Electrum is a naturally occurring alloy of gold and silver, with trace amounts of copper and other metals. It has also been produced artificially, and is often known as green gold. The ancient Greeks called it 'gold' or 'white gold', as opposed to 'refined gold'. Its colour ranges from pale to bright yellow, depending on the proportions of gold and silver.
The gold content of naturally occurring electrum in modern Western Anatolia ranges from 70% to 90%, in contrast to the 45–55% of gold in electrum used in ancient Lydian coinage of the same geographical area. This suggests that one reason for the invention of coinage in that area was to increase the profits from seigniorage by issuing currency with a lower gold content than the commonly circulating metal.
Electrum was used as early as the third millennium BCE in Old Kingdom of Egypt, sometimes as an exterior coating to the pyramidions atop ancient Egyptian pyramids and obelisks. It was also used in the making of ancient drinking vessels. The first metal coins ever made were of electrum and date back to the end of the 7th century or the beginning of the 6th century BCE. For several decades, the medals awarded with the Nobel Prize have been made of gold-plated green gold.
The name "electrum" is the Latinized form of the Greek word ἤλεκτρον (èlektron), mentioned in the Odyssey referring to a metallic substance consisting of gold alloyed with silver. The same word was also used for the substance amber, likely because of the pale yellow colour of certain varieties, and it is from amber's electrostatic properties that the modern English words "electron" and "electricity" are derived. Electrum was often referred to as "white gold" in ancient times, but could be more accurately described as "pale gold", as it is usually pale yellow or yellowish-white in colour. The modern use of the term white gold usually concerns gold alloyed with any one or a combination of nickel, silver, platinum and palladium to produce a silver-coloured gold.
Electrum consists primarily of gold and silver but is sometimes found with traces of platinum, copper, and other metals. The name is mostly applied informally to compositions between about 20–80% gold and 20–80% silver atoms, but these are strictly called gold or silver depending on the dominant element. Analysis of the composition of electrum in ancient Greek coinage dating from about 600 BCE shows that the gold content was about 55.5% in the coinage issued by Phocaea. In the early classical period, the gold content of electrum ranged from 46% in Phokaia to 43% in Mytilene. In later coinage from these areas, dating to 326 BCE, the gold content averaged 40% to 41%. In the Hellenistic period, electrum coins with a regularly decreasing proportion of gold were issued by the Carthaginians. In the later Eastern Roman Empire controlled from Constantinople, the purity of the gold coinage was reduced, and an alloy that can be called electrum began to be used.
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Electrum was much better for coinage than gold, mostly because it was harder and more durable, but also because techniques for refining gold were not widespread at the time. The discrepancy between gold content of electrum from modern Western Anatolia (70–90%) and ancient Lydian coinage (45–55%) suggests that the Lydians had already solved the refining technology for silver and were adding refined silver to the local native electrum some decades before introducing the pure silver coins cited below.
In Lydia, electrum was minted into 4.7-gram coins, each valued at 1/3 stater (meaning "standard"). Three of these coins (with a weight of about 14.1 grams, almost half an ounce) totaled one stater, about one month's pay for a soldier. To complement the stater, fractions were made: the trite (third), the hekte (sixth), and so forth, including 1/24 of a stater, and even down to 1/48 and 1/96 of a stater. The 1/96 stater was only about 0.14 to 0.15 grams. Larger denominations, such as a one stater coin, were minted as well.
Because of variation in the composition of electrum, it was difficult to determine the exact worth of each coin. Widespread trading was hampered by this problem, as cautious foreign merchants offered poor rates on local electrum coin..
These difficulties were eliminated in 570 BCE when pure silver coins were introduced. However, electrum currency remained common until approximately 350 BCE. The simplest reason for this was that, because of the gold content, one 14.1 gram stater was worth as much as ten 14.1 gram silver pieces.
- Tumbaga – a similar material, originating in Pre-Columbian America; typically with a higher copper content and not necessarily containing significant amounts of silver;
- Shakudō – a Japanese billon of gold and copper with a dark blue-purple patina;
- Shibuichi – another Japanese alloy known for its patina;
- Orichalcum – another distinct metal or alloy mentioned in texts from classical antiquity, later used to refer to brass;
- Corinthian bronze – a highly prized alloy in antiquity, which may have contained electrum;
- Thokcha – an alloy of meteoric iron or "thunderbolt iron" commonly used in Tibet;
- List of alloys
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