Last updated on January 29, 2021
As a symbol of ultimate value, gold is a precious metal that comes in a variety of colours and different levels of purity. However, the actual price of a jewellery piece depends on its level of fineness. Here is a guide on how to test gold purity level in case your piece of jewellery does not have a purity mark.
How Gold Purity is Measured
The purity of gold is measured using the karat system. This system has been developed to measure the ratio of pure gold to other metals or alloys in a piece of gold jewellery.
Although the term “karat” sounds similar to the term “carat”, these two measurements are completely different. Karat system is used to measure gold purity while carat is a unit of mass used to weigh diamonds and other gemstones. The measurement used to weigh gold is the troy ounce (1 troy ounce = 31.1035 grams).
Karats, marked as “k”, “K” or “Kt”, are measured on a scale from 0 to 24, the latter being the purest form (pure gold) with no other metals mixed. The higher the karatage, the purer the gold. For example, 14k gold consists of 14 parts of gold and 10 parts of other metals or alloys.
The minimum gold purity for an item to be called gold varies by country. For example, in the US the legal minimum standard gold purity is 10k. In the UK, France and Portugal, the lowest permitted gold purity is 9k while in Greece and Denmark, the minimum standard is 8k.
Fineness is another way of measuring gold purity popular in the West which is expressed in parts per thousand.
In this system, pure 24k gold is expressed as 1000 parts out of 1000. For example, to calculate 24k gold fineness you need to divide 24k by 24k and multiply by 1000 which will give you a fineness of 1000. However, in practice, pure gold fineness is marked as 999.9 because there is likely to be a slight impurity in any gold. If you divide that number by 10, you will get the percentage value – 99.9% which indicates the pure gold content in a piece.
Following the same logic, 14k gold fineness should be marked as 583 (14/24 x 1000 = 583.333), but most manufacturers have adopted the European practice of making 14k gold slightly over 583, thus 14k gold fineness is marked as 585.
It is worth mentioning that accepted tolerances on gold purity vary from country to country. For example, in the US the permitted negative tolerance is 0.3%, while in China 1% is legally allowed.
How to Test Gold Purity at Home
To test gold purity at home, you will need to buy a gold testing kit. The kit typically comes with a black touchstone and several bottles with nitric acid of varying concentration. Each bottle is labelled with corresponding karat number, such as “10K”, “14K” etc.
First, you need to rub the questionable gold piece on the testing stone in the kit. The gold will leave a little streak on the stone’s surface. Then you need to drop a little acid from one of the bottles on the mark. If the mark changes colour significantly or disappears gradually, then the karat of gold is less than the label on the bottle.
If the mark changes its colour slightly, then it has the same karat as what the label on the bottle (expect an accuracy of about +/-1 karat). And finally, if the mark does not change its colour, then the gold purity of the piece is higher than the number on the bottle’s label. In this case, you need to try the next bottle with higher karat number mark.
As a general rule, it worth to start with the lowest karat number bottle and test until you find the actual karat of the piece.
Always remember to read the instructions that come with the testing kit, as specific instructions and details may apply.
How to Test Gold Purity with Electronic Tester
Electronic gold tester works based on the electrical conductivity of gold. This tool indicates the fineness of the gold item and if it is gold.
To test gold purity, a jeweller first needs to apply a specific liquid or gel to clean the surface and to create a circuit between the tool and the piece of gold. Then he applies an electrical charge and measures the level of resistance. The tester’s output enters a computerised system which displays the results. If the piece is not gold, the tester will indicate “not gold”. If the results are positive, the display will indicate the purity of the item (14k, 18k, etc).
Electronic gold testers are fairly accurate when used properly, and they work on all gold colours as they have manual adjustments to account for the metals and alloys mixed to get a specific colour.
How to Test Gold Purity with XRF Spectrometer
The most accurate tool used to identify gold purity is X-ray fluorescence spectrometer (XRF). This machine can identify pure gold concentration within seconds. However, it costs thousands of dollars and is mainly used by gold processing facilities and analytical laboratories.
As the name suggests, the XRF spectrometer sends X-ray through the tested item making its atoms become excited and move into higher energy state. When the atoms return to their ground state, they release radiation, allowing the XRF to detect the purity level.
Along with the gold purity level, the machine determines all other alloy elements used in a piece of jewellery, meaning it gives full information on what the item is made of.