By Duncan Parker
As I refine and polish this column, I reflect on the metals that have been used in the production of jewels throughout the very long history of precious things. The other day, I was reading Pliny the Elder (as one does), whose writing included a description of ‘touchstones’ used for gold testing during his time. Around 2000 years ago—just before Pliny died in the eruption of Mount Vesuvius that also buried Pompeii—there was no standard of assay or alloying. Even in those days, however, the Romans had figured out rubbing a piece of gold on a touchstone could help determine whether something had higher or lower gold content.
We’ve come a long way.
Most jewellery (though not all) is composed of precious metal. The metals we use seem simple, but there is a complex set of mixtures and compounds. The big three—gold, silver, and platinum—are found in more combinations than any, and are referred to as the ‘noble metals.’
Why are they noble? While they were traditionally owned only by the wealthiest individuals in a community (or the nobles, as they may have been known), that isn’t the real reason for the name. Metals are called ‘noble’ if they don’t break down under normal conditions and aren’t easily damaged by solvents or acids. (Mind you, I’ve still heard of a lot of broken-down nobles…but never mind that.)
Mixing it up
Gold, as mined, is generally not pure. It is naturally an alloy with other metals, such as silver and platinum. In fact, the pure and unalloyed forms of metals are rarely used in jewellery. Silver and gold are too soft for most applications, and platinum can be difficult to work in its pure form.
These days, we have established very precise methods of mixing metals and producing consistent alloys. Around the world and over the years, however, we’ve seen dozens of variations. In the 19th century, for example, Great Britain used 15- and 22-karat versions of its standard alloys. Today, these have changed to nine- and 18-karat, largely due to lower costs (thanks to the reduced gold content) and the need for harder-wearing alloys (22-karat was quite soft).
Canada’s Royal Canadian Mint, on the other hand, is world famous for its ability to produce highly pure 0.9999 gold. Often described as ‘four nines,’ this is an alloy containing 99.99 per cent gold. Gold of this purity is used for the production of high-grade bullion coins.
There are also lots of methods of stating the purity of the many metal alloys we make. In North America, for at least the last century, gold has been described by its ‘karat.’ Pure, unalloyed gold is 24-karat.