Types of Assay – How Precious Metals are Tested for Purity

Assays are an important part of being a jeweler or metal refiner. An assay is a test used to analyze the purity of a metal object. There are several types of assays that can be performed and used to determine the value of all kinds of metal objects – from jewelry to bullion bars. Types of assays include:

Stone assay (hand test) – This is the simplest assay to perform and has been in use since ancient times. The central tool in a stone assay is a touchstone (hence the name), which is usually a dark, hard stone with a finely grained surface. When soft metals like gold are drawn across it, they leave a visible line. The color of the line varies with the purity of the gold. The touchstone is also typically treated with chemicals that will only react with certain purities of gold. By observing the color of the line and whether or not a chemical reaction occurs, you can tell the purity the gold.

X-ray fluorescence – Also known as XRF, this type of assay is much more complex than a stone assay. XRF assays are known for being fast and accurate tests that do not damage the metal being tested. Essentially, a metal sample is bathed in X-rays. The metal then emits light (fluorescence) at an energy level specific to its atomic structure. This energy level is measured by the XRF machine and purity of the metal is determined. An XRF is so thorough it can also measure the percentage of impurities at the same time.

Fire Assay – This complex assay is widely considered to be the most accurate way to test for gold purity. However, it’s usually reserved for large lots or bullion because of how destructive it is. First, the sample is mixed with lead oxide and a few control substances are melted together at about 1650 F. The lead in the mixture binds with the gold in the sample. This mixture is poured into a mold and cooled. Because the lead is so dense, it sinks to the bottom of the mold, where it is chipped off and placed into a cupel (a small container made of bone ash) and reheated. The cupel absorbs the lead, leaving only precious metal behind. This is measured against the size of the rest of the sample being to tested to determine its purity.

Manhattan Gold & Silver commonly uses hand testing, XRF, and fire assays to test precious metals. These precision assays are what allow us to make such quick and accurate payouts for our customers.

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