Gold Purity, How Is It Measured?
There are many different ways that the fineness of gold can be measured. This process is known as assaying, and can be done in multiple methods, depending on the accuracy needed and the ease and speed of measuring, which is important in some cases. Also, the equipment needed to make the measurements will affect the outcome of how it is done. There are many publications that describe these methods, including how they work and which ones are most effective in each different aspect, including time, ease, accuracy, and other elements.
Fire assaying is the most accurate method of measuring gold purity. It has an accuracy of 2-3 parts per 10,000, or .02%. A scraping of the gold item in question is taken, which generally weighs about 250 mg, and is then wrapped in lead foil with added silver. The piece is then placed in a furnace which is about 1100 degrees Celsius. This removes the base metals, allowing the piece to be as pure as it can be.
The gold is then put in nitric acid, which dissolves the silver. Once this is complete, the piece is re-weighed to determine the exact purity, based on the difference between the original weight and the weight after the fire assaying process. Some places will omit the first stage of this process, mix it with silver and/or copper, and then dissolving the metals with nitric acid. This is less accurate, and is only a good method when it is guaranteed that no other impurities exist.
Inductively coupled plasma spectrometry is the second most accurate method for measuring gold purity. In this method, a sample of about 20 mg will be placed in acid and then exposed to an ICP Spectrometer, which is a very expensive piece of equipment. It has the accuracy of 1 part per 1,000, but also is cost-prohibitive for many and requires reference samples for comparison. X-Ray Fluorescence is another technique that measures gold purity, and is a non-destructive option. The accuracy is within two to five parts per 1,000, based on the surface area of the item. If it is larger, the accuracy will be better. This takes about 3 minutes and results are automatically printed out. It only measures surface layers, though, so any item that has been chemically treated on the surface will not be substantiated by this method.
Touchstone testing is less effective and an ancient method for testing gold purity. This simply involves rubbing the item on a touchstone, and then comparing it to the reference samples to determine purity. It is not accurate by any means, at only a best of 15 parts per thousand, and is used more for sorting rather than actual purity measurements. There are many other less significant methods of testing gold purity, including the Electronic Gold Tester, but these are less reliable and more likely to cause inaccuracies in the testing methods. The first three methods listed here are the most effective and most commonly used methods for testing gold purity.
With gold coins, only non destructive kinds of testing for purity can be done so as not to damage a coin and decrease its value.
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