Composition Metallurgy

Summary of discussion on southasia-coins eGroup.

From: Kavan Ratnatunga  
Date: 2000 Mar 1, 7:53am

Is there an independent way to age the surface of a metal to check
when it was last in melted form. Something like for "Ceramics".  For
example will electron microscopy show tracks of radioactive decays
which can be calibrated with composition to give an age estimate.
Even being able to say under 100-years or over 200-years will tell a
lot about if a coin is real or a fake.

Is it possible to check the detailed composition using spectroscopy
without destroying the coin. 

I would appreciate getting any reference of research if any that has 
been done in this field. I agree the tests might cost more than the
coin unless one has access to a research facility with the needed

From: Alan VanArsdale Date: 2000 Mar 1, 10:50pm Spectroscopic analysis does not require destroying the coin and is pretty cheap to do. This method, if one has the analysis for authentic coins, will eliminate most forgeries made from modern alloyed metals. Also one can often tell the source of the metals, that is what copper mines produced the copper etc., which is of some help (again with data for various mines, which exists in detail in the literature). The problem remains of old metal being used. When I used to buy junk coins from Bulgaria I would find many which had been worked in preparation for restriking, and discarded for various reasons later in the process. I saw only a few restruck coins for sale, looking quite nice except they seemed much too broad and thin, more so than restruck ancients even. As to some way to knowing when the metal was melted I am not so clear on this topic. Some ancient coins are prepared from native metal which is cold worked and struck so never melted, so this would be of no help in these cases. I do not think there is any method to measure how many atomic particles have passed through the metal and so date the coin. If there was such a method, someone might store their coins near Chernobyl to defeat the method. The method of telling the age of coins I am familiar with is reaction of the surface of the coin with the atmosphere and/or chemicals in the ground. In the case of patinated bronze it is felt widely even a novice can eliminate all forgeries which pretend to have a crystalline bronze patina (there are some baked on glazes which are deceptive, but microscopic examination will discover them or just experience). I have had some correspondance with scientists who have done metallurgical analysis, and read a little on it. Generally this is for academics, who sometimes are called in for example to authenticate or condemn hoards. It is rather expensive and requires some technical expertise. As I understand it the metal can be analised, not only for content but for things like crystallisation, oxidation etc., and in about all cases it can be conclusively determined if the metal is ancient. If the metal is ancient then it is possible to discover alteration as I have discussed before. If there is a lot of interest in this topic I can try to do some research or find some expert to address the group about it. From when I did study it, for cheaper coins (under $300) occurring singly or hoards of very cheap coins it is not economically practical (maybe it would be worth it if someones name were falsely blackenened I suppose, or for an authenticator to defend themselves in a defamation suit, these sorts of things go in in Europe sometimes). This point is interesting, and some feel that this type of analysis holds the key to some day defeating forgeries quickly and relatively inexpensively, only we await better and cheaper technology and some graduate student or academic to develop such a method (and probably make a good living as an authenticator for some time). It seems possible and even likely some methods of relatively cheap analysis will emerge to detect forgeries with time. What I hear is it can run in the area of $400 to $2,000 to conduct such sophisiticated analysis now, as some expert authenticators will work for as little as $25 per coin science is less practical than human expertise for now it would seem.
From: Kavan Ratnatunga Date: 2000 Mar 2, 00:01am vanarsdale wrote: > I do not think there is any method to measure how many atomic particles > have passed through the metal and so date the coin. If there was such a > method, someone might store their coins near Chernobyl to defeat the method. What I was thinking about was decay of trace radioactive isotopes on the surface of the metal. Just a wild idea.
From: Nupam Mahajan Date: 2000 May 1, 11:24am Subj: Gold Content I would like to determine the gold content of the ancient gold coins. I have been using an instrument which use the conductivity of pure gold and the alloys, and thus provide reasonable estimate of gold content of coin in Carats. A good thing is this is non-destructive method and takes just few seconds. But the bad thing is, it tells me the coin being 20-21 Carats or 22-23 Carats and so on, I can never determine the contents like 91% or 96% So the quation goes, is there a reliable and non-destructive method of determination of gold content of ancient coins? PS. I also used scanning electron mictroscope which provided excellent results. It gave not only gold content but all the other metals contents in 0.01% level. Unfortunately, it is bit difficult and can not be done on so many coins.
From: Michael Bates, American Numismatic Society Date: 2000 May 1, 12:21pm Specific gravity measurement, if done carefully, and if you know the alloying element (silver or copper or both) will give you precision within a percent. If the gold content is in the nineties, the other alloying element doesn't matter very much and your precision will be within two percent. But, you need a very precise balance. You could bring the coins to the ANS and do the measurement here--we have an instruction packet and the balance, but you have to do the work (it's a bit boring, especially if you've done it often before as I have). I have heard that there are commercial services that can do an analysis using XRF or a similar method. I'd like to know more about that myself. If it's cheap enough, it would be well worth while. The only other methods known to me require a grant application or a very good friend in a scientific department of a major research university.
