Page 217 - Materials Chemistry, Second Edition

204 3 Metals

3.2.4. Nonferrous Metals and Alloys
Although we have focused on iron for the majority of this chapter, many of the
materials we use on a daily basis comprise other metals. Applications that are
particularly suitable for other metals include those that require more lightweight,
highly conductive, and/or corrosion-resistant materials (often at a lower cost),
relative to iron-based materials. Since metals comprise only one portion of this
textbook, we do not intend on discussing all of the other metal classes in as much
detail as the iron system. We may now expand our discussion a bit since the general
structures and mechanisms involved for alloying, surface/bulk hardening, annealing,
etc. also apply for other metals. For more information about the structure and
processing of other metal classes that are not discussed herein, refer to the Further
Reading section at the end of this chapter.

The Coinage metals
Let us begin our survey of other nonferrous metals with “show me the money!” The
coinage metals consisting of copper, silver, and gold represent the first metals known
to man. The first reports for copper purification date back to 3,500 B.C. in the Middle
East; bronze alloys were first introduced in ca. 3,000 B.C. in India and Greece.
However, it is likely that earliest use for copper may have been much earlier for
weaponry applications. As a testament to the durability of the coinage metals,
5,000 years after an Egyptian Pharaoh had copper pipes installed in his bath, those
same pipes were discovered, dug up, and were still in sufficient shape to carry water!
The first application for gold currency dates back to around 3,400 B.C. in Egypt;
however, gold was probably employed for decorative applications much earlier –
before 9,000 B.C. Although gold comprises an insignificant 0.004 ppm of the Earth’s
crust, its early widespread use was a consequence of its availability as the uncomplexed
element. Hence, simple techniques such as panning along riverbeds were necessary to
isolate the gold, requiring no previously developed knowledge of refining. Discovering
any of the three coinage metals was as simple as noticing colors in rocks!
Needless to say, the surface deposits of the coinage metals have long been
expired, and the metals must now be isolated in small quantities from sulfide-
based ores. As early as 3,000 B.C., a process known as cupellation was used to
isolate the precious metals from their ores – a method still in use today. In this
technique, lead was added to the ore and heated to a temperature of ca. 800 C in air.

Any gold or silver in the sample was dissolved in the liquid metal, separating it from
the undissolved matter. The insoluble material primarily containing iron, copper,
and zinc compounds was discarded, and the remaining precious metals were brought
to a higher temperature by blowing the fire with bellows. This raised the temperature
to the point where lead oxide formed rapidly. This was usually performed in a hearth
comprising clay or crushed bones; the PbO would be absorbed into the hearth, while
the precious metal deposited at the surface.
Another early process known as amalgamation was used by the Romans in the
Middle Ages. This simple procedure consisted of combining a precious metal ore in