Composition Metallurgy

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
equipment.                          



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 barnow-@silcom.com 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.).