What is Gold Parting?
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Due to the fact that gold is rarely found in its pure state in nature, procedures that separate gold from any impurities that may be intermixed with it have been developed over the centuries since the discovery and harnessing of gold as a prime material for currency and art. The process of separating gold from other metals, usually silver or lead is known as gold parting.[1] Gold parting is a procedure which relies on chemical agents or electromechanical methods to help separate gold from other impurities that may be found in ores in its natural, unrefined state. Due to the fact that gold and silver deposits are usually intermixed and are rarely found separately (even in trace amounts) gold parting is necessary in order to develop finer types of either metals. The need for gold parting came about when the metal became the standard form of currency in many realms. Because currency needed to be dependable, different types of gold with varying purities needed to be developed, with the standards of purity systemized. While there were older procedures that could separate gold from other metallic impurities such as cupellation,[2] it is the distinct natural mixture of silver and gold that was very hard to separate.
Prior to the development of the rudimentary methods of gold parting, smelted gold contained trace amounts of silver, and when melted down and made into jewelry or currency, created a natural alloy known to the ancients as electrum. Due to the need for gold of varying purities that could be created as dependable types of currency, the ancients created a rudimentary method of gold parting known as salt cementation. Prior to the advent of coinage, the need for purer gold for the purposes of standardization was unnecessary, and so objects made with gold was simply treated with weak acids to dissolve impurities – a method known as surface enhancement. Later on, the need for standardized gold purities coaxed many ancient civilizations to develop salt cementation, and while this method became commonplace, it was in Lydia that the first record of such a process came to be.[3] It would later be recorded by many ancient historians, more notably Diodorus Siculus, Agatharchides of Cnidus, and of course, Pliny the Elder.[4] These historians, more so the former, and later on the latter detailed the process of salt cementation in their writings, it being the mixing of impure gold ores along with a measure of salt, which would then be sealed in clay pots and heated until the silver separated from the gold and bonded with the salt crystals[5] (once thought to actually ‘dissolve’ the silver and copper impurities). This would later be skimmed off and refined in its own right for further uses, while the gold itself was rendered ‘pure’ by the standards of the ancients.
It is now known that gold purity can be measured, and that the ‘pure’ gold of the ancients was by no means the ‘pure’ (99.999% pure) gold of today’s standards. Other methods involving acids (i.e. nitric acid; aqua regia) was primarily discovered and perfected in the Mediaeval periods and later used throughout the Renaissance, although the salt cementation process still remained a staple of gold parting.[6]
Nowadays, more thorough methods are employed to yield gold possessing the highest possible grade of purity such as the Miller and Wohlwill processes of gold parting. The former, which yields gold with a purity of 99.95% involves the use of chlorine gas which helps to transform gold impurities into chlorides which are then skimmed off the surface.[7] The latter method yielding gold of the highest purity as 99.999% involves an electrolytic process, done with the use of pure gold (or titanium) and chloroauric acid. This is executed by dissolving the impure gold with chlorine gas along with hydrochloric acid. The impure gold is dissolved and rendered pure as it is attracted into a cathode via ion transfer, while the impurities are themselves easily removed. This renders gold of the highest purity, which, since its development in 1874 has been the standard means of gold parting employed in industrial gold mining even to this day.[8][9]
What is Gold Parting? - References:
[1] http://www.mastersoftrivia.com/blog/2011/08/gold-parting-what-is-it-and-how-does-it-work
[2] http://en.wikipedia.org/wiki/Cupellation
[3] A. Ramage and P. T. Craddock (eds) ‘King Croesus’ Gold; Excavations at Sardis and the History of Gold Refining’, page 27
[4] Notton, J. H. F. (1974) Ancient Egyptian Gold Refining: A reproduction of early techniques; page 52
[5] Pliny the Elder; Naturalis Historiae; also: A. Ramage and P. T. Craddock (eds) ‘King Crroesus’ Gold; Excavations at Sardis and the History of Gold Refining’, page 28
[6] http://www.ganoksin.com/borisat/nenam/wgc-refining.htm
[7] A. Ramage and P. T. Craddock (eds) ‘King Crroesus’ Gold; Excavations at Sardis and the History of Gold Refining’, page 69
[8] Rapson, W. S. (1992). Mining, Extraction, and Refining of Gold. ‘Interdisciplinary Science Reviews’ Vol. 17. No. 3 page 210
[9] http://en.wikipedia.org/wiki/Wohlwill_process
Content researched and created by Alexander Leonhart for coinandbullionpages.com © coinandbullionpages.com
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