CHAPTER 6
Metals
DESMOND BARKER
Consultant, Formerly Department of Applied Chemistry,
University of Portsmouth, UK
1 INTRODUCTION
Some of the earliest known metallic artefacts probably originated from nuggets
of pure metals such as copper or meteoric iron. A dagger dating from 3000 BC
is a good example of the latter, which gave a chemical analysis of 90% iron
and 10% nickel. Metals are rarely found in their native form and exist in the
earth’s crust as minerals predominately as sulfides or oxides. Man soon dis-
covered that it was possible to produce molten metal in containers (crucibles) by
making their fires hot enough. This molten metal could then be poured into a
shaped cavity made by placing together two halves of hollowed out clay or
stone mould. The metal took the shape of this cavity when solid. This process is
called “casting”. A bell recovered from 1000 BC is an early example of an
artefact made in this manner. It was a mixture of copper and 13% tin that is
known as bronze. Countless examples of this alloy have been excavated,
although the tin content does vary considerably. Early metallurgists soon
found that by changing the alloy content, the mechanical properties could be
altered. Lowering the tin content, the bronze became softer, while increasing it
the alloy became harder.
By the time that the Romans came to Britain, iron and bronze were being
usedfor weapons and tools; copper for vessels and ornaments; lead for pipes
and coffins; silver and bronze for coinage; and gold, silver and tin for orna-
ments. A considerable number of Roman drinking vessels have been found
made of pewter, which is an alloy of lead and tin. Even the Romans must have
known something about lead poisoning as their superior quality pewter had a
lower level of lead in the alloy; 20% instead of the normal 50%.