Chemistry Reference
In-Depth Information
calcium carbonate out of the water and use it: the animals to make their
shells and the plants their inorganic solid matter. When the organisms die,
their living matter decays. The calcium carbonate that constitutes the shells,
corals, or plants, however, remains in the form of small particles that grad-
ually accumulate as sediments. Over long periods of time, such sediments
become consolidated into beds of cohesive limestone deposits (Boynton
1980).
Both gypsum and limestone are widely distributed throughout the
earth's crust. They are mostly white, as are pure calcium sulfate and calcium
carbonate. Because they often contain impurities, many gypsum and lime-
stone rocks are colored, usually off-white, but occasionally they display
intense colors, bands, and speckles. They are excellent thermal insulating
materials, and although they are strong, they are relatively soft and can be
cut easily, without splitting. These properties makes them ideal for building
sheltering constructions, sculpting statuary, carving ornaments and engrav-
ing epigraphs.
Gypsum and Alabaster
Gypsum, one of the more common sedimentary rocks in the earth's crust,
occurs in varieties that differ from each other in texture rather than in com-
position. In most of its varieties gypsum is very soft and can be scratched
even with a fingernail. One massive and fine-grained variety of gypsum,
known as
alabaster
, much appreciated for designing delicate decorative
objects, has been used particularly for carving and ornamentation.
There is some confusion regarding the use of the term
alabaster
in archae-
ological contexts. Although alabaster (see Fig. 31) is a fine-grained form of
gypsum (composed of hydrated calcium sulfate), much of the stone known
as
ancient alabaster
from ancient Egypt, for example, where it was widely
used for decorative work, consists not of gypsum but of calcite (composed
of calcium carbonate) (Lucas 1962). Ancient alabaster also known as
oriental
alabaster
,
Egyptian alabaster
or
onyx marble
, therefore, is a variety not of
gypsum but of limestone. The appearance, carving, and sculpting properties
of ancient alabaster are, however, very similar to those of ordinary alabaster.
The two types, alabaster and ancient alabaster, can, however, be easily dis-
tinguished from each other by some simple tests. Alabaster (a variety of
gypsum) is so soft that it can readily scratched even by a fingernail; its hard-
ness on the
Mohs scale
is 1.5-2 (see Table 20). Ancient alabaster, whose hard-
ness is about 3 on the Mohs scale, is too hard to be scratched in this way.
Moreover, since ancient alabaster is composed of calcium carbonate, it effer-
vesces when in contact with any acid, whereas gypsum alabaster is unaf-
