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 than geologists. The former use is 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. Geologists define alabaster only as the gypsum type. Chemically, gypsum is a hydrous sulfate of calcium, while calcite is a carbonate of calcium.
Alabaster is a very common microcrystalline variety of the mineral gypsum. Chemically, the alabaster is a calcium sulfate. It has a certain similarity to marble, but in contrast to this is a poor conductor of heat. Alabaster therefore feels warm. Another difference is its lower weather resistance, which is why the stone in sculpture only for interior objects, not for works of art that should be exposed to the weather, can be used. Its color may be white, pale yellow, reddish, brown or gray, depending on where it is found.
The Egyptian alabaster is a variety of calcite with a similar appearance as the gypsum pat variety. However, this is in contrast to the gypsum alabaster water insoluble and harder. These are lime sinter (onyx marble). The term “Egyptian alabaster” for the processed lime sinter from the Wadi Sannur and the Bosra wadi persists in archeology.
Both types of alabaster have similar properties. They are usually lightly coloured, translucent, and soft stones. They have been used throughout history primarily for carving decorative artifacts.
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 is so soft that a fingernail scratches it (Mohs hardness 1.5 to 2), while calcite cannot be scratched in this way (Mohs hardness 3), although it yields to a knife. Moreover, calcite alabaster, being a carbonate, effervesces when treated with hydrochloric acid, while gypsum alabaster remains almost unaffected when thus treated.
Alabaster 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.”
Since alabaster is significantly softer than many rocks, such as marble, but harder than conventional plaster, it was often used for vases and art objects. Alabaster is suitable for the production of small jewelry items as well as life-size sculptures and reliefs. However, from a sculptural point of view, he belongs to the typical interior stones. Alabaster is not weatherproof – such sculptures rely on sheltered spaces. The material would be destroyed after only a few years by the weather. Thin cut alabaster is very translucent and is therefore used in the arts and crafts for lamp shells. In dry areas such. Central Spain, for example, has a tradition as a church window. Less well known but impressive are the altars made of alabaster from the Klettgau in Salemer Münster.
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 decoration and for the rails of staircases and halls.
When dismantling alabaster for commercial and craft purposes, one finds egg-shaped blocks of raw stone of 1 to 3 meters in length. Alabaster is still being promoted and processed in Europe today. A center of European alabaster processing is the Italian Volterra, in whose environment the rock has been used since Etruscan times.
Calcareous alabaster is a calcite, crystal of calcium carbonate, which is effervescence with acid. It is hard enough to break white marble. It always has on its surface rippling species of a honey yellow more or less dark, sometimes drawing on the dark red. It is extremely rare to find perfect limestone alabaster.
Its break is crystalline and striated, which gives it a semi-transparency, because the light has a much easier access in the thickness of this stone so composed, as in that of a marble, for example, whose interior presents an infinity of small slats that break the light rays without allowing them to enter. Well polished, it looks like marble.
Unlike marble, a rock forming kilometers or tens of kilometers deep (it is a metamorphic rock) and whose outcrop at the surface requires the exhumation and erosion of a thick pile of rocks, Alabaster is formed on the surface or very close to the surface on time scales in decades or centuries. The limestone alabaster tends to fill the caves or excavations, transported by the waters which infiltrate into the earth crossing calcareous and ferruginous layers, by taking care of all that they can dissolve since their departure from the surface of the ground to the ceiling of the caves. Thus calcite is dissolved by water acidified by carbon dioxide to give calcium bicarbonate. It can decompose in the opposite direction, giving carbon dioxide and insoluble calcium carbonate which crystallizes in calcite and, after a while, forms a concretion. The drops of water coming from the ceiling give on the spot a stalactite which believes, therefore, from top to bottom. The rest falls to the ground forming a stalagmite from below. These two concretions can end up joining and forming a column. Calcite also covers the walls and floors of these caves, found in almost all limestone-rich countries. They are currently places of visit, because of their diversity of colors and forms, whereas they were places of exploitation in the most distant antiquity, not to mention the prehistoric shelters.
Alabaster, in most cases of its natural occurrence, is a sediment that occurs in large quantities within salt lakes or isolated sea basins in the evaporation of water. This mode of forming can be imagined by the retreat of the sea in trough-shaped depressions; here often in paragenesis with carbonates, halite and other similar minerals. Depending on the viewpoint and deposit situation, this is called a mineral or evaporite rock.
