Chemistry Reference
In-Depth Information
Zinc dioxide is a friable material that could then readily be reduced to
metallic zinc by carbon, the main component of the fuel:
2ZnO
+
C
→
2Zn
+
CO
↑
2
zinc metal
Zinc melts at a fairly low temperature, 419°C, and boils at 918°C, below
the temperature at which zinc oxide is reduced to zinc. If zinc had been
smelted in the way described, the metal would have distilled when formed.
To prevent its reoxidation when in the gaseous condition, the atmosphere
of the furnace would have had to be kept free of oxygen so that the metal
could cool and condense. The earliest available information on zinc distil-
lation, however, is from as late as the twelfth century
C
.
E
. in India; the sug-
gested explanation advanced here does not, therefore, provide an answer
as to how zinc was smelt before that time.
Ancient objects made from zinc are extremely rare, and it seems that
practically no zinc was produced and used as a metal in antiquity. One of
the few ancient objects that are known to be made of zinc is a roughly rec-
tangular fragment of corroded zinc sheet found at the Agora of Athens
(Farnsworth et al. 1949). Even before it was produced and recognized as a
metal, however, zinc was being used for making alloys; it has also been
claimed that mixed zinc-copper ores containing sufficient zinc might have
been used to produce brass on smelting, although it is doubtful whether the
process could have been accomplished in antiquity. When mixed with
copper, zinc forms two alloys known in antiquity:
brass
, which usually con-
tains 15-40% zinc, well known in Europe (see pg. 170), and
paktong
, which
contains 33-46% zinc, in China. Paktong, a silver-colored alloy, also contains
a small proportion of nickel (Gilmour and Worrall 1995; Needham 1980).
Mercury
Mercury (chemical symbol Hg, from the Latin name of the metal,
hydrar-
gyrium
, liquid silver), previously also known as
quicksilver
is, at ordinary
temperatures, a silvery white liquid metal that boils at 360°C. The metal is
occasionally found in nature in the native state. Most mercury has been
derived, however, from the red mineral
cinnabar
(composed of mercuric
sulfide) that was also used in the past as a red pigment known as
vermilion
(see Textbox 41). The Greek philosopher Aristotle, writing in the fourth
