Pure gold is a symbol of ultimate value. But how to figure out if the gold that you have purchased is pure or not. Here is a guide on how to test gold purity.
How Gold Purity is Measured
The most widely used measure of gold purity is karat (K or kt). It is measured on a scale from 0 to 24, the latter being the purest form. For example, a piece of jewellery that has 14K purity consists of 14 parts of gold and 10 parts of other metals or alloys.
Other indications used for gold purity are percentages and parts per thousand.
To convert karats to percentages, you should divide the karat number by 24 and multiply by 100. For example, to convert 14 karats to percentages, divide 14 by 24 and multiply by 100 (14:24×100=58.3), thus 58.3% is pure gold.
Finding Out the Gold Content of Your Jewellery
To see how much gold your jewellery contains, you should check the stamp indicating its karat.
For example, if the marking is 18K, this means that the jewellery piece contains 18 parts of pure gold and 6 parts of other metals, which makes 75% of gold content.
In some countries, especially in Europe, it’s common to measure gold purity in parts per thousand. In this case, you will see a 3-digit number marking.
For instance, “583” mark means that the gold content is 583 per thousand. If you divide that number by 10, you will get the percentage value – 58.3%. And to find the purity in common karats, divide 583 by 1000 and multiply by 24 – the result is 14 karats.
Here is the list of the most common parts-per-thousand and percentage purity marks corresponding to standard karats:
- 999 = 99.9% = 24K
- 917 = 91.7% = 22K
- 833 = 83.3% = 20K
- 750 = 75.0% = 18K
- 583 = 58.3% = 14K
- 417 = 41.7% = 10K
How to Test Gold Purity Yourself
To test gold purity at home, you will need to buy a gold testing kit.
The tool contains 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 gold piece on the testing stone in the kit. The gold will leave a little mark on the stone’s surface. Then you 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 vary.
What Does “KP” or “P” Marks Mean?
“KP” or “P” marks stand for “plumb”, meaning that the piece is guaranteed to have at least the indicated karat value.
For example, if a piece of jewellery is marked 18KP, it has a gold purity of no less than 18 karats.
No doubt, you are curious to know what for is this additional marking if the karat number itself is supposed to indicate the purity?
The reason is that there are gold sellers who are allowed to put a karat number that can differ slightly from the actual content of pure gold.
No worries, this is totally legal. In the US, the permitted deviation is 0.5 karats. This means that a ring stamped with 18K mark may actually be 17.5K.
However, if the ring is stamped with “18KP”, you can be sure that the gold purity is at least 18 karats.
Cover image credit: Leonhard Foeger / Reuters