Dating Metals?

[Michael Blalock]

Is there an empirical means to determine the age of a metal object? Can the determination be made by the patina or chemical make-up of the corrosion? In this case it’s copper or a copper alloy I’m looking for. I'm talking about millennia here, not years or centuries.

[Rivkin]

In short - no, it can not. The only analysis I'm aware of is some basic empirical comparison to other pieces of the same age while asking a question "does this rust looks natural - like in other pieces of the same time ?".

[fearn]

Hi Michael,

If you're thinking about millenia, then I'd suggest carbon-14 dating of any carbon containing compound that's positively associated with these specimens. In other words, talk to an archeology department at a nearby school and find out where they do their dating.

F

[ariel]

I am not sure carbon dating will work.
It is based on uptake of C14 by a living organism . As soon as the organism dies, it stops absorbing carbon at all and then we can measure the ratio of C14 : C12. From this number we can calculate the age of the remnant knowing the rate of decay of C14 into C12.
Metals are not living creatures. Alloys such as steel were prepared using carbon existing at the time of manufacture and the dating will reflect the age of, say, coal(several millions of years) or wood/leaves (days to tens of years). If I use coal made out of 500 yo wood (such as many olive trees), the sword made in 2005 will reflect the age of that particular wood.
Also, the precision of the method is too crude to be of major help to us: it is relatively easy to distinguish an object that is 2,000 years old from the one that is 100 y.o. However, we are mostly interested in "is it 19th or 18th century?" type of questions. That falls well below the assay limit of resolution.
Ann Feuerbach would know the answer much better and correct me if I am ignorantly wrong.

[Michael Blalock]

I was thinking along the lines of 2000 years old vs 100 years old. I have a South Arabian bronze sculpture that must be 1500+ years old which i am certain is not a fake.
I would think this situation comes up often with bronze swords that may be ancient vs. fake. Especially with all the replica bronze swords from china on eBay. I take it from the answers that they only way to make an educated guess is to have someone with lots of experience examine the bronze and compare the condition of the metal to known pieces.

[fearn]

Hi Ariel,

To be more precise, I was thinking of an archeological situation, where charcoal from an old fire, the rags wrapping a statue, or some other source of organics were found with the statue. Or there were remnants of a wood hilt on an old knife.

If it's a statue that was found in an antique shop, forget about it. I suppose you can get some estimate by observing rates of corrosion in an environment similar to whatever it was in for the last two millenia, but absent that, it's wild-eyed (ahem) guess time.

F

[Marc]

There's being some success right now in dating ferrous metals through the carbon found in the trapped slag inclusions and even through the carbon from the carbides in the steel itself, using Accelerator Mass Spectrometry(AMS). See, for example, the works of R. G. Cresswell and, more recently, of Andrea C. Cook and Jeffrey Wadsworth. The use of AMS allows to work with much smaller C amounts than needed before, so it’s opening up the path for analysing a much wider spectra of samples.

Of course, this assumes that the carbon present in the steel to be analysed comes from organic sources, i.e. charcoal. The good news is that a significant majority of antique metals were smelted and worked using charcoal, coal smelting only starting to be prevalent in Europe at the end of the 18th c. Although this is far from being definitive (you knew I was going to say that, did you?), sometimes it is found that some coal was indeed used at some point in the manufacture of the item, then the resulting date jumps back thousands of years, and as the academic community, so far, seems somewhat unconvinced of the existence of an Hyborean Era, one has to conclude that the most likely reason for such inconsistence is the use of mineral fuel.

There are also other things to be taken into account, like the early use of coal in places like China to smelt and cast iron, etc. Caution is always needed when analysing archaeological samples... And keep in mind that radiocarbon dating has quite a margin of error, normally, in this cases, of some decades, so one can’t pretend excessive precision in the results.

For nonferrous metals... well knowing your patinas is a must for any collector of Cu-based alloys items. The techniques to fake patinas are many and well-developed, so, well, you know the mantra: caveat emptor. For a collector’s opinion on this regard, I would recommend taking a look to the interventions by Kenneth Blair in this thread in SFI. Also, this one may be illustrative.

Beyond "field" experience, and as corrosion is extremely variable depending on the particular conditions every item has been subjected to, a moderately well-equipped archaeological/archaeometallurgical lab has enough resources to tell old from ancient. Sometimes, especially if the patina of the item has been messed with, this is not easy and some invasive techniques (i.e. taking samples of the actual metal) must be used. The degree and morphology of the corrosion is studied, the nature and concentration of elements in the patina is checked, the coherence of the traces left by the manufacturing methods are contrasted... well, the whole nine miles, if done properly. This is, also, somewhat expensive. Some items are not worth enough (in market value) to have them go through such exhaustive testing.

In short, there's no simple answer. There's the means, indeed, but, as always, in many occasions no easy way is available. The best bet, for a change, is experience. Becoming acquainted with the basics of the corrosion processes, with the most usual accelerated corrosion and overall faking techniques and the results they yield, with the details of historical manufacture and the features they leave in the final product... in short, as always, educating oneself.

Hope this helps…

[Michael Blalock]

Thanks for all the information. Here is a photo of the little fellow I am talking about. He came from Yemen and has been handled a lot for the last 35 years so some of the patina has worn off, especially on the nose were bits of the copper alloy are showing through.