Featured Lies, damned lies, and Specific Gravities - my journey to date

Discussion in 'Coin Chat' started by AussieCollector, Jun 2, 2018.

  1. AussieCollector

    AussieCollector Active Member

    Hi all

    I just wanted to share some learning/experience of specific gravity tests for my coins.

    This is not intended to be a best practice overview of conducting specific gravity tests. Rather, I am just sharing my journey to date, in the hope that it may be of interest and/or benefit to fellow collectors.

    Context to my coins

    My coin collection (or amassed coins to be more accurate) is somewhat eclectic. I am attempting to build a coin collection that more or less spans from the dawn of coins, to pre-modern/decimal. My passion is most definitely in the age of sail/discovery, closely followed by Byzantium and ancient coins.

    With that in mind, the structure of my collection is as follows:

    Ancient – 500BC to 500AD (pretty much Roman and Greek coins)

    Medieval – 500AD to 1500AD (which solely consists of Byzantium coins at this stage, as other medieval coins are both small and expensive)

    Age of sail and discovery – 1500 to 1862 (weird that I have such a precise end date - I know - but that’s the generally accepted date of when the age of sail concluded)

    European settlement of Australia – 1788 to 1910 (finishing in 1910, which was when we started to mint our own coins)

    Australian pre-decimal coins – 1911 to 1966 (finishing with the conversion to the dollar)

    I’m going to focus on the first 3 categories, as they’re the most interesting (in my view) and the coins are more difficult to authenticate.

    Equipment

    Pocket scales (to two decimal places)

    Cup of water

    Thin floss

    Jerry-rigged wire to suspend coins (towards the end – see troubleshooting)

    Method

    Simple water suspension method, with weight of coin (dry weight) divided by coin suspended in water (suspended).

    Results

    Alrighty, now for my trial and error.

    8 Reales, Potosi, 1666
    Dry weight - 27.06g
    Suspended 2.64g
    SG - 10.25

    So far so good, SG is very close to what you’d expect. Perhaps a bit under.

    8 Reales Mexico 1821
    Dry weight - 27.01
    Suspended - 2.63
    SG - 10.27

    Seems ok, even better than the cob in terms of expected SG.

    Leeuwendaalder 1641
    Dry weight - 26.73
    Suspended - 2.67
    SG - 10.01

    Given the Leeuwendaalders were only 75% silver, this is about what you’d expect – perhaps a smidgen under, but even then – that’s assuming the remainder of the composition is copper, which it may not be entirely. More on this later.

    My first real surprise was the Ducat.

    1648 Dutch Ducat
    Dry weight - 3.48g
    Suspended - 0.17g
    SG - 20.47

    See…. here’s the thing…. fun fact, 24 carat gold is 19.4. So this is… more than pure gold?? Now the Ducat is 98.6% gold (unlike many other gold coins of the period, which were closer to 90%) – so you’d expect a high SG. But not THIS high.

    Another fun fact, if the suspended weight was 0.18, the SG would be 19.3 – which is almost exactly as you’d expect.

    0.01 – it’s important. More on this under troubleshooting.

    My second surprise was the Hekte.

    El Hekte – Lesbos, 377 to 326 BC
    Dry weight - 2.56g
    Suspended - 0.21g
    SG - 12.19

    Again, there’s a problem here. My understanding was that Hektes, being electrum, were a 50-50 (ish) gold silver mix, which should put the SG at about 14 to 15. I stressed over this result for almost a week, trying to find information on what the reading should be

    More on this under troubleshooting.

    Denarius – Marcus Aurelius 140 to 144 AD
    Dry weight – 3.52
    Suspended weight – 0.36
    SG – 9.78

    Not terrible, but not exactly what you’d expect either. More on this one under troubleshooting too.

    Hyperpyron – Andronicus II & III
    Dry weight – 3.89
    Suspended weight – 0.31
    SG – 12.55

    Like the Hekte, this was significantly under what I was expecting – particularly because this was a more modern coin, supposedly consisting of 11 or 12 carats, with the rest being silver and copper (which by my calculations should have come in at 14 or something).

    More on this below.

    Troubleshooting

    Dip and let it settle

    So often the scales will fluctuate for a bit when using floss in particular, and you just need to wait a few of seconds to let it all settle down to get an accurate reading. Sometimes the non-settled weight of a suspended coin can be +/- 0.02 – which is significant on smaller coins.

