Precious Metal Refining

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The refining of precious metals such as gold and silver has long been practiced since the ancient times. Because gold and silver are rarely found in their pure state, and because it is rarely (if ever) made into any object in its pure form, precious metals had to undergo a process of refining either to leech away any impurities that would debase the overall value of the metal, or to otherwise remove any metallic compounds that have been alloyed onto it so as to reuse it for further purposes.^[1]

While the term ‘precious metal’ varies depending on the purpose and the culture, gold and silver have always been the commonly accepted ‘universal’ precious metals, and since the dawn of recorded history, various methods of precious metal refining have been developed either to increase the aesthetic properties of gold and silver, or to help purify it to better comply with the standardizations required for monetary purposes. One of the earliest methods of precious metal refining was cupellation, a process that was used by the ancients, particularly the Lydians, Romans, and Greeks to obtain silver from lead deposits. This was done by heating the metal in high heat to separate the noble metals, with the gold or silver that remains intact is simply removed away from the molten lead which was absorbed the specialized linings in cupellation hearths which were made of calcareous clay or bone ash.^[2] While cupellation is still a method that is employed (in mass-scale procedures) to this day, small-scale cupellation is also undertaken, although its purposes serve more of an artisan rather than an industrial purpose.

Another method of precious metal refining is known as parting, a process which involves using a chemical or an electromechanical agent to help separate metals into their specific constituents. Commonly employed to separate gold and silver in order to be refined, parting is a relatively ancient process that dates back to the early 1st century.^[3] The earliest methods of parting usually involved adding chemical agents, most commonly salt, to impure metal ores. The most rudimentary of parting processes known as salt cementation involved interleaving metal ores requiring purification in a ‘cement mix’ of crushed bricks or limestone along with salt, which was then sealed up in a clay pot and heated to very high temperatures to allow the impurities found in the metal to cling to the salt cement, while the noble metal (in this respect, gold) was simply removed after the pot had cooled, while the impurities (mostly silver) that attached itself to the salt cement and was smelted off to be recovered for further use.^[4] During the Middle Ages until the latter periods of the Renaissance, precious metal refining took another turn with the discovery of potent and powerful acids such as Aqua regia which was highly employed in the refining of noble metals.

While the use of salt cementing, acid purification, and cupellation rendered desirable variances in gold and silver purity, it was only pure to such an extent. Despite being technically ‘pure’, that is, free from any significant adulterants that would have otherwise clung to the noble metal, it could not be termed wholly pure of any impurities of a minute scale. It was not until the latter part of the 1800s that a process to render gold perfectly pure (at 99.999% purity, the highest grade of pure gold known to man) was developed. Known as the Wholwill electrolytic process (invented by one Emil Wholwill,^[5] it was (and still is) employed for mass-scale refining and commonly used to achieve the purest state of gold. It involved purifying the gold through the use of the electrolytic procedure, whereby impure ores were suspended in an acidic solution where a charged cathode (which acts as a catalyst) was placed to attract the noble metals, leaving behind any impurities. To this date, this process renders the purest example of gold to date, while rendering silver absolutely pure is done through the use of both traditional and modern processes. Refining has become a highly profitable business, with many large and influential companies offering their services to mining industries and even to individuals who wish to engage in the business and or investment of precious metals.

Precious Metal Refining - References:

[1] http://www.britannica.com/EBchecked/topic/377665/metallurgy/81862/Refining
[2] http://en.wikipedia.org/wiki/Cupellation
[3] http://web.comhem.se/vikingbronze/soderberg_situne_dei_2011.pdf
[4] http://hist-met.org/hmsdatasheet02.pdf
[5] http://books.google.com.ph/books?id=E_u9ARrm37oC&pg=PA244

Note - this site provides general information about gold, silver, coins and bullion. None of the contents of this web site should be seen as financial or investment advice.