However, alabaster may also be formed by weathering as sinter deposit or by oxidation processes in sulphide ore deposits.
Alabaster consists of calcium sulfate (gypsum) and water of crystallization.Locations for crystals include Romania (Cavnic), Poland (Tarnobrzeg), Spain (Gorguel) and Mexico (Naica, Chihuahua). The fine-grained aggregates are found, inter alia, in Italy.
A large mining area lies between Sulzheim and Bad Windsheim in Lower Franconia. There, gypsum has been mined for centuries and alabaster in the form of potato-sized tubers found in the gypsum.
The Alabaster Caverns State Park, Oklahoma, is home to one of the largest gypsum caves in the world, built as a show cave, with a length of just over one kilometer. The walls of the cave are lined with pink, white and the rare black alabaster.
Other varieties of Gipsspat are Marienglas (selenite) and fiber plaster.
Black alabaster is a rare anhydrite form of the gypsum-based mineral. This black form is found in only three veins in the world, one each in Oklahoma, Italy, and China.
Alabaster Caverns State Park, near Freedom, Oklahoma is home to a natural gypsum cave in which much of the gypsum is in the form of alabaster. There are several types of alabaster found at the site, including pink, white, and the rare black alabaster.
In order to reduce the translucency of the alabaster and to obtain an opacity which gives the impression of being marble, the statues are immersed in a water bath and gradually heated to near the boiling point; this operation requires great care, because if the temperature is not carefully controlled the stone acquires a white appearance as the chalk. The effect produced by the heating seems to be a partial dehydration of the gypsum. If correctly treated, the final appearance is that of true marble, known as Castellina marble. The alabaster may also be treated so as to produce a material that imitates the coral (alabaster coral).
Today, gypsum stone (alabaster) is basically a raw material for the production of gypsum – a powdery binder material, obtained by heat treatment of natural two-water gypsum CaSO4 * 2H2O at a temperature of 150-180 ° C in apparatus communicating with the atmosphere, before converting it into semi-aquatic gypsum CaSO4 * 0,5H2O – gypsum of β-modification. The product of crushing gypsum of β-modification into a fine powder before or after treatment is called building gypsum or alabaster, with finer grinding, a gypsum is obtained or, with the use of raw materials of higher purity, medical gypsum.
At low-temperature (95-100 ° C) heat treatment in hermetically sealed devices, α-modification gypsum is formed, the product of crushing is called high-strength gypsum.
In a mixture with water, the gypsum powder quickly hardens (20 … 60 min.), Turning again into a two-water gypsum, with the release of heat and a slight increase in volume, but this secondary gypsum stone already has a uniform fine-grained structure, the color of various shades of white (depending on the raw material ), opaque and microporous. These properties of gypsum find application in various spheres of human activity.
Alabaster is mined and then sold in blocks to alabaster workshops. There they are cut to the needed size (“squaring”), and then are processed in different techniques: turned on a lathe for round shapes, carved into three-dimensional sculptures, chiselled to produce low relief figures or decoration; and then given an elaborate finish that reveals its transparency, colour, and texture.
Typically only one type is sculpted in any particular cultural environment, but sometimes both have been worked to make similar pieces in the same place and time. This was the case with small flasks of the alabastron type made in Cyprus from the Bronze Age into the Classical period.
When cut in thin sheets, alabaster is translucent enough to be used for small windows. It was used for this purpose in Byzantine churches and later in medieval ones, especially in Italy. Large sheets of Aragonese gypsum alabaster are used extensively in the contemporary Cathedral of Our Lady of the Angels, which was dedicated in 2002 by the Los Angeles, California Archdiocese. The cathedral incorporates special cooling to prevent the panes from overheating and turning opaque. The ancients used the calcite type, while the modern Los Angeles cathedral is using gypsum alabaster.
Calcite alabaster, harder than the gypsum variety, was the kind primarily used in ancient Egypt and the wider Middle East (but not Assyrian palace reliefs), and is also used in modern times. It is found as either a stalagmitic deposit from the floor and walls of limestone caverns, or as a kind of travertine, similarly deposited in springs of calcareous water. Its deposition in successive layers gives rise to the banded appearance that the marble often shows on cross-section, from which its name is derived: onyx-marble or alabaster-onyx, or sometimes simply (and wrongly) as onyx.
Egypt and the Middle East
Egyptian alabaster has been worked extensively near Suez and Assiut.
