Over the centuries, gold has played a unique cultural role in societies across the world. Something about the metal gives it a transcendant symbolic value: from the legendary treasuries of the Aztec and Mayan empires to the Golden Calf of the Book of Exodus, gold is implicated in the most dramatic tales of ancient human history. Even today, gold retains its cultural signficance. For an Olympic athlete, a single gold medal represents the zenith of achievement. The Palme dOr, or Golden Palm, is a symbol of supreme attainment in the film industry. Only a handful of British monarchs have managed to stick it out for half a century and earn their Golden Jubilee. And clearly there is a reason why credit card companies designate their high-end offerings as “Gold Cards”. But what is it about real gold that captures our attention? What gives this yellow metal such value?
Like anything of real value, gold is rare. Only 3 particles per billion in the Earths crust are gold. Although the worlds oceans hold vast amounts of gold, it is so dilute that attempts to recover it from the salt water have never been successful. If all the gold ever refined were formed into a ball, it would be less than 25 metres across. However, some gold does exist naturally in a relatively pure state (as opposed to in an ore), enabling humans to gather it in small quantities without the need for chemical extraction. And once harvested, gold is one of the most versatile metals on earth. It has been crafted and shaped ever since the ‘caltholithic’, or ‘copper’ age, as early as 4000 BC, making it one of the very first metals to be used by humans. The ancient peoples of Egypt and the Middle East used gold for religious rites, as well as for ornamentation. The first recorded use of gold coins was during the reign of King Croesus of Lydia around 600 BC. Gold is also more malleable and ductile than any other metal. A single gram of gold can be beaten into a sheet one metre square. It is one of the heaviest elements; heavier, even, than lead. Ten teaspoons of gold would weigh as much as seventeen teaspoons of lead.
It has been customary throughout history to test the purity of gold by biting on it. Since pure gold is soft enough to show teeth-marks, and cheaper alloying metals are usually harder, this method does give some indication. More reliable tests, however, involve the use of nitric acid. The content of gold in alloys is measured in karats (k). Pure gold is 24 karat, with the number of karats decreasing proportionally as other metal is added.
Although too soft for ordinary use in its pure state, gold can be extremely hard and durable when alloyed with other metals. Alloying also has other advantages besides increasing its hardness. The distinctive yellow colour of gold is recognised throughout the world. However, a fascinating spectrum of other colours can be created by combining small quantities of other metals with the gold. Alloyed with copper, it has a rosy colour, with nickel, silver, or palladium it is white, with iron, blue, and with aluminium, a purple hue. Pure silver gives the alloy a greenish tint, and rhodium or ruthenium produce black. Gold in such forms is most commonly used in jewellery. Colloidal gold is micro-particles of gold suspended in water, producing an intense red colour. Colloidal gold is used in stained glass, and in certain applications for medical and scientific research.