Hi all For interest, I have been conducting specific gravity tests on my silver ancient coins. I primarily do it as another authentication indicator, but I figure I should post the results for the interest of others. The results have been interesting, and mostly as you'd expect. Essentially, the Roman Republic produced very fine silver coins, and the Empire progressively got worse. Greek coins are much the same, with the general rule being the older the Greek coin, the more pure it is. Here is a table of silver specific gravity, with a copper mix. Unfortunately it's not directly translatable, as we cannot assume it's a silver/copper alloy. However, it's the best table I could get my hand on, so it's the comparative I have used. Silver (copper mix) .999 = 10.49 .925 = 10.36 .900 = 10.31 .850 = 10.23 .800 = 10.17 .750 = 10.08 .600 = 9.84 .500 = 9.68 .400 = 9.53 .300 = 9.38 .200 = 9.23 The tested coins are included below by date. Comments, questions, musings, and thoughts welcome. Thessaly Hemidrachm 480 to 400 B.C Obv: Head of Athena, in close-fitting crested helmet Rev: Horse's head Dry weight: 2.9g Suspended weight: 0.28 Specific gravity: 10.35 Outcome - this is a fine silver coin, as you would expect of ancient Greek coins, at just a smidgen under 0.925 pure. Athens Tetradrachm 350-300 B.C Obv. head of Athena to right wearing ivy crested helmet, eye in profile, Rev. owl standing to right, head facing, in erect posture, olive twig and crescent behind, to right, **AQE*, all in incuse square Dry weight: 17.09 Suspended weight 1.65 Specific gravity: 10.38 Outcome - also pretty darn fine. If we assume a silver/copper mix (which we can't, but I don't have another SG table) the SG would make it just over 0.925 pure. Seleucid Tetradrachm Seleucus I Nicator Tetradrachm, 295 to 291 BC, Susa mint Obv: Head of Hercules, in lion skin headdress Rev: Zeus seated, with scepter and eagle. Dry weight: 16.78 Suspended weight: 1.61 Specific gravity: 10.42 Outcome - This coins is next to bullion silver, at around 0.960 pure silver. It is the most pure ancient silver coin in my collection. Thasos Tetradrachm After 148 BC), silver Tetradrachm, (16.65g) Obv: Wreathed head of Dionysus Rev: Herakles, standing left, resting on club, lion skin draped over left arm Dry weight: 16.65 Suspended weight: 1.62 Specific gravity: 10.28 Considering this is towards the end of Greek silver coins, this Tetradrachm has a very decent (assumed) silver purity. Roman Denarius Roman Republic, 122 BC Obv: Helmeted head of Roma right; X below chin, RVF behind Rev: The Dioscuri riding right. Q MINV below, ROMA in exergue Dry weight: 3.9 Suspended weight: .38 Specific gravity: 10.26 This is a high purity silver coin, as would be expected from the Roman Republic. Illyrian Drachm 150-50 BC Obv: cow standing left with calf Rev: double stellate pattern in square Dry weight: 2.45 Suspended weight: .30 Specific gravity: 8.16 The specific gravity of this coin is problematic, so much so that I question the authenticity. There is almost no percentage of silver that would result in a specific gravity like this. However, it is unlikely to be a silver/copper mix, so perhaps it is authentic with a very low silver value. Comments welcome. Roman Empire Denarius Antoninus I Pius, 138-161 Obv: Laureate head of Antoninus Pius Rev: Vesta, draped, standing left, holding simpulum in extended right hand and palladium at shoulder in left Dry weight: 3.28 Suspended weight: .33 Specific gravity: 9.94 Outcome: this is quite a low specific gravity, particularly for this period of the Roman Empire. A specific gravity of 9.94 would put it at approximately 70% pure. But again, that's assuming a copper mix, which is would almost certainly would not be. Thoughts and comments welcome on this result. Roman Empire Denarius Marcus Aurelius (under Pius), 140 to 144 AD Obv:bare head of Marcus Aurelius Rev: knife, sprinkler, ewer, lituus and simpulum Dry weight: 3.52 Suspended weight: .35 Specific gravity: 10.01 Outcome: this is what I would expect of a Roman Empire Denarius of this period. Roman Empire Denarius 196 AD Obv: Julia Domna Rev: Juno, standing Dry weight: 3.2 Suspended weight: .33 Specific gravity: 9.45 Outcome: there was significant debasement during this period, so possibly as expected. However, the SG does seem on the low side. Comments and thoughts welcome.
