The determination of the sources of copper ore used in
the manufacture of copper and bronze artifacts of Bronze
Age civilizations would add greatly to our knowledge of
cultural contacts and trade in that era. Researchers have
analyzed artifacts and ores for their concentrations of elements,
but for a variety of reasons, these studies have generally failed
to provide evidence of the sources of the copper used in the
objects. Elemental composition can vary within the same
copper-ore lode, usually because of varying admixtures of
other elements, especially iron, lead, zinc, and arsenic. And
high concentrations of cobalt or zinc, noticed in some artifacts,
appear in a variety of copper-ore sources. Moreover, the
processing of ores introduced poorly controlled changes
in the concentrations of minor and trace elements in the
resulting metal. Some elements evaporate during smelting
and roasting; different temperatures and processes produce
different degrees of loss. Finally, flux, which is sometimes
added during smelting to remove waste material from the
ore, could add quantities of elements to the final product.
An elemental property that is unchanged through these
chemical processes is the isotopic composition of each
metallic element in the ore. Isotopic composition, the
percentages of the different isotopes of an element in a given
sample of the element, is therefore particularly suitable as an
indicator of the sources of the ore. Of course, for this purpose
it is necessary to find an element whose isotopic composition
is more or less constant throughout a given ore body, but
varies from one copper ore body to another or, at least, from
one geographic region to another.
The ideal choice, when isotopic composition is used to
investigate the source of copper ore, would seem to be copper
itself. It has been shown that small but measurable variations
occur naturally in the isotopic composition of copper.
However, the variations are large enough only in rare
ores; between samples of the common ore minerals of copper,
isotopic variations greater than the measurement error have
not been found. An alternative choice is lead, which occurs
in most copper and bronze artifacts of the Bronze Age in
amounts consistent with the lead being derived from the
copper ores and possibly from the fluxes. The isotopic
composition of lead often varies from one source of
common copper ore to another, with variations exceeding
the measurement error; and preliminary studies indicate
virtually uniform isotopic composition of the lead from a
single copper-ore source. While some of the lead found in
an artifact may have been introduced from flux or when
other metals were added to the copper ore, lead so added in
Bronze Age processing would usually have the same isotopic
composition as the lead in the copper ore. Lead isotope
studies may thus prove useful for interpreting the
archaeological record of the Bronze Age.