Alabaster

Alabaster is a mineral or rock that is soft, often used for carving, and is processed for plaster powder. Archaeologists and the stone processing industry use the word differently from geologists. The former use it in a wider sense that includes varieties of two different minerals: the fine-grained massive type of gypsum and the fine-grained banded type of calcite.''More about alabaster and travertine'', brief guide explaining the different use of these words by geologists, archaeologists, and those in the stone trade. Oxford University Museum of Natural History, 2012

/ref> Geologists define alabaster only as the gypsum type. Chemically, gypsum is a hydrous sulfate of calcium, while calcite is a carbonate of calcium.

The two types of alabaster have similar properties. They are usually lightly colored, translucent, and soft stones. They have been used throughout history primarily for carving decorative artifacts."Grove": R. W. Sanderson and Francis Cheetham. "Alabaster", Grove Art Online, Oxford Art Online, Oxford University Press, accessed 13 March 2013
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The calcite type is also denominated "onyx-marble", "Egyptian alabaster", and "Oriental alabaster" and is geologically described as either a compact banded travertine or "a stalagmitic limestone marked with patterns of swirling bands of cream and brown". "Onyx-marble" is a traditional, but geologically inaccurate, name because both onyx and marble have geological definitions that are distinct from even the broadest definition of "alabaster".

In general, ancient alabaster is calcite in the wider Middle East, including Egypt and Mesopotamia, while it is gypsum in medieval Europe. Modern alabaster is probably calcite but may be either. Both are easy to work and slightly soluble in water. They have been used for making a variety of indoor artwork and carving, and they will not survive long outdoors.

The two kinds are readily distinguished by their different hardnesses: gypsum alabaster ( Mohs hardness 1.5 to 2) is so soft that a fingernail scratches it, while calcite (Mohs hardness 3) cannot be scratched in this way but yields to a knife. Moreover, calcite alabaster, being a carbonate, effervesces when treated with hydrochloric acid, while gypsum alabaster remains almost unaffected. Endnotes:
* M. Carmichael, ''Report on the Volterra Alabaster Industry'', Foreign Office, Miscellaneous Series, No. 352 (London, 1895)
* A. T. Metcalfe, "The Gypsum Deposits of Nottingham and Derbyshire," ''Transactions of the Federated Institution'', vol. xii. (1896), p. 107
* J. G. Goodchild, "The Natural History of Gypsum," ''Proceedings of the Geologists' Association'', vol. x. (1888), p. 425
* George P. Merrill, "The Onyx Marbles," ''Report of the U. S. National Museum for 1893'', p. 539.

Etymology

The origin of "alabaster" is in Middle English through Old French "''alabastre''", in turn derived from Latin "''alabaster''", and that from Greek "''ἀλάβαστρος''" ("''alabastros''") or "''ἀλάβαστος''" ("''alabastos''"). The Greek words denoted a vase of alabaster.

The name may be derived further from ancient Egyptian "''a-labaste''", which refers to vessels of the Egyptian goddess Bast. She was represented as a lioness and frequently depicted as such in figures placed atop these alabaster vessels. Ancient Roman authors, Pliny the Elder and Ptolemy, wrote that the stone used for ointment jars called ''alabastra'' came from a region of Egypt known as Alabastron or Alabastrites.

Properties and usability

The purest alabaster is a snow-white material of fine uniform grain, but it often is associated with an oxide of iron, which produces brown clouding and veining in the stone. The coarser varieties of gypsum alabaster are converted by calcination into plaster of Paris, and are sometimes known as "plaster stone".

The softness of alabaster enables it to be carved readily into elaborate forms, but its solubility in water renders it unsuitable for outdoor work. If alabaster with a smooth, polished surface is washed with dishwashing liquid, it will become rough, dull and whiter, losing most of its translucency and lustre. The finer kinds of alabaster are employed largely as an ornamental stone, especially for ecclesiastical