If you buy scrap gold or precious metals jewelry from the general public, undoubtedly you have tested the metals in some way to determine purity. In this article we explore the 6 most common ways to test your precious metals.
Testing Acids – Rub the piece on your touchstone and create a residue “streak” a couple inches long. Apply three different acid spots; the karat you suspect the scrap gold piece is, and then the acid to test the karat above the piece and below the piece. If you suspect the piece is 14kt, the 10kt acid shouldn’t dissolve it, the 14kt acid should dissolve it, but the 18kt acid will definitely dissolve it. If you test 14kt but the 10kt acid dissolves the streak, you know that the piece is under 10kt. Acids are good at giving you a general idea that a piece is gold and a range of its karat. Below are a few things to keep in mind in regards to using acids.
- Don’t scratch from common solder spots. Many times repair spots don’t match the karat of the original piece and can give you false results, usually towards the lower end but I’ve known a few jewelers who only use 18kt solder to save on having to buy more than one type, which could lead to a higher than accurate result.
- Make sure acids are good. About once per month, perform an acid test on your test needle kit that may have come with your test kit, or if you don’t have the needle kit, have some pieces that you have verified the karat of and use those to verify your acids have retained their potency.
- Remember that acids can only tell you if a piece is over or under a specific threshold. You could have a stamped 14kt piece that doesn’t dissolve under the 14kt acid but does dissolve under 10kt. All you know is that the piece is somewhere between 10kt and 14kt.
Magnetic Test – All karat quality scrap gold will not be magnetic. This is called a negative result test….it confirms that something ISN’T what you’re being told, but can’t confirm that it is. If a piece isn’t pulled by a strong earth magnet…..it may or may not be gold, but if it does pull on a good magnet, you know for a fact it’s not gold.
Hallmark Stamp – Look for a hallmark stamp. This stamp is to denote what a piece is made of. The stamp is usually found on the clasp of a necklace or bracelet, or on the inside band of a ring. American made products are marked in Karat, while European made products are usually marked with their purity. You might see 10k, meaning the piece is American in origination, or at least for the American market, or you could see .415, meaning it’s 41.5% pure and for the European market. They both are the same purity though. One thing to keep in mind, the hallmark act went into effect in 1973, so it’s possible to find some pieces that don’t have a hallmark if they were produced before 1973.
Spotting or discoloration – The look of a piece can tell you a great deal. If you see blackened or greenish spots, than odds are the piece is plated. You should also look for areas on a piece that look worn and the gold doesn’t look uniform. That’s also an indicator of a plated piece.
Heaviness – Gold is a very heavy metal. It is more than twice as heavy, by volume, as many other metals used in jewelry, such as tin, brass, bronze and iron. This is not an accurate way to test gold, but it will help lead you in the right direction. If a piece feels “hefty”, it’s a better chance that it’s gold, than if a piece feels light. An easy “negative test” is to drop your piece into a container of water, if it floats then there’s something wrong with it.
Quality of craftsmanship
Class vs. Trash Test. This is an observational, opinion based test. It will not determine gold, but it will help lead you in a direction, similar to the “heaviness test” above. Jewelry is expensive. Most manufacturers have stringent quality control and care how their pieces look. If you’re looking at a piece, and it looks like trash….I’d spend extra time in my testing methods. There are a few areas to look at as well. Many plated castings are rough. Manufacturers aren’t going to spend the time on the unseen finishes of a piece to make them look smooth and beautiful. You can also look for corrosion at the base of posts…this shows that at least a portion of the piece is plated.
Bite test for scrap gold
Everyone has seen the bite test in a movie. Someone grabs a coin, bites down on it and then looks surprised. There’s truth to that test. Gold is a very soft metal. If you put medium pressure on it, you should dent the metal. I don’t recommend biting into every Tom, Dick or Harry’s ring though…I don’t think I want to allow my mind to imagine where that ring has been. I always recommend having a small hammer and nail set on hand. Put the nail set against the piece and give it a little hit. You should see the gold dent. If nothing happens to the metal, odds are it’s fake. The problem with this test is it’s reliant upon you applying the right amount of power to dent gold, but not so much that you’d dent a metal like stainless steel.
Determining specific gravity
Density is the weight of an item within a specific volume. Gold is one of the densest metals that is both known to man and readily available. The density of pure gold is approximately 19.32 g/ml. To put this in perspective, brass, a common metal used in fakes, is only 8.4-8.7 g/ml. Measuring the specific gravity can help you determine if your metal is gold. So how can you measure specific gravity? Follow the steps below. One warning….if there are gemstones in the pieces, it can skew your results, so this test is better suited for sold metal pieces.
- Weigh your scrap gold piece in grams
- Have a vial that has milliliter markings on the side and fill it to a designated spot with water. It doesn’t matter the amount of water. You just need two things; when the piece is put in, it doesn’t overflow and you have the starting amount of water
- Place your piece in the vial and figure out how much water was displaced. If your gold weighed 38 g and it displaces 2 ml of water, than you know the density (g/ml)…..38/2….19 g/ml. Refer to the list below for common alloys and karats.
- 10K yellow – 11.57 g/ml
- 10K white – 11.07 g/ml
- 14K yellow – 13.07 g/ml
- 14K white – 12.61 g/ml (can vary based on palladium alloys)
- 18K yellow – 15.58 g/ml
- 18K white – 14.64 g/ml
- 22K yellow – 18.58 g/ml
Scientific Testing Equipment
- XRF machines – X-ray guns or machines that scan the top couple layers of microns and determine the molecular make-up of the material. The problem is x-ray has a 2-5% margin of error. You can calibrate your software to account for that discrepancy, but it still exists. The other issue you have to account for is that x-ray is simply a surface test. You would need to melt your material into a homogeneous bar and then x-ray the bar to maximize the test and avoid inaccuracies.
- Specific Gravity testers – There are testers that will tell you the specific gravity or density of your items. These are very accurate because you can’t “fool” density. The only downside is these testers tell you the density of an item, not what the chemical composition is. So if you see the specific density showing as 13.07 g/ml, the machine is telling you that the item has the same specific density as 14kt gold….not that the item is actually 14kt gold. These are very accurate simply due to the fact that gold has such a unique density.
- Electrical current testers – These are testers that work by sending an electrical current through a conductive material and analyzing the resistance. Because precious metals are extremely conductive, this is a fairly accurate form of testing. The downside to this tester is that it is only determining the conductivity signature of a material, not the actual material itself. Someone could “fool” a tester by manipulating the other metals in an alloy, such as copper, aluminum and silver.
These are some techniques that can help improve the accuracy of your scrap gold buying process. Ultimately, the proof of the pudding is in the eating though. The only 99.99% accurate way to determine the amount of pure gold in your material is through a true fire assay that physically parts out the metals into their individual forms.
Scott Girouard has an extensive history in the metals industry, ranging from carbon steel through precious metals, as well as precious metals commerce and its impact on US and global markets.
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