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ABOUT GOLD PURITY:
As a measure of purity, one carat is 24 purities by mass. Therefore, we have X=24 Mg/Mm, where X is the carat rating of the material, Mg is the mass of pure gold in the material, and Mm is the total mass of the material. As such, 24-carat gold is 99.9% fine gold, 21-carat gold is .875 gold, 18-carat gold is 75% gold, 14-carat gold is .585% gold, 12-carat gold is 50% gold, 10-carat gold is .417% gold, 9-carat gold is .375% gold and 8-carat gold are .333% gold. If you are up to it, you can determine the purity of your gold at home by using a popular method of relative accuracy.
EFFECTIVE AND INEXPENSIVE: GOLD ACID TESTING AT HOME.
Starting with the Gold Testing Acid Kit.
To assess the purity of an unknown gold item, two reliable methods are commonly used: acid testing and needle comparison. Acid testing involves applying specific acids to determine gold’s karat by observing how the metal reacts, especially starting with lower karats. This is often used to verify the authenticity of a piece, such as a hallmark-stamped jewel or coin. The needle comparison method, on the other hand, uses gold needles of known purity, allowing you to visually compare the test item’s reaction alongside a declared standard. Both methods offer practical, accessible ways to gauge gold purity without advanced equipment.
To test the purity of unknown gold, start with a 10k acid solution. Rub the gold piece on a testing stone and apply a drop of the 10k acid over the mark. If the line remains undissolved, the piece is at least 10k gold. Next, move up to the 14k solution and repeat; if the line stays, continue testing with 18k acid, progressing to higher karats until the line finally dissolves. The karat level of the solution that dissolves the line indicates the gold’s maximum purity, with the previous level being the true karat. Once testing is complete, apply neutralizer to the stone, wipe it clean, and you’re done.
Testing directly for 10K, 12K, 14K Scratch the gold piece to be tested on the stone.
This test requires a set of testing needles with different tips of exact karat, a testing stone and some acids. The testing solutions are meant to be used on finished jewelry because there are general guidelines about 10, 14 18k alloys and such. With a nugget, the content could be anything. That being said, the testing solutions will work the same way. They don’t dissolve the gold, but the alloy metals mixed with the gold. The gold will flake off, but won’t dissolve. So, the more gold, the more prominent the mark will remain. Low karat marks have little gold and will dissolve away, but high karat marks will remain “brighter” if I can use that word. the acid test kit, the acidic solutions contain nitric acid & hydrochloric acid. Base metal alloys will be dissolved by the acid whereas real gold will survive the corrosive acid test.
Procedures For Testing Gold, Platinum & Silver Using A Gold Testing Acid Kit:To test for the karat value of gold, platinum and silver, you will need the following materials and tools:
- Black acid testing Stone that is washed thoroughly with water prior to each test.
- Acids
- Gold testing needles with gold tips used for comparison with test pieces.
Wether you want to check the suposed purity- Next to this position, scratch the appropriate needle (10, 12 or 14K). Place a drop of the appropriate acid on the stone where the gold was rubbed off. If the gold is the same karat or higher, the color of the scratch mark for the gold piece will appear the same as the mark from the needle. If that gold piece is a lower karat, the scratched deposit will become fainter and eventually disappear.
The next step is to use the acid. Carefully apply a drop of acid on each line on the stone. In this case, we are testing specifically for 14K because that is what the hallmark claims. When there’s no hallmark, I usually scratch my item being tested between 2 needle marks (10k and 14K) and judge the reaction. If I am not using needles, I use the different acids and judge the reactions
Below you can see the immediate reactions to the 18K acid. The test needle (left) and the ring (center) don’t react or fade, but if you look at the non gold item (right) it has a slight green tint and is fading.
The next photo shows you what the testing stone looks like about a minute after the acid is applied. Notice that the left and center lines are still strong, while the line on the right has completely disappeared everywhere the acid has touched it.
- If the test scratch stays very strong, it is AT LEAST the karat of the acid. It may actually be a higher Karat and you should test with a higher acid to be sure. In this case, there is no need to test higher because the hallmark clearly states 18K. If there was no hallmark, I would then proceed to test with 22K acid just to be sure.
- If the test scratch turns green, brown, or fizzes it is not gold.
- If the test scratch fades, it could be a lower purity and you should check with a lower acid.
Testing For 18K gold
Scratch the test piece on the stone and apply 18K acid. Any gold that is less than 18K will disappear in less than 30 seconds. Gold that remains on the stone is 18K or higher.
Testing For 20K & 24K gold
Scratch the gold piece on the stone. Next, scratch any item of known karat (coin or needle) on the stone. Apply one drop of acid to area. The material that starts to disappear first has the lower karat.
Testing For Platinum
Scratch the test item on the stone and apply one drop of acid to the application on the stone. If the material is platinum, it should keep its white, bright color.
Testing For White Gold
The same procedure for platinum can be used for 18K white gold. The material on the stone should start changing to a light bronze in about three minutes.
