Chemistry Reference
In-Depth Information
Arsenical and Antimonial Bronze.
The addition of
arsenic
or of
antimony
to copper produces hard alloys, which hold a cutting edge better and are less
likely to bend in use than copper. Arsenic often occurs as an impurity in
copper ores, and small amounts of arsenic in early smelted copper were
probably not intentionally added or particularly noticeable in the final
product. When the proportion of arsenic in the alloy reaches over 2%,
however, the effects are quite noticeable, and ancient copper arsenic alloys,
which are generally known as
arsenical copper
or
arsenical bronze
, were pre-
sumably intentionally made. In western Asia and in Europe, for example,
copper arsenic alloys were regularly produced between approximately the
middle of the fourth and the second millenniums B.C.E. (Lechtman 1991). The
results of many analyses of ancient copper alloys seem to suggest that tin
gradually replaced arsenic in copper alloys. On the reasons for this gradual
change opinions vary. One suggested view is that the gradual cessation of
the use of arsenic lies in the extreme
toxicity
of all arsenic compounds, par-
ticularly the arsenic oxides, that are readily formed when arsenic-containing
minerals are processed. This view is challenged by another asserting that
metal workers at the time learned to control the dangers associated with
arsenical compounds (see text below).
Copper-Iron Alloys.
Iron may enter copper during smelting operations,
and many samples of ancient copper also contain varying amounts of
iron. A study of a large number of samples of ancient copper and
copper-iron alloys of different origin revealed that changes in the
relative amount of iron may be indicative of changes in the smelting tech-
nology of the copper. Many early samples of copper, which were apparently
made using rudimentary smelting technologies, contain relatively small
amounts of iron. Others, made at later times and apparently using
more advanced technologies, contain relatively higher amounts of iron,
some as much as 30-40%. The iron was probably introduced deliberately to
render the resulting alloys suitable for
minting
coins (Craddock and Meeks
1987).
Shakudo.
Shakudo
is a copper alloy used in Japan since the third century
B.C.E. for making decorative and ornamental objects. In addition to copper
it also includes gold (1-4%), silver (about 2%), and lead (about 1%). Occa-
sionally gold replaced part or even all of the silver. Exposed shakudo sur-
faces acquire a layer of patina consisting mainly of the mineral
cuprite
(composed of cuprous oxide), which exhibits a characteristic shine. Varying
amounts of gold in the alloy are said to have a marked effect on the color
and the shine (Notis 1988; Oguchi 1983).
