to temperatures in excess of 200°C. This is called annealing and if the shaping
of metals was performed at these elevated temperatures, it was referred to as hot
working. For steel on heating to red heat, it becomes soft enough for large
changes of shape to be made with relatively small forces, hence the blacksmiths
craft. The temperature of hot working is peculiar to each metal, with tempera-
tures in excess of 550°C being required for copper, while the annealing tem-
perature for lead is room temperature. This is why it is impossible to harden pure
lead by cold work.
Metals and alloys that were shaped by mechanical means were called
wrought alloys, with wrought iron and wrought brass being good examples of
this class of materials.
For some alloys or the early impure single metals, it was virtually impossible
to shape them by either cold or hot working. Provided the metal or alloy could
be kept molten, it was possible to pour the molten metal into specially shaped
moulds where it would eventually solidify into a similar shape to that of the
mould. This is called casting and relies on the ability of a molten metal to take
up the shape of vessel (mould) containing it. The moulds were made from clay
or sand and were destroyed after each pouring. Casting can fairly easily produce
complicated shapes and it would be very difficult if not impossible to manu-
facture the same item by mechanical means. This is the reason why large
wrought iron split-ring guns were superseded by cast iron ones at the beginning
of the sixteenth century!
If a polished piece of metal is placed in a suitable chemical (etchant), grains
will be visible if viewed under an optical microscope. The size of these will vary
between a few microns (m) up to 3–4 mm in diameter. If an alloying element
is added to the parent metal and can fit into the original crystal structure, the two
metals are said to be soluble in one another (similar to milk in tea). Under the
microscope, the grains will appear the same as the pure metal. These alloys are
often referred to as single-phase alloys. The addition of alloying elements will
increase the hardness without sacrificing too much loss in ductility, provided
the alloying additions are able to dissolve in the crystal structure of the parent
metal. Arsenic (4%) dissolved in copper (often referred to as arsenical copper
alloy) had a hardness approximately double that obtained from pure copper.
Pure silver was too soft for coinage so copper was added to increase the hard-
ness and wear resistance. A typical analysis of coinage silver, which did not vary
for many centuries, was 6.19% copper, 0.8% lead and 0.3% gold together with
other trace impurities such as arsenic and antimony. Brass is an alloy of copper
and zinc in which the zinc is completely soluble in the copper lattice up to
30 wt%. The advantages of adding zinc to copper was that the tensile strength
and hardness of the resultant alloy was superior to that of pure copper. Depending
upon the impurities present in the alloy, the ductility was also improved. As
already stated, the term wrought brass was often applied to this type of alloy.
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