    The differences on the SG reading can be massive.

    Ancient coins and SGs

    Before I get into the floss vs wire, the context of ancient coins and SGs is important.

    As it turns out, it’s not as simple as “this coin is 50% gold, 25% silver, and 25% copper and therefore should be X specific gravity”.

    Specific gravity as a test is great for single or binary alloys. But not so much for 3 or more, and especially not so much for ancient coins, where there are also unknown metals and materials present.

    I found some research on Hekte coins here:
    https://www.scribd.com/document/81156859/Electrum-coins-and-their-specific-gravity-Barclay-V-Head
    (I found the table at the end to be particularly useful)

    And electrum and other gold alloy coins here:
    https://pdfs.semanticscholar.org/f3a9/a9d85d3afb43ea83442445306867bf6afaf7.pdf
    (really good discussion in this article on the impact unknown metals and compositions have on SG)

    Floss vs suspended wire

    FLOSS MAKES A DIFFERENCE – and not in a good way.

    You’d think it wouldn’t, but it most definitely does. The smaller the coin, the bigger the difference it makes. On the larger crown sized coins, the difference becomes almost a non-issue (almost). But on the small coins, it can mean the difference between an acceptable range, and suspected forgery – either modern or contemporary.

    Using the wire method is by far superior, because you can tare the weight once you know how much water it displaces. With floss, this isn’t possible, because the floss will just float at the top.

    I haven’t listed all the coins I tested above, but here are some SGs using suspended (and tared) wire:

    Ducat – measurements came in at 3.48/0.18 = 19.30. It came in exactly as expected using the tared suspended wire technique.

    Hekte – measurements came in at 2.56/0.20 = 12.80. A significant and important difference, with the assumed gold content jumping from ~30% to ~40%.

    Denarius – measurements came in at 3.52/0.35 = 10.01. Again, the Denarius shot up from what was a bit low, to exactly where it should have been.

    Leeuwendaalder – measurements came in at 26.73/2.66 = 10.05. Not a huge difference (as the coin is larger, and therefore the impact of the floss on the SG is smaller), but I still saw a difference and a shift up. And it’s achingly close to exactly what a 75% silver 25% copper mix should be – 10.08.

    Hpyerpyron – measurements cam in at 3.89/0.29 = 13.41. This is a really significant difference, with the assumed gold content of the coin jumping significantly.

    Learnings

    Your method of suspension is important

    Overall, switching from floss suspension to wire suspension results in a shift in exactly the right way on my coins. Coins that were too high, came down, and coins that were too low, went up. It's just more accurate.

    The science of SG for ancient coins

    Due to the nature of SG testing on ancient coins, I have spend undue hours wondering/stressing about the authenticity coins. I thought it would be a more precise science than it actually is.

    Accuracy

    I also need to get scales with 3 or even 4 decimal places. 2 decimal places are plenty for dry weight, but it isn’t for suspended weight.

    0.01 can make a big difference when calculating the SG of smaller coins. Again, on the Hekte, a lack of clarity of 0.01 potentially results in an artificial low of 12.19, or an artificial high of of 13.63. The margin of error is unacceptably huge!

    I have somewhat gotten around this by multiple tests and finding the mode, but it’s not ideal.

    Question

    Finally - Question, question, question. Test. Fail. Have another go. Get it wrong. Try again.

    Hope that was of interest to some, and let me know if you have any comments, musings, corrections, or recommendations.

    Cheers

    AussieCollector
     
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  3. GDJMSP

    GDJMSP Numismatist Moderator

    Interesting experiments and well written. I find 2 things I would address, well 3 really but only 2 that you actually mentioned.

    This is imperative. But there's more to it than just the number of decimal places, the scales also have to be extremely accurate if you wish to test SG and quite frankly most of the ones you can buy today are not. They'll get ya close but how close is close enough ?

    What a lot of folks don't realize, understand, or are simply unaware of is that even 2,000 years ago it was a precise science. They seem to think that just because it was so long ago that people could not have done what they do today. But they are wrong on that count. As a matter of fact the mints of ancient and subsequent periods followed much tighter tolerances than any of the mints of today.

    The ancients and all those who followed them had the ability to refine and create alloys of gold and silver to whatever degree of fineness they wished, and out to 3 or 4 decimal places. They also had very accurate but simple and quickly performed testing methods. Methods that are still in use even today. But they had them 2,000 years ago, and they used them.

