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Types of Assay – How Precious Metals are Tested for Purity

5 Types of Precious Metal Assays

Updated January 2022

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.

Here are the 5 most used methods for analyzing precious metals:

Touchstone (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 of the gold.

Electronic Tester

Electronic testers work by measuring the electrical resistivity of the test sample. Resistivity quantifies how strongly a given material opposes the flow of electric current and is measured in ohms (Ω). The tester uses a signal to measure the surface resistivity of the sample. Then it sends another signal completely through the sample to measure the through-resistivity and thickness of the sample. By checking the sample’s through-resistivity against its surface resistivity, the verifier can detect whether a sample is plated or filled. If the material is consistent all the way through the sample, the tester algorithmically correlates the resistance level of the sample to known precious metal alloy conductivity signatures to determine metal type and purity level.

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.

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.

Ultrasound Assay

This assay uses ultrasound to measure changes in the consistency of the material being tested. To conduct the test, an ultrasonic flaw detector generates an ultrasonic pulse and emits it via a transducer that is pressed against the sample. Ultrasonic waves travel through any medium in the same direction until they meet a different material, which causes them to reflect back to their source. If there are changes in the consistency of the metal (i.e. changing from pure gold into tungsten), the frequency of the ultrasonic pulse also changes. The transducer detects any changes in the ultrasonic pulse to find out if the outside and inside of an object are made of the same material. Ultrasonic assays are recommended for use on pure samples of uniform structure. This makes it the ideal test for authenticating bullion.

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


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