Carbon dating coins

This is in response to the below question posted on the Proculus thread:

"Is there a scientific test to know the exact age of a coin ?"

I think you're thinking of something along the lines of carbon dating. Not with today's technology but perhaps not terribly far-fetched at some point in the future.

You can use carbon as a dating tool for figuring out the approximate date of manufacture because a particular isotope of this element, carbon-14 if memory serves, is radioactive. Since radioactive elements decay at a steady rate you can compare the amount of C14 (or whatever that particular isotope is) to the amount available in nature and use the difference to calculate the period necessary to reach the depletion rate observed.

Problem is, carbon is by definition a molecule of organic origin where metals are inorganic. There being no carbon inside a coin, logic goes, you can't use this method. The atoms that make up a coin were formed in supernovae billions of years ago and remain stable to this day. It doesn't matter - again, just using basic common sense - whether those atoms were in an ore vain or inside a coin.

But, as a thought experiment at least, I'd like to challenge the standard thinking. There are at least three possibilities that come to mind.

First, coming back to carbon dating, the deciding factor on whether an object can be dated this way largely falls on whether there's a big enough piece of it to test. If it's wood a splinter won't do, you need x grams' worth. If it's a piece of cloth a single thread is insufficient, you need a patch of a certain size and so on. This is because the instrument that is going to analyze the sample needs a minimum threshold amount below which the amount of radioactive carbon is undetectable.

That highlights that this is simply a matter of a practical, and not a theoretical, limitation. There is nothing that suggests to me that it is absolutely impossible that the amount of carbon accidentally trapped in the alloying process (say from workshop smoke) will be forever below the limits of detection. There may come a time in the not distant future when just a nanogram or two of carbon is sufficient to make such an analysis.

Second, and in a similar vein to carbon, there are many other elements with radioactive isotopes. A volcanic eruption may create an unstable form of potassium which lingers in the atmosphere. A few of these atoms could, and likely are, continuously trapped in everyday objects as they're made (potentially including coins). In so doing this creates the opportunity for a very sensitive instrument to potentially be able to measure the amount of radioactivity vis a vis background radiation and give us a probable date range of manufacture. So we're not limited to just carbon.

Lastly, I think perhaps one of the most promising avenues could be acoustic analysis. When a metal is cooled from a molten state its internal composition assumes a latticed structure according to its crystalline habit. When this structure is compressed (as would occur when it's hammered) at a microscopic level you find that the shock waves create layering and fractures; alternating areas of higher and lower spacing. In effect, some of the energy transferred in the blows has not dissipated immediately in the form of heat but is stored like an accordion or a spring. These atoms and molecules slowly lose that energy and relax to their initial state. This 'relaxation' happens at a steady rate that can take place immediately or span thousands (millions?) of years depending on the material. It is this steady rate that could potentially be exploited as a means of dating.

By knowing the alloy's makeup and its atomic organization in its most relaxed state (molten), measuring the current level of stress and, finally, the rate of this particular alloy's relaxation (or elasticity) you can then make a computation that would give you such a mint date.

The first two of these values can be determined today. There are several commercially available methods for determining exact metallic composition of an object, this is the easy part. Stress can be analyzed with ultrasound and there are several available methods that can give you acoustic analyses today. The last variable would be the rate of elasticity for any given metal or combination of metals. This would take some engineering or computational modelling to figure out but is a fairly straightforward project. That there is as yet no purpose built analyzer that incorporates all three of these to spit out a number has everything to do with there not being a commercial market to justify the R&D but the tech is there now, it's not science fiction.

 

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Thanks for the write up.

 

If there was such I'd suspect much of what we think we know of ancient numismatics would be upended. Interesting.

 

Well, I seem to have read somewhere about metallurgic analysis deciding the whereabouts of the mine where the metal of a silver or gold coin comes from. That would be a great help. Though I don't know if you have to damage the coin for this.

 

Thanks for the overview! Here's an interesting example of an isotopic analysis used to compare silver from Roman Republican coins pre- and post- 211 BC reform:
https://www.geochemicalperspectivesletters.org/documents/GPL1613_noSI.pdf

I have to say that the description of the method - with words like polishing, leaching, and extraction - doesn't make me want to push my favorite coin for analysis:
"Upon later polishing of the coins, no trace of their having been leached for Ag and Pb extraction was visible. This technique therefore holds potential for analysis of rare exhibit-quality coins in the future."

 

Thanks to both! Chemical analysis tells us much, just as simple weighing reveals fundamental facts about the economic context.

One aspect that needs emphasis is that the physical analysis tends overwhelmingly to support the historical context. My recollection is that seldom does a chemical analysis or metrological study reveal a startling new fact contradicting what we previously thought to be true. I am sure that it happens, of course. We discover better knowledge all the time. But my point here is that first history is a science and that the physical studies of coins tend to provide more facts that support the histories.

