Alabaster
Alabaster (sometimes called satin spar) is a varietal name applied to two distinct minerals; gypsum (a hydrous sulphate of calcium) and the other calcite (a carbonate of calcium). The former is the alabaster of the present day; the latter is generally the alabaster of the ancients.
The two kinds are readily distinguished from each other by their relative hardnesses. The modern alabaster is so soft as to be readily scratched even by the finger-nail (hardness 1.5 to 2), whilst the stone called alabaster by the ancients is too hard to be scratched in this way (hardness 3), though it yields readily to a knife. Moreover, the ancient alabaster, being a carbonate, effervesces on being touched with hydrochloric acid, whereas the modern alabaster when so treated remains practically unaffected.
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A modern lamp made completely from Italian alabaster (white and brown types). The base is 5 inches (13 cm) in diameter.
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Ancient Alabaster
This substance, the "alabaster" of scripture, is often termed Oriental alabaster, since the early examples came from the East. The Greek name alabastrites is said to be derived from the town of Alabastron, in Egypt, where the stone was quarried, but the locality probably owed its name to the mineral; the origin of the mineral-name is obscure, and it has been suggested that it may have had an Arabic origin. The Oriental alabaster was highly esteemed for making small perfume-bottles or ointment vases called alabastra; and this has been conjectured to be a possible source of the name. Alabaster was also employed in Egypt for Canopic jars and various other sacred and sepulchral objects. A splendid sarcophagus, sculptured in a single block of translucent Oriental alabaster from Alabastron, is in the Soane Museum, London. This was discovered by Giovanni Belzoni, in 1817, in the tomb of Seti I, near Thebes, and was purchased by Sir John Soane, having previously been offered to the British Museum.
When cut in thin sheets, alabaster is translucent enough to be used for small windows in medieval churches, especially in Italy.
Oriental alabaster is either a stalagmitic deposit, from the floor and walls of limestone caverns, or a kind of travertine, similarly deposited from springs of calcareous water. Its deposition in successive layers gives rise to the banded appearance which the marble often shows on cross-section, whence it is known as onyx-marble or alabaster-onyx, or sometimes simply as onyx -- a term which should, however, be restricted to a siliceous mineral. The Egyptian alabaster has been extensively worked near Suez and near Assiut; there are many ancient quarries in the hills overlooking the plain of Tell el Amarna. The Algerian onyx marble has been largely quarried in the province of Oran. In Mexico there are famous deposits of a delicate green variety at La Pedrara, in the district of Tecali, near Puebla. Onyx-marble occurs also in the district of Tehuacan and at several localities in California, Arizona, Utah, Colorado and Virginia.
Modern Alabaster
When the term "alabaster" is used without any qualification it invariably means, at the present day, a finely granular variety of gypsum. This mineral, or alabaster proper, occurs in England in the Keuper marls of the Midlands, especially at Chellaston in Derbyshire, at Fauld in Staffordshire and near Newark in Nottinghamshire. At all these localities it has been extensively worked. It is also found, though in subordinate quantity, at Watchet in Somerset, near Penarth in Glamorganshire, and elsewhere. In Cumberland and Westmoreland it occurs largely in the New Red rocks, but at a lower geological horizon. The alabaster of Nottinghamshire and Derbyshire is found in thick nodular beds or "floors," in spheroidal masses known as "balls" or "bowls." and in smaller lenticular masses termed "cakes." At Chellaston, where the alabaster is known as "Patrick," it has been worked into ornaments under the name of "Derbyshire spar" -- a term more properly applied to fluorspar.
The finer kinds of alabaster are largely employed as an ornamental
stone, especially for ecclesiastical decoration, and for the
rails of staircases and halls. Its softness enables it to be
readily carved into elaborate forms, but its solubility in
water renders it inapplicable to outdoor work. The purest
alabaster is a snow-white material of fine tiniforni grain,
but it is often associated with oxide of iron, which produces
brown clouding and veining in the stone. The coarser varieties
of alabaster are converted by calcination into plaster of Paris, whence they are sometimes known as "plaster stone."
On the continent of Europe the centre of the alabaster trade is Florence, Italy. The Tuscan alabaster occurs in nodular masses, embedded in limestone, interstratified with marls of Miocene and Pliocene age. The mineral is largely worked, by means of underground galleries, in the district of Volterra. Several varieties are recognized -- veined, spotted, clouded, agatiform, etc. The finest kind, obtained principally from Castellina, is sent to Florence for figure-sculpture, whilst the common kinds are carved locally, at a very cheap rate, into vases, clock-cases and various ornamental objects, in which a large trade is carried on, especially in Florence, Pisa and Leghorn.
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Detail of base of alabaster lamp
Larger version
In order to diminish the translucency of the alabaster and to produce an opacity suggestive of true marble, the statues are immersed in a bath of water and gradually heated nearly to the boiling-point -- an operation requiring great care, for if the temperature be not carefully regulated, the stone acquires a dead-white chalky appearance. The effect of heating appears to be a partial dehydration of the gypsum. If properly treated, it very closely resembles true marble, and is known as marmo di Castellina. It should be noted that sulphate of lime (gypsum) was used also by the ancients, and was employed, for instance, in Assyrian sculpture, so that some of the ancient alabaster is identical with the modern stone.
Alabaster may be stained by digesting it, after heing heated, in various pigmentary solutions; and in this way a good imitation of coral has been produced (alabaster coral).
See also: list of minerals
This article incorporates text from the public domain 1911 Encyclopaedia Britannica.