How can you test if gold is pure? Some methods are more destructive than others

The conversationWhen it comes to gold, how pure is pure? And how does anyone know?

As recent revelations about the Perth Mint have shown, gold buyers and sellers take purity very seriously. Questions have been raised about impurities found in around A$9 billion worth of gold sold on the Shanghai Gold Exchange.

While the gold reportedly met the industry standard for purity of 99.99% (or “4N”), it failed to meet additional agreed specifications for silver content.

The question of testing the purity of gold has been a hot topic for millennia, and increasingly precise methods are being developed. But despite these techniques, the gold industry still thrives on trust and reputation most of the time.

A eureka moment

The ancient Greek mathematician Archimedes is said to have stumbled upon a way to test the purity of gold while stepping into the bath.

The story goes that the king of Syracuse asked the mathematician to determine whether a gold crown was made of pure metal or whether a dishonest goldsmith had mixed in impurities.

After pondering the problem, Archimedes took a bath and noticed that the water level rose as he stepped in. He immediately jumped out and ran into the street shouting “Eureka!”. (or “I found it!”).

He had realized that by immersing the crown in water, he could determine its volume and thus its density. Because gold is denser than most other metals, this can be used to measure crown clarity.

There is debate as to the historical accuracy of the story and the exact mechanism of the test it would have used, but the gist is in line with the principles set out by Archimedes in his Treatise on Floating Bodies.

fire and fluorescence

Despite the ingenuity of Archimedes’ method, it is no longer used today. Some of the most common methods used in the modern gold industry are fire assay, X-ray fluorescence, and inductively coupled plasma (or ICP-MS) mass spectrometry.

Fire assay is the traditional method used in the hallmarking industry (e.g. to certify gold in jewelry as 9 karat or 18 karat) and is also commonly used in gold mines to test the quality of ores.

This is a destructive method, so it wouldn’t have worked for Archimedes. They take a small amount of metal from the object to be tested, mix it with various chemicals, and melt it down in a furnace or crucible.

The process is designed to remove everything but gold. So if you weigh the sample you put in and the gold you take out, you can find out how pure the sample was.

However, the fire assay only tests the amount of gold – it doesn’t tell you what else is in the sample.

Another common test (this one is non-destructive) involves X-ray fluorescence. You fire X-rays at the object you want to test, which excites the atoms in your sample and causes them to spew out X-rays of different wavelengths. Analysis of these wavelengths can tell you what’s in the sample. The machine will show you the amount of gold, silver, copper and so on.

The gold standard

To get true precision, there are several different, more sophisticated tests you can try. In Australia, the “gold standard” would be what is known as inductively coupled plasma mass spectrometry (ICP-MS).

This process effectively vaporizes a sample and then weighs the different atoms within it. It can tell you the composition of a sample with an accuracy of parts per billion.

However, it is an expensive, bespoke process. You wouldn’t use it for jewelry. It is mainly used by scientists or mining companies who want to determine the exact composition of a sample.

You would typically only use this type of test if you have a strong need for accuracy or have reason to be suspicious of a sample.

The importance of trust

In practice, these tests are not typically used by people buying gold. The gold industry runs largely on trust.

Generally speaking, if a reputable seller tells you they are giving you gold that is 99.99% pure, believe them. They don’t test every coin or bar.

For whatever reason (perhaps their additional terms regarding silver grade), the Shanghai Gold Exchange has not taken the Perth Mint’s word on the purity of the gold supplied.

Now the Perth Mint could have a trust issue. And when trust is destroyed, it’s not that easy to restore.The conversation

Michael KortieEmeritus Professor, University of Technology Sydney

This article was republished by The Conversation under a Creative Commons license. Read the original article.

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