and then tempering at 200–300°C. This produces the ideal mechanical prop-
erties for cutting instruments such as knives, blades, scissors, razors, etc.,
which require sharp cutting edges and do not blunt easily.
Some of the non-ferrous metals and alloys have already been mentioned such
as copper and brass. Archaeological bronze is an alloy of copper and tin. The
composition of these alloys ranges typically from 3% to 14% tin together with
trace impurities such as lead and iron, depending on the chemical content of the
original ores. Alloys with a tin content, up to 6%, were capable of being cast and
subsequently hammered into their final shape. This is due to the tin being sol-
uble in the copper crystal structure, which allows the alloy to be deformed at
room temperature.
With higher levels of tin, the melting point of the alloy is lowered, with a
corresponding increase in fluidity, thus making these alloys ideal for shaping by
casting. This is the method used for producing the range of bronze cannons
recovered from the Mary Rose. Adding zinc improved the fluidity of the alloy
still further and allowed less tin to be used. This was particularly important if
there was a shortage of tin or it became too expensive. These ternary alloys were
the precursor to modern Admiralty Gun-Metal, which is a 88/10/2 alloy of
copper/tin/zinc.
Lead was one of the first metals to be smelted and used by man. The low
melting point (327°C), softness, malleability and ductility indicated the ease
with which it could be cast and formed. As the annealing temperature for lead is
room temperature, it is impossible to harden pure lead by cold work. Alloying is
the only way to increase the hardness of lead. Lead was used for weights
(density 11.35 g cm^3 ), sheeting, piping, net sinkers, etc.
The major alloy of tin recovered from archaeological sites is pewter. This can
be divided into those containing lead and lead-free alloys. The former could
have a lead content ranging from 67% (equivalent to plumbers solder) down to
15%. The French in Elizabethan times kept the lead of their wine goblets to
below 18% as above this, the wine would become tainted! As the lead and tin are
insoluble in one another, they are classed as a two-phase alloy and articles
could only be manufactured by casting. The lead-free pewter was invariably an
alloy of tin with a small amount of copper (0.5–7% for pewter recovered from
the Mary Rose). The copper dissolved in the tin crystal structure resulted in a
single-phase structure, which was considerably harder than pure tin. Hence this
class of pewter could be subjected to a limited amount of mechanical working
to achieve the final shape.
2 Corrosion
The aqueous corrosion of metals is due to an electrochemical cell being formed
between two different metals in electrical contact (galvanic corrosion) or two
Metals 131