For 14K white gold, the material on the stone should disappear in 15 seconds.Testing For SilverPlace a drop of acid on the object to be tested.
- 90 to 100% silver, the acid will show a creamy color.
- 77-90% silver, the acid turns a gray color.
- 65-75% silver, then the acid will show a light green color.
Here, we’d like to give you a step by step tutorial on how to use an acid testing kit (one of the main ways a professional jeweler determines the karat and worth of a piece of jewelry).
Safe and cheap Acid Neutralization for Gold Testing.
One of the best acid neutralizers for use with gold test kits is sodium bicarbonate (NaHCO₃), also known as baking soda. When using acids like aqua regia from a gold testing kit, simply sprinkle some baking soda over any acid residue. The baking soda will absorb the moisture and form a bluish paste, which you can then rinse off with water. This method works on your test surface, testing stone, accidental spills, or even on skin in case of accidental contact—though safety precautions are essential. Always wear goggles and use protective gear when handling acids. Baking soda or washing soda is inexpensive and readily available, making it a practical and effective choice for neutralizing acids.
Cost breakdown: approximately $10 for a magnet, $20 for a testing kit that includes the stone, acid solutions from 10k to 22k, and neutralizer, totaling around $30.
Caution: Gold Testing Limitations with Plated Items
Be aware, however, that this method is not foolproof. It may show gold-plated items as high-karat since the surface layer rubbed onto the stone can be genuine gold, even if it’s only a thin layer over a base metal. In most cases, a careful visual inspection before testing should reveal whether an item is plated, especially if it’s older or shows signs of wear.
Rub a small piece of your item on the test stone and leave a visible deposit. Select the needle that is the nearest to the karat value you believe the gold to be and rub it on the test stone, leaving a visible mark next to your first deposit. Leave a drop of the acid approximating the karat that you believe your item to be.
The image you see above is a typical testing kit containing a testing stone. I prefer a natural stone, like the one above. Some kits come with a glass stone. The kit also comes with acids (10K, 14K, 18K, 22K, silver, and platinum), and testing needles. Also pictured are the items to be tested.
The photo below shows the Hallmark (14K) and the Maker’s Mark of the item being tested. These marks can be very small and we generally use a loupe (a jeweler’s tool to magnify images) to view them properly.
This next picture shows a set of test needles.
Test needles have a known Karat on the tips – this set has 8K, 10K, 12K, 14K, and 18K tips. Notice the corrosion caused by being exposed to the testing acids – I’m pointing this out again because I want any beginner to really understand that care needs to be taken when using these kits.
I suggest using the test needles for people starting out, but they can be expensive. If you don’t want to put the money out for them, you can test the item with several different strengths of acid to determine the purity.
The next photo shows where I scratched the items being tested, along with the testing needle, on the test stone. From left to right – test needle, ring being tested, and a non gold piece I used to illustrate the result for you.
To scratch your item on your stone, put a little pressure on the item and rub it up and down the stone about 5 or 6 times (until there is a good, defined line).
GOLD TESTING Question & Answer.
My gold sample has no reaction to the acid:?
this will reveal that your sample is of higher karat than the acid. In this case you can test again, using a higher test needle and acid until there is a reaction. This way the karat of the gold item can be determined to a closer approximation.
Why my test sample mark reacts sooner than your needle mark?
your test sample has a lower gold content. When your needle mark reacts sooner than your test sample mark, your test sample has a higher karat value. You can repeat these trials until you are able to find a close approximation of the karat of the gold item being examined.
Why two marks react the same way to the acid:?
then you will know that the gold content (karat value/purity) of your test sample is the same as that of the needle.
I store my acid kit in a sealed jar in a cool place away from the window and any light source; but how can I smell the vapors from the acid in the jar? They are all sealed tight. ?
Acids are corrosive and even when sealed in their original factory the top dispenser can leak. Placing them closed and sealed into an air tight container will limit their exposure to their immediate environment but nor prevent 100% transfer. I suggest always wearing disposable latex gloves, a mask and keeping arm’s length distance from applying acids to testing stones. Flushing residential acids from testing stones with tap water and disposing of disposable towels into a sealed bag in the trash afterwards to prevent additional fumes.
What happen with something gold plated with either 14 or 18 karat gold?
You should file it down a bit to be sure you don’t test the gold plating. Things are also only really gold plated with 24k gold. They can’t really gold plate with 18 karat gold or lower purities.
There is no certain amount of time that have to passes for the line to disappear
the acid is simply strong enough to eat away at it at all.
How do you acid test a jewel without scratching it?
You have to scratch the jewel on the stone. If you don’t want to scratch it, you can use an XRF spectrometer.
How to check if my gold coin is real without scratching it or pouring acid on it?
Weigh it and measure the height and diameter and compare that against the official specs. You can also use an XRF spectrometer. You can do a ping test.
i tested my gold. One batch dont not even dissolve under 22k solution
If you still have a strong gold mark left behind after using the 22k solution, them that is a good sign that’s your gold is pure, at 24k, which one don’t contain alloy metal.