Orichalcum or aurichalcum // is a metal mentioned in several ancient writings, including the story of Atlantis in the Critias of Plato. Within the dialogue, Critias (460–403 BC) claims that orichalcum had been considered second only to gold in value and had been found and mined in many parts of Atlantis in ancient times, but that by Critias's own time orichalcum was known only by name.
The Romans transliterated "orichalcum" as "aurichalcum", which was thought to literally mean "gold copper". It is known from the writings of Cicero that the metal which they called orichalcum resembled gold in color but had a much lower value. In Virgil's Aeneid, the breastplate of Turnus is described as "stiff with gold and white orichalc".
In later years, "orichalcum" was used to describe the sulfide mineral chalcopyrite and also to describe brass. However, these usages are difficult to reconcile with the claims of Plato's Critias, who states that the metal was "only a name" by his time, while brass and chalcopyrite were very important in the time of Plato, as they still are today.
Joseph Needham notes that Bishop Richard Watson, an eighteenth-century professor of chemistry, wrote of an ancient idea that there were "two sorts of brass or orichalcum". Needham also suggests that the Greeks may not have known how orichalcum was made, and that they might even have had an imitation of the original.
In 2015, 39 ingots believed to be orichalcum were discovered in a sunken vessel on the coasts of Gela in Sicily which have tentatively been dated at 2,600 years old. They were analyzed with X-ray fluorescence by Dario Panetta of Technologies for Quality and turned out to be an alloy consisting of 75–80% copper, 15–20% zinc, and smaller percentages of nickel, lead, and iron.
According to the Critias of Plato, the three outer walls of the Temple to Poseidon and Cleito on Atlantis were clad respectively with brass, tin, and the third outer wall, which encompassed the whole citadel, "flashed with the red light of orichalcum". The interior walls, pillars and floors of the temple were completely covered in orichalcum, and the roof was variegated with gold, silver, and orichalcum. In the center of the temple stood a pillar of orichalcum, on which the laws of Poseidon and records of the first son princes of Poseidon were inscribed. (Crit. 116–119)
Pliny the Elder points out that orichalcum had lost currency due to the mines being exhausted. Pseudo-Aristotle in De mirabilibus auscultationibus (62) describes a type of copper that is "very shiny and white, not because there is tin mixed with it, but because some earth is combined and molten with it." This might be a reference to orichalcum obtained during the smelting of copper with the addition of "cadmia", a kind of earth formerly found on the shores of the Black Sea, which is attributed to be zinc oxide.
In numismatics, the term "orichalcum" is used to refer to the golden-colored bronze alloy used for the sestertius and dupondius coins. It is considered more valuable than copper, of which the as coin was made.
Whether, if a person should offer a piece of gold to sale, thinking that he was only disposing of a piece of orichalcum, an honest man ought to inform him that it was really gold, or might fairly buy for a penny what was worth a thousand times as much