Metallurgical assay

A metallurgical assay is a compositional analysis of an ore, metal, or alloy.

Some assay methods are suitable for raw materials; others are more appropriate for finished goods. Raw precious metals (bullion) are assayed by an assay office. Silver is assayed by titration, gold by cupellation and platinum by inductively coupled plasma optical emission spectrometry (ICP OES). Precious metal items of art or jewelry are frequently hallmarked (depending upon the requirements of the laws of either the place of manufacture or the place of import). Where required to be hallmarked, semi-finished precious metal items of art or jewelry pass through the official testing channels where they are analyzed or assayed for precious metal content. While different nations permit a variety of legally acceptable finenesses, the assayer is actually testing to determine that the fineness of the product conforms with the statement or claim of fineness that the maker has claimed (usually by stamping a number such as 750 for 18k gold) on the item. In the past the assay was conducted by using the touchstone method but currently (most often) it is done using X-ray fluorescence (XRF). XRF is used because this method is more exacting than the touchstone test. The most exact method of assay is known as fire assay or cupellation. This method is better suited for the assay of bullion and gold stocks rather than works of art or jewelry because it is a completely destructive method.

The age-old touchstone method is particularly suited to the testing of very valuable pieces, for which sampling by destructive means, such as scraping, cutting or drilling is unacceptable. A rubbing of the item is made on a special stone, treated with acids and the resulting color compared to references. Red radiolarian chert or black siliceous slate were used to view the resultant treated streak of the sample. Differences in precious metal content as small as 10 to 20 parts per thousand can often be established with confidence by the test. It is not indicated for use with white gold, for example, since the color variation among white gold alloys is almost imperceptible.

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