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Precious Metals Assaying

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Singapore Assay Office

A division of STS, the Singapore Assay Office (SAO) is an independent body that tests and certifies precious metal articles such as gold, silver, platinum and palladium. SAO provides assaying, hallmarking and X-Ray Fluorescence Material Analysis services.

In 1978, SAO achieved recognition as an Acceptable Assayer by the London Gold Market (now known as London Bullion Market Association). This award is only given when the highest levels of assaying expertise is attained. SAO was the first recipient of that award.

X-Ray Fluorescence Material Analysis

X-Ray Fluorescence Material Analysis is the latest method used by SAO to do testing of precious metal articles. This method is non-destructive, so samples will not be damaged or destroyed. It is also a very quick method, taking about 20 - 30 minutes per test for an accurate result, as compared to the traditional methods taking up to 5 hours to complete. All alloying metals present in the sample can be analyzed, which is to say that not only the gold content can be analyzed, but also other metals present in the sample.

The coating thickness of the sample can be measured with this machine. With this method, jewellery pieces with exquisite designs can be tested, which cannot be done when traditional methods were used due to the high costs of repairing or redoing the item.

X-Ray Fluorescence is used by many assay offices worldwide and is quickly becoming one of the more commonly used methods for testing.

Fire Assay Cupellation

A sample must be taken from the article and that maybe done through cutting, scraping or drilling.

Silver is added to the samples before wrapping it in pure lead foil and placing it in a special porous crucible called a cupel. This is placed into high temperature furnace at about 1100°C. The silver is added to dilute the gold content (known as inauartation). The samples are oxidized to form a liquid slag. This slag is absorbed into the cupel to leave a small gold-silver alloy bead. On cooling, this bead is rolled flat into a thin strip to increase its surface area. It is coiled loosely into a “cornet” and the silver is dissolved out in 2 stages of nitric acid. Once the silver has been removed from the samples, the remaining gold content is pure and can be washed, dried and weighed.

ICP, Inductively Coupled Plasma Using Optical Emission Spectrometry

In Optical emission spectrometry (OES), the sample is subjected to temperatures high enough to cause not only dissociation into atoms but to cause significant amounts of collisional excitation (and ionisation) of the sample atoms to take place. Once the atoms or ions are in their excited states, they can decay to lower states through thermal or radioactive (emission) energy transitions. In OES, the intensity of the light emitted at specific wavelengths is measured and used to determine the concentrations of the elements of interest.

For more information, please visit www.assayoffice.com.sg.