Chemistry Reference
In-Depth Information
Blue Pigments
Mountain blue
,
ultramarine
, and
glaucophane
were the most widely used
natural ancient blue pigments.
Mountain blue
is made by crushing and reduc-
ing to a powder the bright blue mineral
azurite
(composed of a basic car-
bonate of copper) that occurs in only a few regions of the world from where
it was and still is exported in the form of massive rocks.
Ultramarine
is the
name of the dark blue pigment obtained when lapis lazuli is crushed and
powdered. Lapis lazuli, a rare rock of gemstone quality, is a mixture of min-
erals that include
calcite
,
muscovite
, and
lazurite
(composed of sodium calcium
aluminum silicate sulfate). The source of most lapis lazuli used in the ancient
world seems to have been mines in the Badakhshan region of Afghanistan,
from where it has been continuously exported for over six millennia (von
Rosen 1990).
Glaucophane
, a blue mineral (composed of sodium magnesium
aluminum hydrosilicate), was used by the Greeks as a blue pigment as early
as the seventeenth century B.C.E.
Egyptian blue
is an artificial blue pigment that was made in the ancient
Middle East, particularly in Egypt, where it is still made and used. Egypt-
ian blue should not be confused with another, also blue or greenish-blue,
Egyptian faience
, which is made from much the same raw materials (see
Chapter 7).
Egyptian blue
is made by first preparing a mixture of sand, natron,
and copper filings and then heating the mixture to about 850°C. The raw
materials melt at this temperature, reacting with each other to form a defi-
nite chemical compound (composed of copper calcium tetrasilicate). As the
melt cools, it does not crystallize but solidifies into a
frit
(see Textbox 20),
which is then powdered (Tite 1986).
Another artificial pigment, a particular light blue, is
Maya blue
, which
was made by the Maya Indians in pre-Columbian Mexico. Maya blue is not
a frit, however, as is Egyptian blue, but probably a lake. Although its precise
components are still unknown, it may have been prepared by thoroughly
mixing a clay (such as attapulgite or palygorskite) with blue indigo dye and
then heat-treating the mixture before use (Reyes-Valerio 1993).
1.9.
ABRASIVES
Cutting, grinding, and shaping stone, and in particular burnishing and
polishing the surface of stone as well as metals, requires the use of
abrasive
materials that are harder than the solids to be cut, ground, burnished, or pol-
ished. Sapphire and ruby, two very hard gemstones, for example, can be cut
or polished only with the assistance of diamond powder, an abrasive
that is harder than sapphire or ruby. Diamond is the hardest material