    So if what I just said is true, why then would your findings, your results of testing, and not just yours but the reported tests of many others as well, be what they are ? How and why would the expected or specified fineness of the metal be off, and less than they should be ? The answer is really quite simple, it is precisely because they did have the ability to produce coins of whatever fineness they desired. The coins turned out exactly as they wanted them to be.

    You see, they intentionally cheated ! They quite often produced coins with a fineness lower than specified. And they would also produce coins of lower weight at times. But usually in tiny amounts to make it less likely for them to get caught.

    Now sometimes the minters of years gone by did get caught and sometimes they suffered severe penalties. But other times they got away with it. And in either case, caught or not caught, many of those debased coins still exist today and when they are tested we get the results we get. And because most are unaware of the things I have explained they draw the wrong conclusion and think the ancients just couldn't be as accurate as they needed to be.

    The 3rd thing I mentioned above is this, SG only tells you so much in regard to authenticity. Specific gravity can give you an indication of authenticity but not proof of it, in either direction. There's a couple of reasons for this, one is the accuracy of your testing, between the scales you use, your methods, your materials, it's easy to make mistakes and have skewed or unexpected results. Another is tolerances that were in effect at the time.

    Sure, we do the research and find out what the specified fineness and weights were, but very, very few ever do the research to find out what the tolerances were. And there were always tolerances specified. I'll use Netherlands gold ducats as an example because I know more about than anything else. The specified fineness is well known, for centuries it was 0.986. But what few know is that the tolerance specified by law was 0.981. So coins produced of metal with a fineness anywhere between 0.981 and 0.986 (or slightly above and some were) were perfectly legal. And in later years the specified fineness itself changed, it was dropped to 0.983, and the tolerance for that was dropped to .0980.

    So, if you're testing the SG of Netherlands gold ducats you absolutely MUST know these things, otherwise you'll have no idea if your results are accurate or not !

    And you have to realize and understand that it's not just Netherlands gold ducats, these things are true with the coins of all countries and all time periods. The fineness, the weight, the tolerances - all of these things changed in different years and at different mints ! One mint could be required to follow one set of rules and another mint might follow another - with both producing the same coins ! How, why ? Because each mint can be under the minting authority of different entities, and in different time periods.

    And lastly, when it comes to determining the authenticity of a coin there is always one more thing you have to be aware of. It was not uncommon for one government to counterfeit the coins of another government. But the counterfeit coins would the exact fineness, the exact weight, the exact size, and the same design - they were supposed to have ! So you could run the most accurate specific gravity tests that could possibly be done, get exactly the results you're supposed to get, and the coin can still be a counterfeit !

    So unless you know all of things as well, or even if you do know them, you still can't prove authenticity with SG. At best, SG is merely an indicator of authenticity.
     
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  4. Nathan401

    Nathan401 Supporter! Supporter

    Super interesting!! Thanks to both of you!
     
  5. AussieCollector

    AussieCollector Active Member

    Thanks for your details response GDJMSP.

    Yes indeed. Both the decimal places and the accuracy are important.

    Testing my scales on Geleverd Door J.B. Delius & Co scale weights, it seems my scales are 99.85% accurate. Which is probably not good enough for SG.

    At the moment I only have a cheap set of pocket scales, but I think I'm going to upgrade in terms of both quality and decimal places.

    Any recommendations?

    In terms of fineness and variation, thank you for the information. Yes, I often find coins to be out slightly (just ever so slightly), which I had put down to the fact that my measurement must be slightly out.

    On another note, can I get your view on something?

    I didn't cover this in my post above, as it was beside the point to my learnings on SG. I bought a Illyrian silver Drachme (you know, the 150BC type) from a reputable dealer on a reputable website (there are only a few that fit this description, but I'm still not going to name them).

    I bought it early in my collection days, as it is a very accessible type of ancient coin. I put it in the album and didn't think much of it, until I took it out one day (not too long ago) and immediately noticed it did not have the weight of silver.

    I then tested the SG, and found it wanting (and then some!) at 8.2.

    It was well past the guarantee period, but I raised it with both the dealer and the website as an issue. The dealer maintained that it was authentic, and that he would send me a certificate - which is nice, but anyone can print out a certificate (and I never actually got said certificate).