From: Nupam Mahajan, Date: 2000 May 1, 4:00pm I do have a sensitive balance which reads upto 0.001 gms. Indded, another one is in my lab which can do 0.0001 gms. I was wondering if there is a possibility where I (and perhaps some members may have interest) can get a copy of instruction booklet that you have in ANS thus allowing me/us to determine the gold content of coins of our collection. Secondly, does this method needs any other instrument apart from balance?
From: Alan VanArsdale Date: 2000 May 1, 4:34pm Keep in mind specific gravity is not very effective in the area of forgeries. Fortunately the ancients did not have access to many metals of density similar or higher than gold, still lead versus silver versus copper content can throw the results significantly when using specific gravity, the lower the gold content the more uncertain the result if the ratios of other metals are unknown. Platinum can also be present sometimes, as an impurity in the gold or from actual naturally occurring platinum. Nobody can complain too much about intrinsic value about platinum (or its common impurity palladium which has risen now sharply in value in recent years), and it is not too often in modern or ancient times platinum is at a sharp discount to gold. Another factor is bubbles in the metal, this can be a problem with cast ancient coins, or even struck depending on the methods used, any bubbles will make the gold content appear lower based upon strict reliance of specific gravity. There are acid test kits which can test pretty accurately up to about 22 carats. Also some electronic gold testers with experience are fairly accurate in higher carats (they are more expensive). In general without spectral analysis evaluating gold purity is a somewhat imprecise art. I also encounter gold coins with significant deposits or even patinas if lower carat, this is another factor that will indicate gold content lower than actual by specific gravity test alone. Specific gravity is an excellent back up to electronic and acid testing both. This is because the earths gravity can feel the center of the coin, and this is the only test which can without damage to the coin. There are many methods to make the center of a coin less pure in gold, or even not gold at all (maybe leaded bronze for example), and not all such ancient work is fouree, I am sure there are many "gold" coins about which have plugs inside of lower purity than the surface of some millimeters thickness. This problem in both silver and gold is one reason we see so many ancient coins cut deeply (and undoubtedly many more were subsequently melted because they were cut, whether bad or good).
From: Kavan Ratnatunga Date: 2000 May 1, 7:25pm Could I please get a good reference, which describes the best non destructive methods for measurement of composition. I am in the Physics Dept. of a research university and can probably find what is needed if I know what to look for. I am interested in getting the composition not just of Gold but of a wide variety of copper, lead silver and gold coins. Say about 50 coins. Thanks.
From: Michael Bates Date: 2000 May 3, 10:19am I don't know what is the best way. Different approaches have different advantages, I suppose. There is x-ray florescence, x-ray spectroscopy, and neutron activation analysis, to name three that come to mind now. I'm not an expert on coin analysis. There are frequent articles in the journal Archeometry which CMU must have. The French, thanks to their CNRS which links archeologists, numismatists, and scientists into "equipes" to investigate material objects, seem to be the leaders in this respect. There must also be experts in metal analysis on the faculty of CMU. The methods for coins are the same as for any metal objects made of gold, silver, or bronze/copper alloy, I assume. In reality, you are in a much better position than I am to research this question. The Royal Numismatic Society has published at least two volumes of collected papers with the series title Metallurgy and Numismatics, which ought to be most useful. I should say, the "best" method for analysis is the one you have available. And if you have two or more methods available, use them both and make a comparative study of the results, which could be published.
From: Michael Bates Date: 2000 May 3, 2:04pm This is all certainly true if you want to know the intended alloy of a series: you will find that the individual coins vary somewhat for the reasons that Mr. VanArsdale indicates and because they actually do vary somewhat. Before the nineteenth century it was difficult to evaluate gold alloy precisely and it was difficult to make a pot of absolutely homogenous molten gold, so the pots varied somewhat in average alloy and the individual flans poured out of the same pot varied somewhat. However, it is possible to measure the specific gravity of a clean gold coin quite precisely and repeat that measurement carefully to get nearly the same value time after time. In my unscientific mind, that's pretty effective. If you know or can assume what the alloying element was, you can estimate the fineness quite precisely from the S.G. measurement. You would then have to do the same measurement for more coins in the same series, hoping to find--as if often the case--that they come out to be close, say from 95 to 97% estimated gold content. You might then conclude that the intended fineness was 23 carats (95.833...%) or 23.5 carats (97.91666...%) or even 23.25 carats (96.875%), depending on how precisely you think the minters could define their alloy standard. One thing for sure: they calculated in carats, not in per cent. There's no point in trying to be terribly precise. The method is not very useful for silver and useless for copper, so far as I know. If you know that the gold coins of a series run 95-97% or something of that kind, and you find one that seems to be 85%, for example, that's good enough to know something is wrong (first, recheck your measurement of the S.G.).