Gold coin tested (sigma vs xrf): the results surprised me
Ppp
Posts: 517 ✭✭✭✭
FYI-I do not have a vested interest in either device or company I just want to share information I found interesting even thou I do own, because of expense, a sigma.
Since the largest item I ever test is a ten ounce bar I was told I didn’t need the sigma pro (which measures through the entire coin or bar).
A friend of mine showed me a beautiful 1862 one dollar gold coin. It looked really good, fields were clear and details crisp. My gut told me to perform some diagnostics.
-It was non-magnetic,
-It weighed 1.7 gms (unfortunately I didn’t have my other scale with me that measures two decimal places)
-I tested the coin on my sigma, on the 90% gold setting, and I was surprised that an error (arrow to far right) showed up on both sides of the coin. I ran the test with and without the calibration disk with the same results. Conclusion, according to the sigma the coin was not 90% gold. Note, I then tested a coin I knew to be gold which the sigma reconfirmed.
-unfortunately I didn’t measure the coin’s dimensions
The coin was returned to a very reputable dealer who was surprised and refunded my friends money. The dealer had the coin tested using an Xrf gun and the results were real 90% gold (unfortunately, my friend didn’t write down what the other 10% was ie: if all copper, or some lead mixed in, or…).
Results: sigma concludes not real and xrf gun concludes real. Hmmmmm 🤔
Granted old gold could have other stuff mixed in but the sigma should not have given an error message, maybe one or two blocks out of range to the right (never to the left) which would signal to conduct other tests (which should be done regardless).
I was perplexed so I did a little research and was told the xrf gun measures only the surface (I was told up to 15 microns) of a metal where as the standard sigma measures 180 microns deep. I was surprised because I thought xrf gun besides giving a percent breakdown of material in a coin was better than the sigma. I never researched an xrf unit before because it was cost prohibitive for me so I believed others that said xrf gun was the best.
If correct then for coins the sigma is the better option (and less expensive).
Anyone have similar experiences?
If someone thinks or knows the above is incorrect then please explain why so we can learn.
Thank you in advance for your comments.
Comments
Sigma's are better, absolutely, because of the aforementioned factors RE depth of testing.
"It's like God, Family, Country, except Sticker, Plastic, Coin."
It may have been gold plated. Can we get pics?
Worry is the interest you pay on a debt you may not owe.
"Paper money eventually returns to its intrinsic value---zero."----Voltaire
"Everything you say should be true, but not everything true should be said."----Voltaire
Did you use the wand attachment with the Sigma?
A gold dollar has a diameter of 13mm.
According to Sigma's website, the minimum sample diameter for the traditional Sigma "on board" sensor is 24mm, or about the size of a quarter eagle. They offer that as a minimum size for testing a tungsten plated piece.
The smaller wand attachment is capable of accurate tests at 8mm diameter.
I am a big proponent of the Sigma. But I would be very wary of using a Sigma as a final answer in determining the authenticity of numismatic gold--especially pretty early and very small US gold.
That's what our hosts are for.
https://www.sigmametalytics.com/original
--Severian the Lame
24 mm is about the size of a quarter dollar, not a quarter eagle. A Type Three gold dollar is 15 mm.
I would have done a very careful specific gravity test on the coin before returning it.
Sorry, I don’t have pictures and I am not able to get them.
Yes, I used the small wand.
I have a Sigma tester, and do like it. However, with the size of the sample, I think the XRF gun is far more likely to be correct in this case. I think the Sigma gives a lot of false negatives.
Here is something to try. Take a clad quarter, or half, and set the Sigma to 90% silver. See what happens.
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Yes, I have seen clad half’s (especially 40% ag Kennedy’s) and quarters read as positive for 90%. That’s why you need to do other tests such as weight, visual, magnet, and dimensional.
If I remember correctly it’s the high copper content in clad that can throw the unit off but the weight will be a tell that it’s clad because 90% will weigh more. Note, this assumes dimensional and visual are accurate.
Capt is correct. A specific gravity test should have been done. A 90% gold coin should have a specific gravity of 17.2.
And for those that wonder a 90% silver coin should be 10.31.
Courtesy of Gainsville Coins: https://www.gainesvillecoins.com/blog/counterfeit-gold-and-silver-coins
bob
I have a Sigma tester, it seems to work good but I don't test too many.
