Gold testing acids can be purchased Online, and normally come in small plastic 2-ounce dropper bottles, sometimes even including color-coded labels and lids. Each dropper bottle is labeled with a corresponding gold karat purity, the common ones being 10k, 14k, 18k, and 22k.
Here's the KEY to understanding gold test acid and how it works...
It all starts out as concentrated Nitric Acid. This acid is measured and diluted in very specific increments to give you the varying purities, or acid "strength". So - 10k gold test acid is much "weaker" than the 14k, 18k, or 22k test acids. The are all the same acid, but the 10k bottle has been diluted more. As such, the 10k acid won't dissolve as many metals as the others will which is how you can tell that a jewelry item is "at least 10k gold, but definitely less than 14k pure".
Concentrated Nitric Acid will dissolve and/or react with almost all metals, including all but the most pure gold. Some metals like copper and brass turn blue/green, while others dissolve completely.
By diluting this acid, we vary the degree at which it will dissolve metals and can then make a rough estimate of an item's gold content, if any.
To look at it another way, 10k gold test acid IS weak Nitric Acid, 14k gold test acid is slightly stronger Nitric Acid, 18k test acid is slightly stronger... and so on.
How to Test an Item Using Gold Test Acids
There are two standard approaches to testing a piece of jewelry (or any gold for that matter) using gold test acids. Each approach, of course, has its own methods of execution, which are mostly a personal preference but some methods of execution are much more destructive to the jewelry in question than others. We'll try to stick with the less destructive methods.
The "method of execution" we'll use in these examples involves firmly grasping the piece of jewelry and then rubbing it back and forth across the surface of a test stone several times in order to produce a metal line. Gold test acid is then dripped onto the test stone's surface to observe any reactions between the acid and metal line created from the jewelry.
NOTE: Sometimes items may have an extra thick plating (silver-plated flatware, for example). In these cases, the "rub test" can still work and save you from filing into an item. Simply rub a good solid line on the test stone (as described above) then move the piece of jewelry to a different spot on the test stone and make a second, good solid line -- making sure to continue rubbing IN THE SAME SPOT on the jewelry. By rubbing two lines onto the test stone, both from the same spot on the jewelry, you'll almost guarantee the metal that makes up the second line on the test stone has come from underneath any plating the item may have and will reveal the true metal content of the item in question.
The first approach for testing gold jewelry is when the karat purity is known (or at least "reasonably assumed")
In this case, you'll make a metal marking on your test stone (as described above), then start with the gold test acid that matches the purity of the item. If you're testing a 14k wedding band, make a mark on your test stone, then start with the 14k test acid.
Each test acid is just powerful enough to dissolve metals "up to, but NOT INCLUDING" the specified purity. So if you have a mark on your test stone made from a 14k gold ring, the 14k test acid SHOULD NOT dissolve this marking. After the acid has been in contact with the metal for some length of time, the metal may darken and discolor, but the 14k test acid will not dissolve gold-old-honest 14k solid gold.
Using this logic, we can then move up or down the acid potency scale to get a better idea if our ring is real and how much gold it contains.
Ex/ If the 14k test acid DISSOLVES the metal line from a "suspected" 14k gold ring, we would rub another line on the test stone and use the 10k test acid on this line. If the 10k acid DOESN'T DISSOLVE the new metal line, our jewelry in question IS REAL GOLD, and it is AT LEAST 10k pure, but definitely LESS THAN 14k. If the new metal line IS DISSOLVED by the 10k test acid, the item is not solid gold at all (at least, not by American Jewelry standards, which require a 10k minimum to be considered "solid" gold. On the other hand, some antique pieces were made from 8k and 9k gold so all this test tells you is that the piece is "less than 10k pure gold").
The second approach for testing gold jewelry is when you don't know the purity (and might not even know if the item is real gold)
In this approach, we rub the jewelry on our test stone as before, then start with the weakest test acid... 10k. If this test acid dissolves our metal line made by the jewelry, the piece is not gold (or more accurately, it's "less than 10k pure gold"). Almost all, if not all of the gold jewelry you will find today is at least 10k gold but it is still helpful to look at this from both viewpoints.
So if the 10k acid reacts with the metal line, the test is over and the item is a dud as far as scrap jewelry is concerned.
If, on the other hand, the 10k test acid doesn't dissolve our metal line, we're onto something and should move up the acid test scale.
So the metal line withstood 10k gold test acid... make a new test line with the jewelry and then drop a little 14k test acid on there. What happens?
If the metal line dissolves this time, you likely have 10k gold.
If the metal line persists, you might have 14k (or better) gold.
I say "might" there because, while Nitric Acid is, well, an acid... it doesn't dissolve everything so the acid test is not always 100% accurate. (For instance, none of the test acids seem to do anything to a metal test line made from an aluminum pop can.)
This is one VERY VALUABLE reason to keep the higher karat (aka, more potent) gold test acids around. When testing a piece of gold, ideally you want to find that one test acid WILL DISSOLVE the metal, while the NEXT STEP DOWN WILL NOT. If you work your way up the test acids all the way to 22k and still can't dissolve the metal test line, it'd be a good time to start being leery of the piece in question (unless it's marked 22k or a gold nugget).
Hopefully by the time you hit the 18k test acid, you'll have a good idea if the item is real and if so, what's the gold PURITY. My favorite acid tests are when you work all the way up the scale on an unknown piece and then BAM, the 22k test acid dissolves the line, signaling you've found a nicely valuable piece that's at least 18k (75%) PURE GOLD.
So now you know way more than most about gold test acids, what they are, how they're used, and what you can (and can't) learn from them. This will give you a good basis to go on but nothing beats practice and experience.
If you can, get your hands on a gold test kit as soon as possible along with some jewelry pieces of known and unknown precious metal content. When you've got the test kit, the more you use it and the more jewelry pieces you "test", the more knowledgeable and proficient you'll become.
Or if nothing else, start by testing a few "known purity" items, that way you can see how the acids react to real gold, and will be better equipped to identify real gold jewelry in the future.
Keep at it!