We date coins by the archaeological context in which they were found. Then, in addition, we read the coins for their symbols to add understanding, as we know that coins are transportable and durable, of course. The case in point is the paradigmatic study of the very first coins and coin-like objects, the dedications from Ephesus.

 

I've heard about the technology of "BLUE RAYS " to assess the date of objects.

 

But when it comes to coins, many times we want to know down to the month or even day it was minted. Think of the last portrait coins of Julius Caesar for example. I dont think these tests would be able to be that precise.

 

It is worth pointing out that some ancient coins were made of lead.

References:
https://coinweek.com/featured-news/heavier-metal-ancient-coins-made-of-lead/
https://www.scientificamerican.com/article/ancient-roman-lead-physics-archaeology-controversy/
https://wis-wander.weizmann.ac.il/chemistry/lead-artifacts-disclose-their-age

 

I can set up a meeting with Material Science & Engineering professors at Georgia Tech and see if they are willing to take up the project or provide the resources needed.

Unfortunately, this process won’t be able to analyze cast coins, like Chinese coins or large Roman Republic coins.

 

But they would tell you if the coin is ancient or not

 

https://www.thoughtco.com/luminescence-dating-cosmic-method-171538

 

TypeCoin971793 this would be wonderful. Imagine if from a forum post were born a new process that could be useful to so many!

 

Archeology has had to adjust the way C-14 dating is interpreted, as the technique is based on a natural level of C-14 in the atmosphere at the time of stopping accumulation of such, and no one had thought that there might be variations in the level of C-14 in the earth's atmosphere, such that during the last 50,000 years ( approximate practical limits of c-14) the level north of the equator and south of the equator could be different, but recent studies indicate they can be such now and in recent times, no one knows about beyond current records. About the only way carbon could get into most coin metal is from the smoke as mentioned, but if done with coal wood from plants that had died and become buried, and then re-exposed and used as fuel could throw doubts on specific aging. I doubt it was recorded what the fuel was and how obtained..
Also on the acoustic methods, it was mentioned that the metal coined with 'hammer' or similar could leave acoustic waves that declined over time. I do not have knowledge or specific papers on this, I would appreciate if you do. My thought is that metals are heat smelted or produced in crystals with high purity ( gold, silver , copper) have gone through processes that would have left natural acoustic wave patterns much stronger than those of human production and I can not think of a way that one could tell the difference. It will be a long and shaky road at best.

Be interesting what the college professors have to say. Jim

 

Back about 1975 or so, a libertarian friend of mine recommended Industrial Research magazine (later Industrial Research & Development). Though focused on chemistry, it was a good general read across disciplines. They had an iconoclastic columnist whose opinions made people cancel their subscriptions. One I remember well was just that: Carbon-14 dating is (was) inaccurate because C-14 was (and is) created at differing rates over time.

 

I think you misunderstood, Jim. It wasn't about the hammering process leaving wave-like imprints (or at least that's not where I was going with that). I meant using acoustic signature tools to give a mapping of the interior of the coin analogous to x-rays. A couple of different but related techniques are linked below for anyone interested.

Phased array ultrasound
Nondestructive characterization
Acoustic wave emission detection

 

I am satisfied such mapping can be done by the references, but how would that help determine the age of the coin; such as how a difference in acoustic signature of coins , which might have ores from different areas as well as age? What would be used as a reference coin as the age of such a coin would have to have a providence that was absolute dated, or it seems. I appreciate your research in the area~ Thanks, Jim

 

I've worked on trying to use acoustic testing to find internal defects in parts and have always had poor results. There seemed to be too much variability in the microstructures and lots of inhomogeneity. I would think that ancient planchets would have all kinds of variability due to the metallurgy. It would be great if it works, but my guess is it would be challenging

 

I don't know guys, for me this is just speculation based on a a basic understanding of metallurgy. It may well prove fruitless but, to use a cliche, today's science fiction is tomorrow's fact. And maybe it's useless to date but ends up being a useful tool in authenticating..... who knows :- )

 

There are a few tests which use radio isotopes and could potentially be used on coins - certainly, there is a test which can determine when gold was last melted - it's based on (IIRC) small amounts of thorium in the gold which decays, releasing helium (alpha particles being helium nuclei), which is trapped in the gold. Analysis of the thorium vs helium ratio can tell how long since the gold was molten (when the helium would have escaped).

Ah - here - Google knows all...

https://link.springer.com/article/10.1007/s13404-018-0238-z

It's also true that modern steel made since atmospheric nuclear explosions began in 1945 is contaminated by radioactive isotopes, typically cobalt, so at least steel can be dated to pre-1945 and post-1945 and probably more accurately. Old battleships were an important source of such "pre fallout" of "low-background" steel, but of course ancient coins won't have been made from steel:

https://en.wikipedia.org/wiki/Low-background_steel

ATB,
Aidan.