Alabaster

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Materials and Technology

Materials

Alabaster is a term that is often used to refer to two distinct varieties of stone when used as a material for art and artifacts: alabaster and travertine.

Gesso di Volterra, alabastro di Volterra, from Volterra, Pisa, Tuscany, Italy, fine-grained compact gypsum (alabaster) © Oxford University Museum of Natural History

Alabaster (Gypsum Alabaster)

True alabaster, in geological terms, is a sedimentary rock consisting primarily of gypsum, a hydrous sulfate of calcium. Alabaster is a fine-grained aggregate that occurs in layers resulting from the evaporation of sea water. The purest alabaster is white, but its association with iron oxides produces brown clouding and veining in the stone.

Egyptian alabaster, alabastro cotognino, from one of several quarries at Hatnub, Mallawi, medium-grained compact banded travertine © Oxford University Museum of Natural History

Travertine (Calcite Alabaster)

The other material often referred to as “Egyptian Alabaster,” “Oriental Alabaster,” or “Onyx Marble” in archaeological, art historical or decorative contexts, is comprised of the mineral calcite, a carbonate of calcium. Geologically, calcite alabaster is travertine, a sedimentary variety of limestone that appears as a dense, non-porous, fine-grained stone deposited as flowstone, stalagmites and stalactites. Rocks deposited in cold-water cave environments tend to be relatively pure in composition and are typically colorless,white or brown. Travertines deposited by hot springs may be richly patterned and tinted red, yellow and brown by iron oxides, the result of the action of cyanobacteria living in the warm water.

Alabastro a rosa, alabastro fiorito, locality unknown, compact banded travertine © Oxford University Museum of Natural History

Identification

Visual identification of alabaster may be difficult and objects may require sampling or other destructive techniques for positive identification. For example, the types of alabaster can be distinguished from one another by differences in their relative hardness. Gypsum alabaster can be scratched with a fingernail (Mohs hardness of 1.5 to 2), while calcite alabaster is soft but requires more effort to scratch or carve (Mohshardness of 3). Because calcite alabaster is a carbonate, it will effervesce with hydrochloric acid.

Not only are the two varieties (gypsum and calcite) often confused, alabasters are often misidentified with other stones such as marble, steatite and onyx especially when highly polished. Marble, a metamorphic calcium carbonate stone, is generally darker in color and is harder (Mohs hardness of 3-5) and more opaque than alabaster. However, when heated, alabaster loses its translucency and can have an appearance quite similar to marble. Steatite or soapstone, composed of talc, is much softer than alabaster (Mohs hardness of 0-1). True onyx is a banded chalcedony composed largely of silicon dioxide and is much harder than all of these stones (Mohs hardness of 7).

Technology

The softness of both gypsum and calcite alabasters enable them to be easily carved into elaborate forms; however, the solubility in water, particularly of gypsum alabaster, renders it unsuitable for outdoor work. It can be polished to a high gloss and its translucency makes it ideal for objects such as vases, lamps, and lantern shades.

Deterioration

Although the surface can appear hard and glass-like especially when polished, alabasters are easily broken, bruised, abraded and scratched. The stone itself is water soluble and polished surfaces are particularly vulnerable to moisture and aqueous solutions, which can turn the surface dull and opaque.

Since gypsum alabaster is a hydrated form of calcium sulfate, exposure to high heat can cause the conversion of gypsum to the opaque anhydrite.

Alabaster is less porous than some marbles due to its uniform, fine grain, but it is still permeable by water and soluble salts. It can become stained and discolored from airborne pollutants, particulate material, and chemical or coating applications that become ingrained within the stone and are therefore difficult to remove.

Conservation and Care

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.

Documentation

Preventive Conservation

Interventive Treatments

Cleaning

Stabilization

Structural Treatments

Aesthetic reintegration

Surface treatments

Other treatments

References

Aston, B.G, Harrell, J.A., and Shaw, I. 2000. "Stone: Alabaster." In Nicholson, P.T. and Shaw, I (Eds.) Ancient Egyptian Materials and Technology. Cambridge: Cambridge University Press, 21-22.

Aston, B.G, Harrell, J.A., and Shaw, I. 2000. "Stone: Travertine." In Nicholson, P.T. and Shaw, I (Eds.) Ancient Egyptian Materials and Technology. Cambridge: Cambridge University Press, 59-60.

Gänsicke,S and Hirx, J.W. 1997. "A Translucent Wax-Resin Fill Material For The Compensation Of Losses In Objects."Journal of the American Institute for Conservation 36(1): 17-29.

Griswold, J. 2000. "Care of Alabaster"Conserv O Gram. National Parks Service. 15/1.

Please note: This article incorrectly identifies alabaster as a metamorphic rock.

Griswold, J and Uricheck, S. 1998. “Loss Compensation Methods for Stone.” Journal of the American Institute for Conservation 36(1): 89-110.

Price, M.T. 2012. More about alabaster and travertine.Oxford University Museum of Natural History, 1-2.

Further reading




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