    I strongly argued that certificate or not, authentic or not, with a reading of 8.2, there is simply no way it can be silver.

    Am I correct? Or am I being too simplistic/black&white?
     
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  6. GDJMSP

    GDJMSP Numismatist Moderator

    Part of the reason I explained what I did was to point out that testing specific gravity only does so much for you, can only do so much for you - unless you have a great deal of other important information. Information that could easily explain your testing results being skewed one way or the other. Given that, unless you have that information it's really not going to matter much how accurate your scales are.

    So should you spend the money to buy more accurate scales ? Well, it's certainly not going to hurt anything if you do. But then it's not to help much either because you are still stuck with the same problem - all the other things you don't know. So is it worth it to spend that money ? I'd have to say no.

    Actually I don't know. I couldn't tell one ancient coin from another, I've never collected ancients. I know a bit about the minting processes, the testing methods used by ancients (touch stones and karat needles), the habits of assayers and mint masters, stuff like that. But that's about it when it comes to my knowledge of ancients.

    This, this an example of what I was talking about. The SG being too low doesn't really prove anything. The coin could easily be a debased coin, but still genuine. Debased coins were extremely common, but just because they were debased that doesn't mean they are fakes. They were produced at genuine mints of the period and sent out to be used in commerce - so the coins are genuine even though they were debased. Or, you could be right, the coin could be a fake.

    You see, knowing the specific gravity of a coin doesn't prove anything one way or the other. That's because there's more than 1 reason the SG could be low. The coin could be debased, the tolerance level for that specific time period may have been different, and it's also possible the coin was produced from a bad alloy mix - by accident. Any of these different reasons, by themselves and or coupled with one or more of the other reasons, would explain the SG being low.

    You also need to understand I am not talking about this one specific coin you're talking about - I'm talking about the concept, the idea, of using the specific gravity test the singular testing method to prove or disprove the authenticity of a coin. All by itself, SG is not enough to answer the question. You need more, other diagnostics to go along with it. And not just for ancients, but for all coins from any time period.
     
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  7. AussieCollector

    AussieCollector Active Member

    My apologies GDJMSP, for some reason I thought you collected ancient coins.

    Probably a moot point now, but the Illyrian Drachm I am talking about is the suckling cow and calf coin, struck between 150BC and 50BC after Illyria was conquered by the Romans. It’s an interesting coin really, because it was struck under Roman authority, in Greek style.

    Here is a link to some examples on Wildwinds (see Ceka 92):

    http://www.wildwinds.com/coins/greece/illyria/dyrrhachium/t.html

    And sorry, I should have expressed that better. My comments regarding the SG do not relate to the authenticity of said coin. Rather, they relates to the material of the said coin.

    My question is not whether the SG reading for this coin confirms authenticity, it’s whether or not it rules out silver content.
     
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  8. Kentucky

    Kentucky Supporter! Supporter

    Nah, he's just ancient :):):)
     
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  9. Kentucky

    Kentucky Supporter! Supporter

    Not an experienced ancients collector, but did you check the dimensions and weight vs an authentic example?
     
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  10. AussieCollector

    AussieCollector Active Member

    Thanks for the response Kentucky.

    Yes, the dimensions and design of the coin check out - although it's always so hard because of the huge variation you get among these types of coins. But it is definitely within the expected style and size.

    It's just the SG that threw me, which screams out (as an amateur collector) that it's not silver. Which again, is fine, and is not necessarily indicative of a modern fake - but it was sold to me as a silver drachm.
     
  11. GDJMSP

    GDJMSP Numismatist Moderator

    Assuming your testing was performed according to specifications, and I've no reason to say it wasn't, there is no doubt that the coin is not made of the material it was supposed to be made of.

    But rule out any silver content at all ? I don't think so, with emphasis on the think part. What I'm saying is there could be some silver in it, just no where near what there is supposed to be. There could also be no silver at all in the coin, it could be a modern German silver alloy of some kind making your conclusion absolutely correct.

    For one thing specific gravity is tricky, from what I know of it a 1% error in any one thing (weight, scales, water used, temperature, air bubbles, etc etc) can cause a 10% error in results. And multiple 1% errors compound that, so 2 1% errors could equal a 20% error in the result.

