Brass is an alloy of copper and zinc and although Roman coins have been
found manufactured from this alloy (71% Cu, 28% Zn), it was not until the
fourteenth century that reasonable quantities of this alloy could be made.
Iron and its alloys (ferrous metals) were slow to develop because of the dif-
ficulty in winning the metal from its ores. Most of the early metals and alloys
had relatively low-melting points (e.g. lead 327°C), while iron has a melting
point of 1535°C. This temperature was impossible to achieve until the nine-
teenth century. Hence, only solid masses (blooms) were produced that were
subsequently hammered into shape – forming wrought iron. By the sixteenth
century, it was observed that the formation of an iron/carbon alloy (cast iron)
lowered the melting point to approximately 1200°C. This allowed the pro-
duction of cast iron guns, which were considerably cheaper than their bronze
counterparts. Cast iron was very brittle and cracked fairly easily if subjected to
impact loads. This was due to the high carbon content (4% carbon) in the
alloy, which was predominately in the form of graphite flakes. It was not until
the middle of the nineteenth century that large quantities of iron with less than
1.7% carbon could be melted to produce an alloy called steel. The result of this
advancement in technology signified the beginning of the industrial revolution
and the commencement of the wide-scale use of metals and alloys.
The minerals from which the metals are extracted, existed for millions of
years in the earth’s crust and are the most stable form of the metal. A consider-
able amount of energy is required to convert this mineral into the metal. Once this
pure metal comes into contact with the natural environment such as sea-water or
soils, the metal slowly converts back to its original starting material. Iron, for
example, is obtained from the mineral, haematite, an oxide of iron. Once the
pure iron comes into contact with water and air (oxygen), it slowly converts
back to the oxide. This is called corrosion and the product is familiar to everyone
as red rust. Nearly all metals will corrode in natural environments although the
rates of corrosion will vary from metal to metal and alloy to alloy. In addition,
the rates of corrosion will vary from one natural environment to another. Iron
will corrode at approximately 50m per year in freshwater but at 120m per
year in seawater. The reason for this is due to the difference in chemical compo-
sition between freshwater and seawater. The latter contains salt (sodium chloride)
and this is very deleterious to the corrosion behaviour of the metal. Silver arte-
facts may be excavated after several hundred years buried in soils with only
minimal amounts of corrosion. Those recovered from marine sites after a
similar period of burial, have completely corroded and have reverted back to
100% mineral. This is entirely due to the presence of chlorides in seawater.
For metals to corrode, it is essential for water and oxygen to be present.
Removal of either of these will arrest the corrosion process. This helps to
explain why some artefacts can be recovered after hundreds of years under-
ground or on the seabed and still have a considerable amount of un-corroded
122 Chapter 6