Chemistry Reference
In-Depth Information
from a few hundred to several hundred thousand years. This makes these
techniques applicable for dating archaeologically related natural materi-
als such as stone and sediments, human-made ceramics, and some types of
glass. Ceramics and glass are fired during manufacture at temperatures
exceeding 600°C. Such high temperatures are sufficient to release energy
trapped in the “defects” of minerals components of these materials. Thus,
ceramic-firing and glass-melting operations set the thermoluminescence
“clock” of these materials to zero time and make it possible to date them
using the technique. Animal remains (bone and shell) that underwent
thermal events have also been dated using the thermoluminescence tech-
nique (Troja and Roberts 2000).
It should be mentioned, however, that although thermoluminescence
dating has already been in use for about half a century, the dates derived
with the technique are not yet accurate enough to be relied on in archae-
ological investigations. Thermoluminescence-derived dates are thus still
considered to be in their developmental stage; they are used mainly for
testing the authenticity of archaeological finds, not yet to determine their
exact age (Aitken 1998; McKeever 1988).
2.2.
OBSIDIAN
Obsidian
is a dense volcanic solid often formed in lava flows where the lava
cools so quickly that crystals cannot grow. Although it has a rather definite
chemical composition, obsidian lacks the regular crystal structure charac-
teristic of the minerals and is classified as a mineraloid, more specifically, as
a natural glass. Like flint, when obsidian is struck it also breaks with a
con-
choidal fracture
and the broken pieces have sharp edges; thus, obsidian was
the type of stone most widely used for making lithic tools in areas where
flint was not available. Chemically, obsidian consists of a mixture of metal
silicates, a composition it shares with the mineral rhyolite, which also orig-
inates from molten magma. Rhyolite, a crystalline material, is formed when
erupted magma cools down gradually and, while solidifying, its component
atoms become regularly ordered in both short- and long-range geometric
arrangements. Obsidian, on the other hand, is formed when the magma cools
down rapidly, does not crystallize, and remains as a
supercooled liquid
, that
is, an amorphous, hard, brittle glass (see Textbox 27). Often, microscopic
crystals and gas bubbles locked within the glass render much obsidian
opaque and usually gray or black. Colored obsidian, in a variety of colors,
albeit not common, is also known. Red and brown varieties, for example,
owe their color to inclusions of iron oxides. A semitransparent, smoky