    Again, the purpose of my participation in this discussion was and is not to cast doubt on your conclusions, but rather to let others know that there are many things involved, many other possible reasons for a specific gravity test to not return the expected results. And that's kind of important because for a lot of folks if they run an SG test and do not get the expected results the first thing they think is - the coin is a fake !

    And I was merely trying to point that that's not always the case. With emphasis on not always, the coin very well may be a fake.

    It's kind of a food for thought thing because if you think a coin you bought is a fake then confrontations can sometimes develop between you and the seller - when the seller is actually innocent of any wrongdoing. And if one can avoid those because you have more information, well then that's a good thing.
     
    Last edited: Jun 4, 2018
  12. desertgem

    desertgem MODERATOR Senior Errer Collecktor Moderator

    If you have a good balance ( 0.000) range and can do calculations , etc. the mechanism described previously can be quite accurate, but many non-scientists have problems with strict procedure. There is a device that can measure the sp. gr. from sample within .15g to 30 gr with excellent accuracy very rapidly and less procedural problems. It is here

    http://www.mineralab.com/Hanneman/

    or if one wishes to build it them selves , here is a pdf. originally intended for minerology/gemology
    it does have minerals names on the retail one.

    https://www.gemologyonline.com/HSG.pdf
     
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  13. AussieCollector

    AussieCollector Active Member

    Absolutely.

    This is all helpful discussion.

    The Specific Gravity is what the Specific Gravity is.

    The conclusion drawn form the results of an SG test is something else entirely. And I think your point is, a conclusion cannot be drawn and, rather, it is just another piece of the puzzle.
     
  14. AussieCollector

    AussieCollector Active Member

    Thanks for this, have checked it out. Might look at creating one for myself!
     
  15. calcol

    calcol Supporter! Supporter

    When I actively collected ancients, I measured SG on some, mostly as a learning and fun experience. There have already been excellent comments on equipment and procedure in this thread. There are a couple more things: 1. Draft shield for your apparatus. This can be fancy, like sheets of acrylic plastic, solvent-welded to make a box with an opening. Basically a plastic aquarium on its side. Or simple, like a wire or wood frame covered with thin plastic sheeting.

    2. This is optional: The accuracy of SG measurements is improved by using immersion liquids with a density greater than water. I won't go into the detailed math/statistics on it. Briefly, measurement of SG by immersion weighing is relying on the difference in weight in air and when immersed. The error of the two weights is about the same, so if the difference between them can be made greater, then the percentage error of the calculated answer (i.e. SG) will be less. The weight difference will increase with increasing density of the liquid.

    The liquid of choice for small metal or mineral objects is perfluoromethyldecalin. It has a density twice that of water, is inert, has low surface tension (few bubbles adhering to the object), and evaporates quickly. Think of it as liquid teflon that evaporates. It's expensive, but will last a long time. Only a small amount is lost with each measurement.

    Cal
     
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  16. Kentucky

    Kentucky Supporter! Supporter

    So you don't have to be overly concerned with calibration of the balance since the error is present in each measurement and cancels itself out.
     
  17. Kentucky

    Kentucky Supporter! Supporter

    Say that 10 times fast :)
     
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  18. calcol

    calcol Supporter! Supporter

    Let me finish my glass of wine first. :happy:

    Cal
     
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  19. calcol

    calcol Supporter! Supporter

    They partially cancel and partially add. The error of each measurement consists of a fixed part and a random part. The fixed parts will cancel; the random parts will add.

    Cal
     
  20. AussieCollector

    AussieCollector Active Member

    Thanks for the tip regarding perfluoromethyldecalin Cal.

    Actually, I was surprised no one brought up the issue of liquid before now.... but I was thinking more along the lines of the use of Distilled Water!

    I can report (from my experience only) that there were no discernible differences between Distilled Water (at room temp ~ 20 degrees) and tap water.
     
  21. calcol

    calcol Supporter! Supporter

    Chances are your tap water has fairly low mineral content (say, compared to sea water), so its density is close to distilled water. I'd use distilled water; it's cheap and won't have much beyond H2O in it to react with the coins.

    With ancient coins, it's possible to have dirt or mineral encrustation in crevices, etc. that dissolves or goes into suspension in water. After the coin has had its dunking and is dry, weigh it again in air to check. If it's lost weight, use the lower value for your calculations. If it's gained weight, there is probably still some water in cracks or crevices. More drying in a warm place is needed.

    Cal
     
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