This is a free writing exercise of stuff rattling around in my brain .
After finishing cleaning and organizing the data of all my scans, I spent yesterday creating scatter plots, bar graphs, and pareto histograms. I explored overall patterns for all cast coinages and whether there are noticeable differences and patterns between the different issues factoring in the different denominations as well. I cannot wait to share all of this here, but I think I best hold back on sharing my slide deck until after the conference. Maybe about Feb 20 I can post here.
Today’s goal is to quantify degree of difference between obverse and reverse readings and ensure my audience can understand why this variation exists.
Short answer: Neither lead nor iron create a true alloy with copper. This means the composition of all aes grave is not uniform through out the individual specimen. There is not a strict purity standard that was or could be enforced given the high use of lead.
Most of the understanding of surface analysis techniques within numismatics derives from the flawed studies of Walker on silver in the 1980s and the robust debunking of those approaches using drilling techniques by Ponting and Butcher. None of this is untrue but bronze as an alloy is another kettle of fish and we need to take our lessons and caveats from bronze specialists not silver. Drilling is still the best way to know what was originally manufactured but given the lack of a uniform alloy throughout the individual specimens even drilling means we cannot know for sure our sample is always representative of the whole.
Is Drilling useless then? Absolutely not! The samples taken can contextualized surface readings and be used for deeper analyses, especially of isotopes. This is shown by historical changes that were noticed in the this study.
Westner, Katrin J., Fleur Kemmers, and Sabine Klein. “A Novel Combined Approach for Compositional and Pb Isotope Data of (leaded) Copper-Based Alloys: Bronze Coinage in Magna Graecia and Rome (5th to 2nd Centuries BCE).” Journal of archaeological science 121 (2020): 105204–.
(Sorry don’t think an open access copy is out there but happy to share a PDF if anyone needs one)
They drilled 5 cast specimens as a point of comparison for later coinages. And, they found really interesting differences between cast and struck. Here’s one visual to give a sense of the value of this work:
But being able to drill 5 specimens is a bit deal! [I must write and ask if they are willing to share data.] There is no way to drill hundreds. So this raises questions of the patterns that emerge. So for instance the drilling of Bars in the BM in the 1980s. The two Bull bars seemed a little lower in lead (18% versus upper 20s). Did this mean a different period of manufacture? A different recipe for the alloy? I might have thought so, but no more. All those bars look just like the aes grave I’ve been testing, high lead, high variation. The question becomes whats a typical versus atypical composition and how much variation seems ‘normal’ or with in tolerance.
And, then there is the question of why any of it historically relevant?
Well for one, I can tell you Roman aes grave is far more consistent in composition than the aes rude found in the region with good archaeological provenance. They circulate together but they are not the same thing.
I’m warmed up. Time to get to work.