Calculating Karats

The importance of determining gold purity is profoundly understood by the world’s refineries and is the basis for all final gold trade. However, in the field, the actual quantity of gold in a piece of dore (raw gold with some impurities) is difficult to ascertain to anywhere near the degree used by the world’s refineries; accuracy can be off by as much as 10 or 15% in the artisanal mining sector. Purity of gold is most commonly expressed in either percentage (%), as in “five nines gold” being 99.999% gold, or in karats, as in 24 karat gold being more than 99.95% pure. Gold considered London Good Delivered is 99.5%.

There are several ways to determine the precise weight of gold in a piece of dore, and thus determine purity. From lower accuracy to higher accuracy these include the water-air method, XRF and fire assays. The water-air method is simple and cheap and rests at the lower end of the accuracy spectrum. XRF (xray fluorescence) is becoming less expensive and is more accurate but still suffers numerous flaws and is far less accessible under field conditions. The fire assays used in refineries all the way to the actual refining of gold to 99.999% provide perfect knowledge of the quantity of gold being traded but are not a technology accessible to artisanal miners in the field.

Since the other methods of determining gold purity are impractical to use in field conditions, the best way to help artisanal miners ascertain their gold purity before taking it to market is with the water air method. The water air method is based on Archimedes principle; that different metals of the same weight have different volumes, because different metals have different densities. The water air method is most amenable to most conditions in the field, and has the following benefits and flaws: Benefits include simplicity and cost. If the impurity ratios are not known, then flaws include empirical uncertainties about the actual quantity and ratio of  impurities in a piece of dore. We need to make an assumption about what the impurities are; however, once the impurities are known regionally they are relatively invariant per deposit. Other flaws include the purity of the water – hardness and saltiness of the water, accuracy of the balance (should be to 5 decimal places, but as accuracy to 4 digits is required). Lastly, it is easy to cheat with: adding salt to the water, for example, will make it appear that the dore is less pure; adding alcohol to the water makes it appear that the dore is more pure.

To implement the water air method, dore is first weighed in air and then submersed in water. Next, either the impurities are known or an educated guess is made based on commonly found impurity ratios, and through calculations the gold purity is determined. Most often gold impurities consist of silver and copper and smaller quantities of other metals in differing amounts. An accurate purity calculation will ultimately come down to good assumptions about the ratios of these impurities. 

Currently the Artisanal Gold Council has an Request for Proposal out in order to develop an online gold purity calculator that miners can use in the field. Submission deadline is September 30th!  


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