Published: 20 Feb 2018

Assaying – A science to analyse the purity of gold

Gold assay - test gold's purity the right way

How do you know that the gold is pure? If you are a jeweller or a metal refiner, ‘Assaying’ is essential to test the gold. It is an assessment that analyses the purity of gold or any other metal object. From jewellery to bullion bars, assay tests are performed on them to determine their value and purity. There are different ways of assaying the metal. Some of them are as below:

  1. Stone assay (hand test)

    Used since ancient times, this is the simplest way to tell the purity of the gold. A stone assay is a hard, dark colour touchstone with a finely grained surface. All the jeweller has to do is draw the metal across it, which leaves a visible line. The colour of the line determines the purity of the gold.

  2. X-Ray Fluorescence (XRF)

    This assay is much more complicated compared to the stone assay. It tests the purity fast with accuracy, without damaging the metal. Here, the metal is bathed in X-Rays, after which the metal emits fluorescent light at an energy level explicit to the gold’s atomic structure. XRF machine measures the energy levels to determine the purity as well as the percentage of impurities.

  3. Fire Assay

    Though complicated, fire assay is considered as the most reliable and accurate way to test the purity of gold. However, because of its destructive nature, it is usually used for large quantities or bullion. In this method, the metal is mixed with lead oxide and a mixture of fluxes (such as silica and borax), and a reducing agent together at about 1650 F (melting temperature for gold). The lead in the mixture fastens with the gold in the sample. The mixture is then poured into a mould and allowed to cool down. Due to its density, lead sinks at the bottom of the mould. This lead is then reheated in cupel, a small bone ash container. The lead is absorbed by the cupel, leaving behind the precious metal.

  4. Wet Assaying

    It includes converting the ore into powder and dissolving it in some acid or mixture of acid. The assayer then analyses the solution chemically. Depending on the metal in question, the assayer uses electrolysis or salts to separate it.

    The purity of gold is specified by karat. 24 karat gold is the purest. The percentage of gold content is calculated by multiplying it with 4.167 (percent per karat). So, for instance, 18 karat gold has (18 x 4.167) 75 percent gold.

Gold is commonly tested with XRF and fire assays. They are precise and allow the jewellers to make quick and accurate payouts for their customers.