Chemistry Reference
In-Depth Information
then easily be separated from the sand. Heating the amalgam, generally in
pottery vessels, resulted in the evaporation of mercury while rather pure gold
was left behind in the vessels. Such a process was described in ancient texts.
Vitruvius (first century B.C.E.) and Pliny the Elder (first century C.E.), for
example, describe how gold in auriferous sand was dissolved in mercury and
the amalgam formed was periodically removed while more mercury was
added to the sand. The mercury was then removed from the amalgam by
heating, and pure gold globules obtained. The evaporated mercury was prob-
ably recovered by condensation on ceramic pipes. Gold amalgam was also
used to gild objects made from lesser metals and alloys, such as copper,
bronze, and brass; see above, discussion on fire gilding.
Platinum
Platinum
(chemical symbol Pt, from the Spanish word
plata
, silver) is a very
rare silver-colored metal; together with gold, platinum belongs to the group
of metals known as the noble or precious metals. Platinum melts at a very
high temperature, 1774°C; since the maximum temperatures which could be
reached in ancient furnaces was lower than that, platinum could have been
neither smelted nor melted in antiquity. It is even doubtful whether plat-
inum was recognized as a distinct metal in the Old World. Still, two out-
standing platinum finds were found in the Old World: (1) a casket found at
Thebes, in Upper Egypt, dating from the seventh century B.C.E., which was
shown to have been made of an alloy of platinum; and (2) a box made of
copper inlaid with silver and platinum decorations (Berthelot 1901). In South
America, Indians from Ecuador and Colombia made small artifacts from
(probably natural) gold-platinum alloys before the discovery of America by
the Europeans (Scott and Bray 1992; Bergsoe 1937).
5.9.
DETERIORATION OF METALS AND ALLOYS - CORROSION
The environment has negative effects on most metals; thus, when metallic
archaeological objects are eventually found, they are generally in an
advanced state of decay. The decay of metals and alloys caused by the chem-
ical action of gases and/or liquids in the environment is known as
corrosion
.
Corrosion
processes are natural destructive processes that result in the waste
of most metals and alloys. The ultimate result of all corrosion processes is
the reversion of most metals from the metallic condition in which they are
used, to the chemically combined form in which they naturally occur in the
crust of the earth.
Rust
, the reddish-brown corrosion product that forms on