This stone variety is the “alabaster” of the ancient Egyptians and Bible and is often termed Oriental alabaster, since the early examples came from the Far East. The Greek name alabastrites is said to be derived from the town of Alabastron in Egypt, where the stone was quarried. The locality probably owed its name to the mineral;[dubious – discuss] the origin of the mineral name is obscure (though see above).
The “Oriental” alabaster was highly esteemed for making small perfume bottles or ointment vases called alabastra; the vessel name has been suggested as a possible source of the mineral name. In Egypt, craftsmen used alabaster for canopic jars and various other sacred and sepulchral objects. A sarcophagus discovered in the tomb of Seti I near Thebes is on display in Sir John Soane’s Museum, London; it is carved in a single block of translucent calcite alabaster from Alabastron.
Algerian onyx-marble has been quarried largely 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 Tehuacán and at several localities in the US including California, Arizona, Utah, Colorado and Virginia.
Gypsum alabaster is the softer of the two varieties, the other being calcite alabaster. It was used primarily in medieval Europe, and is also used in modern times.
Ancient and Classical Near East
“Mosul marble” is a kind of gypsum alabaster found in the north of modern Iraq, which was used for the Assyrian palace reliefs of the 9th to 7th centuries BC; these are the largest type of alabaster sculptures to have been regularly made. The relief is very low and the carving detailed, but large rooms were lined with continuous compositions on slabs around 7 feet (2.1 m) high. The Lion Hunt of Ashurbanipal and military Lachish reliefs, both 7th century and in the British Museum, are some of the best known.
Gypsum alabaster was very widely used for small sculpture for indoor use in the ancient world, especially in ancient Egypt and Mesopotamia. Fine detail could be obtained in a material with an attractive finish without iron or steel tools. Alabaster was used for vessels dedicated for use in the cult of the deity Bast in the culture of the ancient Egyptians, and thousands of gypsum alabaster artifacts dating to the late 4th millennium BC also have been found in Tell Brak (present day Nagar), in Syria.
In Mesopotamia, gypsum alabaster was the typical material for figures of deities and devotees from temples, as in a figure believed to represent the deity, Abu, dating to the first half of the 3rd millennium BC in New York.
Much of the world’s alabaster extraction is performed in the centre of the Ebro Valley in Aragon, Spain, which has the world’s largest known exploitable deposits. According to a brochure published by the Aragon government, alabaster has elsewhere either been depleted, or its extraction is so difficult that it has almost been abandoned or is carried out at a very high cost. There are two separate sites in Aragon, both are located in Tertiary basins. The most important site is the Fuentes-Azaila area, in the Tertiary Ebro Basin. The other is the Calatayud-Teruel Basin, which divides the Iberian Range in two main sectors (NW and SE).
The abundance of Aragonese alabaster was crucial for its use in architecture, sculpture and decoration. There is no record of likely use by pre-Roman cultures, so perhaps the first ones to use alabaster in Aragon were the Romans, who produced vessels from alabaster following the Greek and Egyptian models. It seems that since the reconstruction of the Roman Wall in Zaragoza in the 3rd century AD with alabaster, the use of this material became common in building for centuries. Muslim Saraqusta (today, Zaragoza) was also called “Medina Albaida”, the White City, due to the appearance of its alabaster walls and palaces, which stood out among gardens, groves and orchards by the Ebro and Huerva Rivers.
The oldest remains in the Aljafería Palace, together with other interesting elements like capitals, reliefs and inscriptions, were made using alabaster, but it was during the artistic and economic blossoming of the Renaissance that Aragonese alabaster reached its golden age. In the 16th century sculptors in Aragon chose alabaster for their best works. They were adept at exploiting its lighting qualities and generally speaking the finished art pieces retained their natural color.
In Europe, the centre of the alabaster trade today is Florence, Italy. Tuscan alabaster occurs in nodular masses embedded in limestone, interstratified with marls of Miocene and Pliocene age. The mineral is worked largely by means of underground galleries, in the district of Volterra. Several varieties are recognized—veined, spotted, clouded, agatiform, and others. The finest kind, obtained principally from Castellina, is sent to Florence for figure-sculpture, while the common kinds are carved locally, into vases, lights, and various ornamental objects. These items are objects of extensive trade, especially in Florence, Pisa, and Livorno.
