Gold
This most famous of precious metals has been highly sought after and valued
throughout human history not only because of its rarity and beauty, but also because
of its usefullness and that it is often found highly pure in content.
Gold is very malleable (easy to work with) and durable because it does not
corrode or tarnish. Its malleability can be realised if we think of how just one
ounce od gold can be beaten into a flat sheet of gold leaf measuring 17 square
metres.
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As for its durability, we only have to consider all the historical relics
such as ancient Egyptian artefacts which have survived intact for thousands of
years. Considering this then, one begins to comprehend just how ancient and historically
important the craft of goldsmithing is.
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Pure gold is very soft and therefore liable to damage. For this reason, most gold
jewellery is made of gold alloys. The content of gold alloys consists of gold
and other metals. The gold content of jewellery is measured in carats (expressed
fractionally as 24ths). For example, 24 carat jewellery is pure gold, while 9
carat gold has a gold content of 9/24ths, or just over one third.
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Silver
Though not as highly valued as gold, this soft and lustrous white metal has a
proud history dating back thousands of years. Ornamental and decorative siverware
has been unearthed in royal tombs dating back to 4000 BC.
Although much harder than gold, silver can be easily worked and beaten into
silver leaf for decorative use. And just like gold it is long lasting and hard-wearing.
Silver has been used in jewellery and coinage throughout the ages in the form
of a silver alloy. The fineness of silver items is measured as a proportion of
the silver content. For instance, Sterling silver has a fineness of 925, which
is to say that the silver content amounts to 92.5% of the piece. The other metal
content of silver alloys used in jewellery is usually copper. Most silver jewellery
has a fineness rating of 800, so there is 80% silver content and 20% copper in
most silver jewellery.
High silver content is also evident in white gold. The gold content of white
gold usually amounts to just over half, with silver amounting to one quarter of
the content.
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Platinum
Although artefacts containing platinum have been found dating back to the times
of the ancient Egyptians, it is thought that they were unaware of this precious
metal being present in the mainly gold pieces it has been found in. The first
discoveries which identify Platinum as a distinct precious metal occurred in 16th
century Columbia. Indeed, the name Platinum dates from this time, when Spanish
invaders noted its resemblance to silver ; the word platinum coming from the Spanish,
'platina del Pinta'.
The demand for platinum in jewellery has greatly increased
in recent times, with particularly high demand in Japan.
Most of the world's platinum comes from the Transvaal
area of South Africa and Russia.
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Bronze
TThe history of mankind's use of bronze stratches way back to 3000 BC. Bronze
is often an alloy made up of the metals copper and tin, although in recent times
bronze has also been made using aluminium, manganese or zinc instead of tin.
Being very hard and durable yet easy to cast, bronze has been widely used
throughout history for coinage, tools and weaponry. Iron hjas taken the place
of bronze in many uses over the centuries because it is more widely available,
reather than it being significantly better or more suited to its uses.
Assaying
The content and value of precious metals is determined by processes of chemical
analysis called assaying. The history and formulation of the processes involved
is rooted in the work of ancient alchemists and goldsmiths who would subject base
metals to heat.
Assaying by fire is still the most economical methos of assaying precious
metals, even more so than modern methods such as spectrographic analysis because
of the amounts of the metals needed to carry out the tests.
The fire assaying method consists of 6 steps :
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Sampling. A representative sample is taken.
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Fusion. The sample is then melted along with flexes
and other agents to collect elements of precious metals in droplets of lead. These
are then cooled to produce a 'lead button'.
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Coppelation. The 'lead button' is then melted and the
impurities are oxidised. Beads of precious metals then form.
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Weighing. Beads of gold and silver are then weighed
to determine their gold and silver content. Platinum is present in quantities
to small to weigh however.
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Parting. Each bead is then trated to dissolve out the
silver content.
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Weighing. Finally, the remianing gold content is weighed
andthis mesurement is subtracted from the gold-silver bead weight previously measured.
This calculation then gives us the weight of the silver content.
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