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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
5 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
10 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
15 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.
20 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
25 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.
30 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
35 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
40 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
45 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
50 studies may thus prove useful for interpreting the archaeological
record of the Bronze Age.
(473 words)
4. The author first mentions the addition of flux during
smelting in the last sentence in the last paragraph in
order to
(A) give a reason for the failure of elemental
composition studies to determine ore sources
(B) illustrate differences between various Bronze
Age civilizations
(C) show the need for using high smelting
temperatures
(D) illustrate the uniformity of lead isotope
composition
(E) explain the success of copper isotope
composition analysis
For the following question, consider each of the choices separately and select all that apply
5. According to the passage, possible sources of the
lead found in a copper or bronze artifact include
which of the following?
□A The copper ore used to manufacture the artifact
□B Flux added during processing of the copper ore
□C Other metal added during processing of the
copper ore
6. Select the sentence in the passage that the author
rejects copper as the "ideal choice".
7. It can be inferred from the passage that the use
of flux in processing copper ore can alter the
lead isotope composition of the resulting metal
EXCEPT when
(A) there is a smaller concentration of lead in
the flux than in the copper ore
(B) the concentration of lead in the flux is
equivalent to that of the lead in the ore
(C) some of the lead in the flux evaporates
during processing
(D) any lead in the flux has the same isotopic
composition as the lead in the ore
(E) other metals are added during processing |
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发表于 2011-10-31 08:15:33
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