I am more confused now after reading these posts. I guess that’s why I don’t buy many gold coins not to mention gold is expensive and way over my budget. Silver is more my level of buys. Specific what? 😬
But I do know the definition of a woman. 🙃
Interesting
Successful transactions with : MICHAELDIXON, Manorcourtman, Bochiman, bolivarshagnasty, AUandAG, onlyroosies, chumley, Weiss, jdimmick, BAJJERFAN, gene1978, TJM965, Smittys, GRANDAM, JTHawaii, mainejoe, softparade, derryb
Bad transactions with : nobody to date
How to do a specific gravity test on a coin:
https://www.youtube.com/watch?v=h_2vA1qO2fA
Looks as if all the tests are necessary when confirming gold coins - dimensions, weight, sigma, specific gravity.... Cheers, RickO
You need three things to authenticate gold: a good light source, a good 10X loupe or stereo microscope, and the eye of a connoisseur with decades of experience.
Worry is the interest you pay on a debt you may not owe.
"Paper money eventually returns to its intrinsic value---zero."----Voltaire
"Everything you say should be true, but not everything true should be said."----Voltaire
Visual inspection is extremely important.
I posted previously of a $2.5 gold Indian which according to many diagnostic tests was confirmed to be 90% gold. However, upon visual inspection tool marks were discovered and the coin was deemed counterfeit.
I think because of the explosion of prices (approx $450-) relative to gold melt (approx $210-) content counterfeiters determined that using 90% gold still provided a substantial profit.
Since the 1950's, most counterfeit US gold coins were made with gold that met US Mint specifications for weight and fineness. The counterfeiters profited from the numismatic premium these coins carried in the marketplace. That's why small gold coins were popular targets of the counterfeiters since the numismatic premium was much higher than the gold value. This is especially true of the gold $1 and $3 coins which were heavily counterfeited.
Worry is the interest you pay on a debt you may not owe.
"Paper money eventually returns to its intrinsic value---zero."----Voltaire
"Everything you say should be true, but not everything true should be said."----Voltaire
From personal experience I have found the XRF gun to be wholly undependable for the reasons outlined in the OP. All that's requires to fool it is an extra-heavy plating, the unreliability being accentuated by the high price of the gun. It's actually easier and more dependable to do a scratch test with acid.
Interesting info, thanks for the thread @Ppp
Successful transactions with : MICHAELDIXON, Manorcourtman, Bochiman, bolivarshagnasty, AUandAG, onlyroosies, chumley, Weiss, jdimmick, BAJJERFAN, gene1978, TJM965, Smittys, GRANDAM, JTHawaii, mainejoe, softparade, derryb
Bad transactions with : nobody to date
@Ppp I have used both the Sigma and XRF gun many times, but I have far more experience using the Sigma on a regular basis.
That being said, the Sigma is the tool of choice when buying and selling coins and precious metals. The XRF is useful as a back-up verification tool, and for advanced research (think pattern material analysis).
I have used the XRF (both handheld and box type) for getting estimates on a scrap gold jewelry melt, or similar. With a heavy gold chain that we are going to melt anyway, we cut the chain, or the refinery will drill a small hole in order to get a fresh, deep analysis of the composition. Sometimes in a few different places.
@Weiss is correct, as posted up thread. Also, @PerryHall is correct regarding light, loupe and experience. You have to use the Sigma results (with the wand in this case) and a visual inspection, along with a high quality scale that can read out three places. And don't forget to examine the "third side" of the coin: the edge.
The most common mistake I have seen with new users of the Sigma is the idea of simply placing a small coin on the target and then running the test. The small profile will give you a bogus reading. For example, if you are testing a small $1 gold piece, then the Sigma will show you that it is outside of the brackets and will lead you to believe that it is fake. If you attach the small wand and then run the test, it will be dead center within the brackets.
I got better at using the Sigma after calling the inventor (and manufacturer) a couple of times with genuine issues. He was able to quickly set me straight.
i once saw a dealer cut a 1oz gold panda with bolt cutters to test to see if it was gold throughout. smh
.
YIKES!!!
Dealer speaking to the customer:
The good news is that your 1982 1-oz gold Panda is indeed genuine. Congratulations on that, because I just wasn't certain.
The bad news is that we do not buy damaged coins.