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Saturday, March 10, 2012

Alabaster the Non-Gemstone

Church walls and gold leaf decorations made from calcitic alabaster in Poland
Photo by Beemwej

There are two distinct minerals that are called alabaster, one of them is calcium sulfate commonly called gypsum or anhydrite.  The other is a variety of calcium carbonate that is commonly known as limestone or marble.  Of these two the former is the alabaster of today, and the latter is the alabaster of the ancients.  Both of these materials are easy to work with, and have an attractive appearance making them favorites for small carvings and other works of art.

The formation of alabaster explained in Italian
by Miguel

Since we've already established that there are two different types of alabaster the best way to distinguish between them is by their relative hardness. The first type that is composed of gypsum is so soft you can scratch it would your fingernail because I have a Mohs hardness of 1.5 to 2. The calcite variety of alabaster is too hard to be scratched in this way having a Mohs hardness of 3 although you can scratch it with a knife blade. Because it is a carbonate it will effervesce when a drop of hydrochloric acid is placed on its surface. When the gypsum type of alabaster has a drop of mass placed on virtually nothing happens.

An alabaster lamp
Photo by David Dennis

Although alabaster can come in many different colors most people think of it as white, so much so that it has entered the language as a metonym for weight objects. In this sense it is often used to describe “alabaster skin” that means very light and quite transparent that is derived from the use of alabaster in building tombs.

Although alabaster is essentially too soft for use as a gemstone it often finds uses as a decorative stone for making different kinds of statuary and other objects de art. A very common use today is making the basis for electric lamps.

A piece of raw alabaster
Photo by Ra'ike

The word alabaster itself is derived from middle English as well as several other languages including Greek. The word was used to identify a type of vase made of alabaster. There is a certain amount of evidence that the word can be traced all the way back to ancient Egypt where it was named after the town Alabastron where it was commonly found. It was Alabastron where it was originally quarried, however the name of the mineral is still obscure.

The so-called “Oriental” alabaster was highly esteemed by sea ancients for making such items as small perfume bottles small point that basis that were called alabastra. This thing has also been suggested as a possible source of the mineral name. The craftsmen of this stone in ancient Egypt often used alabaster for use in candlestick chart and various other sacred and sepulchral objects. In the Sir John Soane’s Museum in London is a sarcophagus that was carved from a single block of translucent calcite alabaster that was mined Alabastron.

An alabaster carver at work.
Photo by Zyance

Both types of alabaster are essentially evaporative minerals in the case of the gypsum variety it is the mineral that evaporates from seawater before salt,  In the case of the calcitic variety is has its origin from the evaporation of lime water that has reached saturation.  Any further water loss causes the lime to come out of solution where it is deposited. A lot of this type of alabaster is found on the walls and floor of caves where it also forms the formations we call stalactites and stalagmites.

Ancient alabaster vases
Louvre Museum, Paris, France

Although most people think of alabaster as a white mineral it is also found in many other colors particularly when it is stained by mineral salts. Iron is the most common of these although copper and manganese also one of their own suite of colors to the stone. If you cut large blocks of this material into thin sheets they can be used as translucent windows such as those found in the Beineke Library of Rare Manuscripts and Books at Yale University. Throughout the ages how a master has been used to make many beautiful objects and will continue to do so into the future.

1 comment:

  1. I believe the last sentence should read "alabaster" not "a master." =) Thanks for the nice blog.