraloid B-72 (Wolfe 2009). Mold-taking, casting, retouching and inpainting materials vary.

Surface treatments[edit | edit source]

Protective coatings and water repellents have been employed to protect stone from the damages of handling, graffiti, water flow and infiltration, and other factors that can lead to deterioration of stone surfaces.

Other treatments[edit | edit source]

Buildings, architectural remains, mosaics, and composite objects can contain elements of stone that require their own unique conservation approaches.

Mosaics at Mount Nebo, Jordan

References[edit | edit source]

Attanasio, D., R. Planiata, and P. Rocchi. 2005. The marble of the David of Michelangelo: A multi-method analysis of provenance. Journal of Archaeological Science 32(9): 1369–1377.

Beaubien, H. F., Karas, B. V., and W. Fitzhugh. 2007. Documenting Mongolia's deer stones: application of three-dimensional digital imaging technology to preservation. In Scientific research on the sculptural arts of Asia: proceedings of the Third Forbes Symposium at the Freer Gallery of Art, ed. J. G. Douglas et al. London: Archetype. 133-142.

Beecroft, G. 1976. Carving techniques. New York: Watson-Guptill.

Berry, J., F. David, S. Julien-Lees, B. Stanley, and D. Thickett. 2005. Assessing the performance of protective wintercovers for outdoor marble statuary: pilot investigation. ICOM Committee for Conservation preprints. 14th Triennial Meeting, The Hague. 879-887.

Brus, J., and P. Kotlik. 1996. Consolidation of stone by mixtures of alkoxysilane and acrylic polymer. Studies in Conservation 41(2): 109-119.

Cooper, M. 2005. Laser cleaning of sculpture, monuments and architectural detail. Journal of Architectural Conservation 11(3): 105-119.

Doherty, B., M. Pamlona, R. Selvaggi, C. Miliani, M. Matteini, A. Sgamellotti, and B. Brunetti. 2007. Efficiency and resistance of the artificial oxalate protection treatment on marble against chemical weathering. Applied Surface Science 253(10): 4477 - 4484.

Hansen, E., E. Doehne, J. Fidler, D. Larson, B. Martin, M. Matteini, C. Rodriguez-Navarro, E.S. Pardo, C. Price, A. De Tagle, J.M. Teutonico, and N. Weiss. 2003. A review of selected inorganic consolidants and protective treatments for porous calcareous materials. Reviews in Conservation 4: 13 - 25.

Išik-Yürüksoy, B., and O. Güven. 1997. The preservation of Denizli limestones by in situ polymerization. Studies in Conservation 42(1): 55-60.

MacGregor, N. 2010. A history of the world in 100 objects. London: Penguin.

Marshak, S. 2008. Earth: portrait of a planet. New York: Norton & Company, G3-11.

Nunberg, S., A. Heywood, and G. Wheeler. 1996. Relative humidity control as an alternative approach to preserving an Egyptian limestone relief. Le dessalement des matériaux poreux: 7es journées d'études de la SFIIC, Poitiers, 9-10 mai 1996. International Institute for Conservation of Historic and Artistic Works, section française. 127 – 135.

Pinchin, S. E., T. Curteis, E. Doehne, and D. Odgers. 2008. Understanding the decay of 14th century magnesian limestone carvings in Yorkshire, UK. In 9th International Conference "Art2008", Jerusalem 25-30.5.2008: non-destructive investigations and microanalysis for the diagnostics and conservation of cultural and environmental heritage ed. A. Notea. Israel National Commission for UNESCO. 1-9.

Toniolo, L., F. Cappitelli, C. Sorlini, and D. Gulotta. 2008. The biological approach for the removal of black crusts from stone surface of historical monuments. Proceedings of the 11th International Congress on Deterioration and Conservation of Stone. Ed. J.W. Łukaszewicz and P. Niemcewicz. Torun, Poland. 1077-1084.

Verri, G., T. Opper, and T. Deviese. 2010. The "Treu Head": a case study in Roman sculptural polychromy. British Museum technical research bulletin 4. 39-54.

Wolbers, R. 2010. Personal communication. WUDPAC Inorganic Block lecture on stone properties and deterioration. Winterthur Museum, Winterthur, DE.

Wolfe, J. 2009. Effects of bulking Paraloid B-72 for marble fills. Journal of the American Institute for Conservation 48(2): 121-140.

Wheeler, G. 2005. Alkoxysilanes and the consolidation of stone. Los Angeles: The Getty Conservation Institute.

Wheeler, G. and C. Riccardelli. 2010. Personal communication. WUDPAC Inorganic Block lecture on stone properties and deterioration. Metropolitan Museum of Art, New York, NY.