Pellinore, there are a number of demonstrations on checking specific gravity on YouTube, none of which are very accurate. The demos I watched used scales accurate to only 1/10 of a gram; they should be accurate to 1/100 of a gram. And none of the demos I watched accounted for the weight or displacement of of the suspension tool (usually a thread of some kind). If you watch a couple of these demos you can at least get the mathematical formula for doing the test. Specific gravity can also be checked by using heavy liquids.
If you like building models or making things in your basement or garage shop, then, yes, this is pretty easy. Otherwise, no, it is hard to do well. See an earlier CoinTalk here: https://www.cointalk.com/threads/coin-specific-gravity.262544/ You can goto YouTube and search "specific gravity of coins" and view many. I cannot recommend one over another, but they seem to show the same process. And I must repeat: If you are handy, it is easy. If you are all thumbs, it is impossible to get repeatable results. Let me tell you... As a lab aide in college, I had access to half a dozen scales and other equipment. I weighed my coins often and noted the results. I could get three decimals on a silver dollar: 26.7 grams. The books say 26.73, but that last place, for me, 26.72782...26.73031...26.7416 ...whatever... I worked for Carl Zeiss Industrial Metrology and translated a manual on statistical process control from German to English. I could so that.
Yes, you can do it, and I would encourage you to. I don't think it's difficult to construct something that would get you within an acceptable margin or error, you just need to construct (or jerry-rig) something to suspend your coins (without a weight on the end, like you require for a thread) so that you can compensate for your suspending apparatus. But, as @kaparthy cautions, it's not as precise as you'd like, you need a scale with three decimal points, and it's important not to stress over small differences to what is expected. And even then, for ancient coins, what is expected often differs from reality (as I have mentioned several times in the OP). With pre and post industrial silver coins, we have relative confidence as to what the alloy is, but for ancient coins, we have less of an idea. Yes, we often know the silver or gold content, but we don't always know what other metals are present. This article explores the specific gravity of gold coins, and discusses the challenges of using specific gravity to estimate gold content: https://pdfs.semanticscholar.org/a96b/2376dd42ff8b3894449c8cf1918e303781cf.pdf I did not post this to draw a line in the sand saying that SG is the be all and end all, I posted this for interest and discussion. At the end of the day, a huge amount of ancient coins sit in private hands, and collectors measuring and posting their results can further our understanding of ancient coins.
Thought I'd share this chart I stumbled across. Of interest to me was that the Pius and the Domna denarius' are of lower silver content (or have a range to lower silver content) than what would be expected of the period, which lines up with my findings. Cope, L. Roman Imperial Silver Coinage Alloy Standards: The Evidence. The Numismatic Chronicle (1966-) Seventh Series, Vol. 7 (1967), pp. 107-131 (31 pages)
Specific gravity come into existence from the unseen (see: 2 Corinthians 4:18 of the Bible) a reference of my personal evidence to the ancients. I held a Mexican Cinco Pesos 1977 90% Silver under water with my fingers in a tub of water, and in the unseen the specific gravity can be seen in water in blue text and numbers outside of the coin "specific gravity 10.3" small 3. Density in the unseen is -10.3 of small 3 in white. Specific gravity or Density can be seen without calculating (Dry wt. grams divided by wet wt. grams = SG).
I think specific gravity goes back to Archimedes, mid to late Third century BC, with his discovery of his principle of flotation. Remember that specific gravity is a ratio of density of substances to the density of water. A volume of sliver is about 10 and a half times as dense as an equal volume of water.
