Alabaster is a fine-grained stone prized for its delicate appearance and ability to transmit light, making it a favored material for artists and designers for millennia. Its pale, often snow-white color and subtle translucency lend a sense of softness and warmth to objects crafted from it. The material has been used since ancient times for both religious and decorative objects. The term “alabaster” is confusing, as it refers broadly to two chemically and geologically distinct minerals.
The Two Classifications of Alabaster
The ambiguity of the term “alabaster” stems from its historical application to two separate materials that share similar visual characteristics and softness. The most common modern definition refers to Gypsum Alabaster, a fine-grained, massive form of the mineral gypsum (hydrated calcium sulfate, \(text{CaSO}_4cdot 2text{H}_2text{O}\)). This variety is notably soft, registering 1.5 to 2 on the Mohs scale, meaning it can be easily scratched with a fingernail or carved with simple tools. This extreme softness makes it favored by sculptors for achieving intricate detail and delicate forms.
The second material is Calcite Alabaster, a form of calcium carbonate (\(text{CaCO}_3\)) that is geologically a type of compact banded travertine. This type is significantly harder than the gypsum variety, possessing a Mohs hardness of 3. While still relatively soft, it cannot be scratched by a fingernail. Calcite alabaster is also known as “Onyx Marble” or “Egyptian Alabaster,” as it was the primary material used by ancient civilizations in the Middle East. The distinction can be confirmed by a simple acid test: the carbonate-based calcite variety will effervesce when treated with hydrochloric acid, while the sulfate-based gypsum variety will not.
Defining Physical Characteristics
The appeal of alabaster, regardless of its mineral composition, lies in shared physical properties that suit it for artistic application. Foremost is its distinct translucency, which allows light to penetrate the surface and diffuse through the stone’s fine crystalline structure. This property causes objects crafted from alabaster, particularly when thin, to emit a soft, warm glow when backlit. This is a prized characteristic for lighting fixtures, as the material can allow up to 70% of light to pass through.
While the purest alabaster is snow-white with a uniform grain, it frequently displays subtle coloration and veining. Impurities, such as iron oxides and other trace elements, are responsible for these variations, resulting in hues that range from cream and beige to pale pink, yellow, or brown banding. The relative softness of the material, especially gypsum alabaster, allows for a high degree of workability. Sculptors can carve intricate designs difficult to achieve with harder stones, and the fine texture can be polished to a smooth finish.
Geological Formation and Sources
The two types of alabaster have distinct origins reflecting their different chemical compositions. Gypsum alabaster is a sedimentary rock formed as an evaporite deposit, typically found in thick layers associated with ancient seas that have evaporated. This process involves the crystallization of calcium sulfate dihydrate, forming fine-grained masses suitable for carving. Significant sources are found globally, including the Ebro Valley in Spain, a major modern supplier, and the areas of Volterra in Tuscany, Italy.
Calcite alabaster forms through the precipitation of calcium carbonate from mineral-rich waters. It is classified as travertine, a type of limestone deposited in successive layers in environments like caves or hot springs. This formation process, similar to the growth of stalactites and stalagmites, often results in the banded, layered appearance that gives it the name “Onyx Marble.” Historically, famous sources were in the Nile valley, particularly at sites like Hatnub in Egypt, where the material was widely quarried by ancient civilizations.
Historical and Modern Applications
Alabaster’s combination of softness and translucency has secured its place in art and architecture from antiquity to the present day. Ancient civilizations, including the Egyptians and Mesopotamians, utilized the ease of carving for small sculptures and funerary objects. The calcite variety was used extensively in Egypt to create objects like canopic jars and vessels for ointments. The sarcophagus of Seti I is a notable example carved from a single translucent block.
In the modern era, the material’s light-diffusing property has become its most valued attribute, leading to its widespread use in interior design and lighting. Designers use alabaster for translucent panels, lamps, and decorative fixtures, often backlighting the stone to highlight its warm glow. Its malleability also makes it a favored medium for contemporary sculptors, allowing for the creation of intricate and abstract forms. Because the gypsum variety is slightly soluble in water and both types are relatively soft, alabaster is overwhelmingly used for indoor applications, where it is protected from moisture and weathering.