Detecting Forgeries - Where can I find the data

[Bixley]

Whenever the topic of forgeries comes up on this forum, folks are advised the check the coin's specifications. The advice is to check the weight, diameter and the thickness of the coin. Data for the weight and diameter seems freely available but what about thickness? Places I have checked are the Royal mint and Numista websites, and the publications 'Coins of England' and 'Coin Yearbook'. Ideas anyone?

[vicamy]

So, theoretically we should be able to work out thickness ourselves if we know the weight and diameter and the density of the metal，because a coin is in a cylinder shape, and calculation is to work out the the volume 😂 . If the weight is known, then divide it by the density, we would know the volume. And then divide the volume by the area of coin’s face which is π x radius x radius, then we can get the height which is the thickness. 😂😂😂

However, it will not be 100% accurate though because a coin is not flat on surface, and also affected by number of decimal points chosen. So the result is more for indication purpose. 

Most time if I need to find a coin’s thickness, I just google. Some dealers websites may have information for some coins, eg Sovereign’s dimensions can be easily obtained. However, need to bear in mind that thickness or diameter is not an exact fixed figure. It is a range. 

Therefore, the best way to check if a coin is fake or not is to have it tested, and it is better to use non invasive methods, for example XRF (can work on directly on the metal to show its composition but not good at if the metal is in packaging), Sigma verifier (can work on the metal if it has thin packaging but can’t show composition and only limited to certain alloys or coins), Specific Gravity Test (cheapest method but need a bit time, and result may be affected by various factors in the process). Sometimes one method can’t give you a solid answer, so you still have to do cross check using a different method. 

[KevinFlynn]

Wikipedia has thicknesses, also look at gold.de which lists and gives technical specifications (including thickness) of almost any precious metal coin.

Weight and Diameter are the easiest to measure at home, thickness is so so as already described above. Specific weight will be tricky with smaller coins at home. A ping test can give a general impression (on gold and silver, less on platinum.

[Bixley]

Doh! I never thought to look in Wikipedia. I've looked at the German site, very comprehensive. Thanks for the tips.

[LawrenceChard]

On 27/08/2021 at 07:04, Bixley said:

Whenever the topic of forgeries comes up on this forum, folks are advised the check the coin's specifications. The advice is to check the weight, diameter and the thickness of the coin. Data for the weight and diameter seems freely available but what about thickness? Places I have checked are the Royal mint and Numista websites, and the publications 'Coins of England' and 'Coin Yearbook'. Ideas anyone?

Many sources which you would expect to be accurate and reliable are not, including the Royal Mint. As most journalists, dealers, authors, etc, simply believe what they find on "official" sites, the misinformation gets repeated endlessly, until everybody believes it, often including the original source of the misinformation. 🙂

Thickness might sound simple. If coins were plain flat discs, like blanks or planchets as the Americans call them, and without any milling or bevelling, it would be easy to publish and use thickness data. This also assumes the surfaces are reasonably flat and regular.

As we all know, coins are not uniform flat discs, with the possible exception of 2013 gold and silver Britannias, which come pretty close. 🙂

Even simple dimensions like diameter are not precise. For example, the published data on the diameter of a gold sovereign is 22.05m, a "fact" which many TSF members probably know, but...

This is a 2021 gold sovereign, a proof which as we all know are minted to the highest standards. We calibrated or zeroed the calipers before use. As you can see, the reading was 21.93mm, a discrepancy of .12mm, or 0.544%. As the area is proportional to diameter squared, this would increase the error, to 1.085%. Add in, or rather multiply by any error in the thickness measurement, and any results are likely to be suspect at least.

In case anybody thinks using a £10 vernier gauge does not lend itself to precision, we did the same thing with a Leveridge gauge, a specialised type of micrometer:

With similar results.

Now for the thickness or height measurements:

There is a third height measurement, namely the highest parts of the design on both sides. The height on each side is known as the relief.

Which depth / height measurement would you use?

Perhaps an average?

In which case, mean, median, or mode?

Now add in striking differences, such as rim height, which is known to be subject to considerable variation.

Have I introduced enough variables?

...or too many? 🙂

 

Edited September 23, 2021 by LawrenceChard

[Guest]

@LawrenceChard that leveridge gauge is a fascinating measuring instrument. I work at a UKAS Calibration laboratory and have never seen one before! To how many microns/mm did that micrometer accurately perform during its last annual calibration at 20 mm out of interest?

[LawrenceChard]

4 minutes ago, MattyBoo said:

@LawrenceChard that leveridge gauge is a fascinating measuring instrument. I work at a UKAS Calibration laboratory and have never seen one before! To how many microns/mm did that micrometer accurately perform during its last annual calibration at 20 mm out of interest?

We don't calibrate it annually, as our use of it is not so critical.

We do have a few others which require repair and recalibration. It doesn't seem to do much good when people drop them. 🙂

[Guest]

10 minutes ago, LawrenceChard said:

We don't calibrate it annually, as our use of it is not so critical.

We do have a few others which require repair and recalibration. It doesn't seem to do much good when people drop them. 🙂

Makes sense! I love it's analogue charm - looks particularly endearing measuring that sovereign

Yes, dropping instruments tends to not do them some good! I'm not sure our mechanical cal engineers would repair such an item (I'm just a humble electrical cal engineer), **puts on salesman hat** but we'd be happy to test its accuracy through a calibration if you ever need one doing - please feel free to give us a ring sometime at Poole Instrument Calibration Ltd 😉 Would love to see it in use, maybe photo it on instagram and become famous to my ones-of-followers. Leave my old life of obscurity behind.

 

[LawrenceChard]

13 hours ago, MattyBoo said:

Makes sense! I love it's analogue charm - looks particularly endearing measuring that sovereign

Yes, dropping instruments tends to not do them some good! I'm not sure our mechanical cal engineers would repair such an item (I'm just a humble electrical cal engineer), **puts on salesman hat** but we'd be happy to test its accuracy through a calibration if you ever need one doing - please feel free to give us a ring sometime at Poole Instrument Calibration Ltd 😉 Would love to see it in use, maybe photo it on instagram and become famous to my ones-of-followers. Leave my old life of obscurity behind.

 

They are mainly used by jewellers, diamond merchants, and gemmologists.

Ours all came from here:

https://www.hswalsh.com/product/presidium-dial-gauge-pdg-tg7

But they probably have not sold Leveridge (now Micromat) for about 10 years.

Some of the prices have come down.

This place has more selection

https://septools.com/product/a-d-leveridge-gauge/

But I had to laugh at "Color of the Gouge might vary."

We could probably Photoshop one onto your portrait. 🙂

[LawrenceChard]

On 27/08/2021 at 09:01, KevinFlynn said:

Wikipedia has thicknesses

Yes, I believe they are called editors...

or is it contributors? 😎

Although it is a good resource, I have corrected many errors on it, not all about coins, and spotted many more since.