As much as I trust a good caliper and scale, it would be nice to have a little dropper bottle of acid to do an acid test on gold coins. Any know anything about that? Does it damage the coin's surface in the process?
All I found online is various droppers with acid to test different purities of gold used in jewelry. I'm guessing it damages the surface slightly, which is why I haven't found anything on using it for gold coins. Or maybe the known size/weight (and thus density) is so good for coins, that no one bothers with acid.
I'm thinking of it as a secondary tester in case my scale breaks.
Using Acid to Test Gold Coins
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Re: Using Acid to Test Gold Coins
An acid test would only verify the composition of the very outer surface of the coin and would tell you nothing about the composition of the inner bulk of the coin. A fake coin good enough to pass the caliper/scale test would almost certainly be gold-plated, so you'd be out of luck. Testing the inside of the coin with acid would clearly require one to damage or destroy the coin.TripleB wrote: As much as I trust a good caliper and scale, it would be nice to have a little dropper bottle of acid to do an acid test on gold coins. Any know anything about that? Does it damage the coin's surface in the process?
All I found online is various droppers with acid to test different purities of gold used in jewelry. I'm guessing it damages the surface slightly, which is why I haven't found anything on using it for gold coins. Or maybe the known size/weight (and thus density) is so good for coins, that no one bothers with acid.
I'm thinking of it as a secondary tester in case my scale breaks.
In another thread, WildAboutHarry mentioned the clever "ring test" method of recording the sound of dropping a gold coin on a hard surface and then using free software to analyze the audio spectrum to see if the characteristic peaks are present at the correct frequencies. I've performed a few ring tests myself using the free software, and so far it seems like a reasonably simple and promising approach.
Some more exotic test methods have also been proposed on this forum (such as using a hot plate and an infrared thermometer to measure thermal conductivity), but so far the only methods that strike me as being fairly simple to do in a coin dealer's shop are (1) caliper/scale or equivalent method and (2) "ring test" using a laptop computer and a microphone. I plan to do both next time I visit the coin shop to buy a gold coin. The employees will probably chuckle at my paranoia, but I'm fine with that.
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WildAboutHarry
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Re: Using Acid to Test Gold Coins
I've been working with the "ring" test as well, and it does seem reliable. The one problem I've had is in getting smaller coins to really ring, but the Audacity software still picks up a characteristic audio spectrum for different coin compositions in small-sized coins.
Another useful non-destructive test is the "magnet" test. This works on poor-quality copies made of magnetic material.
I have looked at one of the fake gold American Eagles (copper with a gold plating). The size and strike look real (aside from the "COPY" marks on the coin), but of course it fails the weight test. And it doesn't ring like the real thing.
The one coin parameter that seems to be most variable is thickness. If you think about it, a coin-sized disk of a given weight would also have a fixed diameter and thickness, but after the coin is struck the surface would be pushed into the design, the fields would be flattened, and the rim would be raised. If all of the raised parts equaled the volume of the flattened parts then the thickness should remain constant, but that clearly isn't the case since coins are designed to stack easily (i.e. the rims exceed the design in height). I've seen quite a variation of rim thickness in a variety of coin types. I suspect that is normal, since rims wear, strikes are not necessarily uniform, etc. Further, a fake that met the weight of a real gold coin, say in lead, would need to be quite a bit thicker.
If you are comfortable with a digital scale and caliper (these can easily be carried) then I suspect that you will be able to ID the real thing 99% of the time with just weight and diameter. The real danger is tungsten copies, and I just don't know how likely it is for those to be in the market.
I would like to get my hands on a tungsten copy of a Krugerrand or an American Eagle just to see how it does on the ring test. I suspect that it would be detectable.
Another useful non-destructive test is the "magnet" test. This works on poor-quality copies made of magnetic material.
I have looked at one of the fake gold American Eagles (copper with a gold plating). The size and strike look real (aside from the "COPY" marks on the coin), but of course it fails the weight test. And it doesn't ring like the real thing.
The one coin parameter that seems to be most variable is thickness. If you think about it, a coin-sized disk of a given weight would also have a fixed diameter and thickness, but after the coin is struck the surface would be pushed into the design, the fields would be flattened, and the rim would be raised. If all of the raised parts equaled the volume of the flattened parts then the thickness should remain constant, but that clearly isn't the case since coins are designed to stack easily (i.e. the rims exceed the design in height). I've seen quite a variation of rim thickness in a variety of coin types. I suspect that is normal, since rims wear, strikes are not necessarily uniform, etc. Further, a fake that met the weight of a real gold coin, say in lead, would need to be quite a bit thicker.
If you are comfortable with a digital scale and caliper (these can easily be carried) then I suspect that you will be able to ID the real thing 99% of the time with just weight and diameter. The real danger is tungsten copies, and I just don't know how likely it is for those to be in the market.
I would like to get my hands on a tungsten copy of a Krugerrand or an American Eagle just to see how it does on the ring test. I suspect that it would be detectable.
It is the settled policy of America, that as peace is better than war, war is better than tribute. The United States, while they wish for war with no nation, will buy peace with none" James Madison
Re: Using Acid to Test Gold Coins
My concern is more about having a backup to the scale in an event that I don't have power or electricity. I'm thinking of a scenario where society has collapsed and we are bartering with precious metals.
It looks like the acid testers used for jewelry are only a few dollars, and if they maintain potency, and are effective, it seems like it could potentially be a decent backup.
It looks like the acid testers used for jewelry are only a few dollars, and if they maintain potency, and are effective, it seems like it could potentially be a decent backup.
Re: Using Acid to Test Gold Coins
TripleB reacts to a fake gold coin revealed through the acid test:TripleB wrote: My concern is more about having a backup to the scale in an event that I don't have power or electricity. I'm thinking of a scenario where society has collapsed and we are bartering with precious metals.
Q: “Do you have funny shaped balloons?”
A: “Not unless round is funny.”
A: “Not unless round is funny.”
Re: Using Acid to Test Gold Coins
LOL @ MT
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WildAboutHarry
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Re: Using Acid to Test Gold Coins
Howard made one...TripleB wrote:My concern is more about having a backup to the scale
It is the settled policy of America, that as peace is better than war, war is better than tribute. The United States, while they wish for war with no nation, will buy peace with none" James Madison