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GOLD

Historical notes on gold

The chemists and archaeologists tell us that the earliest gold objects of all ancient civilizations were fashioned directly from native gold. Later, around the middle of the first millennium B.C., a method of purifying native gold came into existence, and at about the same time the practice of alloying metals such as silver and copper with gold and thus debasing the metal came into use. It was apparently Archimedes, in his bathtub, which devised a means of determining the amount of alloyed metals, although it seems probable that his principle was known long before the advent of Greek science.

The practice of alloying metals with gold was called diplosis by the early Egyptian goldsmiths, meaning the art of 'doubling' the weight of gold without materially changing its outward appearance. In Colombia where platinoids were abundant in the early gold placers, the Spanish treasurers encountered no end of troubles with 'doubling' of the gold by individuals by means of alloying with the platinoids, elements unknown in Europe at that time (1750). It is said that to avoid this debasing practice that the treasurers ordered all of the platinum won from the placers to be thrown into the sea or no auriferous rivers.

The early metallurgy of gold probably involved little more than separating the metal by gravity from the dross of the placers and the quartz and other gangue of veins. Later, probably prior to 1000 B.C., it was discovered that gold adhered to mercury, and this developed into the amalgamation process that still finds a use in the treatment of gold ores. In the latter part of the 18th century and the greater part of the 19th the chlorination process was used extensively. Chlorine was passed through the moistened ore forming gold chloride, which was leached out with water; the gold was then precipitated by ferrous sulphate. In 1887, McArthur and Forrest invented the cyanide process of dissolving gold followed by its precipitation with zinc dust. This is the method now commonly used on a large scale for the extraction of the element from its ores.

The extraction of gold and silver from sulphide ores by the process of liquation was probably known to the ancients, as the Greeks seem to have been familiar with it and the Romans are known to have practised the art.

In the process of liquation the copper, arsenic, etc., ores were first smelted often with a siliceous flux, to yield an impure ingot. This ingot was then alloyed with lead if there was insufficient lead in the ores, and the alloy was heated to a temperature between the melting point of lead and copper, during which the lead would liquate out, carrying the silver (and gold) with it. The lead was then cupelled on bone ash.

The process of 'parting' gold and silver by cementation with salt, whereby the silver is converted to chloride and the gold is run off, was apparently known in the 6th century B.C. Parting, employing acids, was unknown to the Romans. As far as can be determined, the first reference to the parting of gold and silver by means of nitric acid is to be found in the treatises of the Moslem alchemists of the thirteenth century. Modern processes of separating silver and other metals from gold involve the treatment of molten gold with chlorine and precipitation of the metal by electrolysis. The latter produces a pure product with only mere traces of silver, copper and other metals.

Native gold is a relatively common mineral and is present in gold-quartz veins, in oxidized zones of many sulphide and other types of auriferous deposits and in placers in streams and rivers in most parts of the world. It would seem that much of the gold of the ancients came first from placers and later from primary deposits and their oxidized zones. One of the richest placer streams which supplied much electrum to the Middle East, and is said to have been the source of the wealth of Croesus, was the river Pactolus (Sarabat), a tributary of the Hermus (modern Gediz) after passing Sardis in Lydia (now Anatolia, Turkey). The Pactolus drains the auriferous region on the flanks of Mount Tmolus (the present Boz Sira Daglari); it is the river in which it is said that Midas, the mythical founder of the Phrygian Kingdom, on the advice of Bacchus, bathed in its water to rid himself of the fatal faculty of turning everything he touched into gold. The other two legendary sources of gold, mentioned repeatedly in the Old Testament, Havilah and Ophir, have not been precisely identified.

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This document is in the public domain.

March, 2011