Tuesday 29 October 2013

Analysing the Leutwitz Apollo (9): Lead Isotopes

Now we come on to the really interesting results. The final series of analyses reported by Bennett in his 2013 book were carried out in June 2005. It was then that Prof. Ernst Pernicka of the University of Tubingen did an analysis of the lead isotopes in the solder "used to join the figure to the base" from four samples supplied to him from the museum, two from the baseplate (?), one from a "toe" and one from "Apollo". Of course Bennett's book provides not a smidgen of information where those samples had actually been taken from. Depending on how you interpret the other analyses discussed so far, there is a distinct possibility emerging in my mind that the solder on the foot of the statue might not be the same as the solder on the base plate.  Bennett (p. 59) says that the results of Professor Pernicka's research show that the lead in the solder "was produced slightly less than one hundred years ago [Ernst Pernicka, "Analysis Report 05-12" June 4th 2005, CMA curatorial files]":
His isotope analysis of four samples of the solder showed that the figure must have been attached to the base nearly a hundred years ago and was therefore outside of (sic) its archaeological context before that time, and probably long before that time".
That statement seems ripe for analysis (initially let us leave aside the question of how this analytical technique works). Let us note in passing that it is not stated whether all four produced exactly the same results.

The first question of course is what is there in the analysis discussed here which allows Bennett to say - apparently on its basis - "and probably long before that time"? Is that in Pernicka's report? If it were to be published, is that what we'd find there? Because if it's not, by what right is Bennett adding it here?

The next question I'd like to learn an answer to is whether there is (or was) a plug of lead in the inside of the foot or feet of the statue. If so, was its isotopic makeup analysed, to 'date' it or whatever, and how does it compare with the solder? Thirdly, we read from the second Northover report that the lead phase of the alloy in places separated out, forming large globules. Was the lead isotopic content of any of these analysed, to 'date' the statue? What about the solder on the "lizard"?

It is one thing to date the production of lead from its ores, and another to extrapolate that date to the use of that lead in a soldered joint. A lead pipe from the renovation of a 1902 villa in Stuttgard could have been re-used by any handyman to make hard-solder in 1930 or 1989. More interestingly if the lead had been obtained from metal detecting the spoil heaps of a lead-production site of a specific locality in order to mislead researchers that the base was soldered on in a specific place (the main use of lead isotope analyses is to establish the origin of the raw materials) and that factory had gone out of use more than 100 years ago, the same effect would be obtained. The date of production of the lead from its ore does not date its use and re-use.

Now for the past two days I have been in correspondence with Dr Pernicka, who has been very helpful and is interested to know how the results of his work were used by Bennett (who seems to have neglected to contact him to run his text by him to check he is not misrepresenting what was said). I will be sending him a scan just after I post this. Unfortunately he considers that contractual obligations with Cleveland restrict what he could tell me on record, but he has been able to tell me a lot which gives grounds for thought. Let us just say that if what I learnt is true, very little significance can be placed on these results due to the way Bennett has presented them.

I challenge Cleveland Museum of Art to publish the FULL report of this key test with all four of the samples properly described and the results of their isotopic makeup detailed. If they have nothing to hide about one of them, then there will be no problems. Or perhaps they would rather not reveal what one of the results actually was, and where it was from? Of course an ideal solution would be to pass the old report back to Dr Pernicka and ask him to edit it in a form which reflects now what he has learnt about the uses to which the results will be put. 

Initially I was a bit sceptical about this isotopic dating method. Lead's natural isotopes are pretty stable and before I wrote to Professor Pernicka (precisely to ask him about this) I could not see how their long halflives allowed dating to such precision. It turns out there is in fact a very neat means of doing this. As Prof. Pernicka notes, perhaps we should not give everything away to the forgers. Indeed not, so I am not going to share with you what I learnt about how this works (its in the specialist publications - which you'll not find referenced by Bennett either).*

*It is a shame the Queens' Rhyton was not tested by it before it went back to Iran.

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