In the 3rd century BC the Etruscans used the alabaster of Tuscany from the area of modern-day Volterra to produce funeral urns, possibly taught by Greek artists. During the Middle Ages the craft of alabaster was almost completely forgotten. A revival started in the mid-16th century, and until the beginning of the 17th century alabaster work was strictly artistic and did not expand to form a large industry.
In the 17th and 18th centuries production of artistic, high-quality Renaissance-style artefacts stopped altogether, being replaced by less sophisticated, cheaper items better suited for large-scale production and commerce. The new industry prospered, but the reduced need of skilled craftsmen left only few still working. The 19th century brought a boom to the industry, largely due to the “travelling artisans” who went and offered their wares to the palaces of Europe, as well as to America and the East.
In the 19th century new processing technology was also introduced, allowing for the production of custom-made, unique pieces, as well as the combination of alabaster with other materials. Apart from the newly developed craft, artistic work became again possible, chiefly by Volterran sculptor Albino Funaioli. After a short slump, the industry was revived again by the sale of mass-produced mannerist Expressionist sculptures, and was further enhanced in the 1920s by a new branch creating ceiling and wall lamps in the Art Deco style and culminating in the participation at the 1925 International Exposition of Modern Industrial and Decorative Arts from Paris. Important names from the evolution of alabaster use after World War II are Volterran Umberto Borgna, the “first alabaster designer”, and later on the architect and industrial designer Angelo Mangiarotti.
England and Wales
Gypsum alabaster is a common mineral, which occurs in England in the Keuper marls of the Midlands, especially at Chellaston in Derbyshire, at Fauld in Staffordshire, and near Newark in Nottinghamshire. Deposits at all of these localities have been worked extensively.
In the 14th and 15th centuries its carving into small statues and sets of relief panels for altarpieces was a valuable local industry in Nottingham, as well as a major English export. These were usually painted, or partly painted. It was also used for the effigies, often life size, on tomb monuments, as the typical recumbent position suited the material’s lack of strength, and it was cheaper and easier to work than good marble. After the English Reformation the making of altarpiece sets was discontinued, but funerary monument work in reliefs and statues continued.
Besides examples of these carvings still in Britain (especially at the Nottingham Castle Museum, British Museum, and Victoria and Albert Museum), trade in mineral alabaster (rather than just the antiques trade) has scattered examples in the material that may be found as far afield as the Musée de Cluny, Spain, and Scandinavia.
Alabaster also is found, although in smaller quantity, at Watchet in Somerset, near Penarth in Glamorganshire, and elsewhere. In Cumbria 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 local alabaster is known as “Patrick,” it has been worked into ornaments under the name of “Derbyshire spar”―a term more properly applied to fluorspar.
Modern building plaster is a powder of white, yellowish, pinkish or light gray color, with a significant admixture of coarse fraction (sand), which is usually distributed in paper bags weighing up to 40 kg. It is used in construction as an air binder for plastering walls and ceilings in buildings with a relative humidity of no more than 60%, as a basis for the production of special building mixtures (putties, plasters), in the production of gypsum partition walls, dry plaster sheets, drywall, ventilating boxes, arbolite, gypsum-fiber and gypsum-cutting plates. When mixed with water, the building gypsum hardens quickly, again turning into gypsum stone, which is used in building, sculptural and architectural works that are undemanding for strength, where it often comes in sacks with the inscription “Medical”. Unlike high-strength gypsum, it has a shortened setting time due to the large content of coarse-grained fraction acting as a catalyst and increased adhesion to the surfaces, which is valuable in construction, but also less durable and greater porosity. Of all the gypsum, this is the lowest-grade and cheap material.
Only recently the alabaster was no longer called the building plaster – now the name “alabaster” is outdated.
Marble imitation: 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 heated gradually—nearly to the boiling point—an operation requiring great care, because if the temperature is not regulated carefully, 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”.
Dyeing: Alabaster is a porous stone and can be “dyed” into any colour or shade, a technique used for centuries. For this the stone needs to be fully immersed in various pigmentary solutions and heated to a specific temperature. The technique can be used to disguise alabaster. In this way a very misleading imitation of coral that is called “alabaster coral” is produced.
High-strength and special gypsum, also made from gypsum stone, are used in sculpture, ceramics production, dentistry and jewelry, in architecture and technology for the manufacture of works of art, volumetric products, foundry and water-absorbing forms, fastening and sealing holes and many other intermediate works .
And, despite the antiquity of the material and technology, even at the current level of development of industry and science, a plaster cast is not replace.