METAL ANALYSIS OF TWENTY
TETRARCHY ARGENTEUS and TWENTY
FOLLIS( A very long post)
I recently
had some
Tetrarchy coins analysed for their metal content. Two groups of coin were tested. The first group was 20 GENIO POPULI ROMANI post reform
billon follis, with assorted levels of surface
silvering still visible, from seven mints (
Treveri x 5,
Heraclea,
Alexandria x 8,
Londinium,
Cyzicus x 2,
Thessalonika and
Antioch x 2), all struck between 295 and 306 ce. The second was 20
argenteus from nine mints (
Treveri x 3,
Rome x 7,
Alexandria,
Siscia x 2,
Cyzicus,
Thessalonika,
Antioch x 3, Aquilea x 1and
Nicomedia x 1), all struck between 294 and 302 ce.
I needed the analysis to be non-destructive ( they were my coins), and couldn't get access to the most sophisticated methods that can penetrate right through the metal, so I settled for a spectroscopic analysis which penetrates about 1.3 mm into the surface. The analysis was done professionally, using a properly calibrated DELTA Premium Handheld XRF Analyser, which is primarily used in the
field for mine ore assay
work, and for the technically minded uses a 4W Rh, Au, or Ta anode X-ray
tube as it's excitation source. The machine claims a penetration of up to 2 millimetres into the metal but the technicians running the test said that in reality about 1.4 millimetres is more realistic.
The current literature says that
average silver content of
a post reform
tetrarchy follis between 293 and 303 ce should be about 3%, and that the
argenteus had a silver content of over 90%, averaging at about 93 %. The raw data from the testing, and a detailed description of each coin tested is in the appendix. The sample size is less than I would have wished for, but it seems worth posting the results and my tentative analysis, as there doesn't seem to be much around recently on this topic, and the relative values of
Tetrarchy Aureus,
Argenteus,
Follis and Aurelianii is a hotly debated subject .
RESULTS
1) The 20
billon follis (
nummus?) GENIO POPULI ROMANI coins.
All the coins tested were confirmed as copper/silver
alloy with trace tin and lead, the copper content ranging from a high of 87.25% to a low of 64%, and the silver from 16.82% down to 3.4%. here was also quite a high lead content, from a low of 1.14% to a high of 17.18%.
The mean silver content was 7.94%
per coin , and copper 80.22%
per coin. There was also a mean of 5.6% lead and 2.49% tin in each coin.
I knew that the results would be of limited use in determining overall silver content for the
billon coins because of the different composition due to
silvering near the surface, and I am aware of the statistical issues with such a small sample but I was interested to see if the least well preserved coins, that
had no remaining visible silver and
had been worn down so that the spectroscopic analysis was of metal closer to their core, showed a silver content that approximated to that predicted in the literature. I was also interested in differences between the coins and if I could find any pattern, and expected the
follis in general to show comparative silver contents roughly proportionate to the amount of visible
silvering. Of the twenty coins tested, five
had full
silvering and the features on the coin were in EF condition so they
had lost little if any of their original content, six
had what I have called half
silvering left , four
had some traces of
silvering but not many, and five
had no visible
silvering at all and were well worn down.
The first hypothesis I tested was that the coins with most or all of their
silvering visible would show a higher silver content with this
type of analysis than those with less visible
silvering. I was surprised to discover that at least with my sample, they did not. The five coins that
had the best visible
silvering and were in the best condition
had silver contents of 4.47%, 3.59%, 13.25%, 9.48%, and 6.12% respectively for an mean
average of 7.38%. The six coins that
had some but not all their visible surface
silvering intact ( the ones I called 'half-silvered')
had silver contents of 8.68%, 16.82%, 10.49%, 11.25%, 9.61% and 8% for an
average of 10.81%. The four coins with bare traces of
silvering left
had silver contents of 8.73%, 3.64%, 6.76% and 8.91% for an
average of 7.01%. The five 'worst' coins, with no
silvering left at all, well-worn and VF at best,
had silver contents of 7.07%, 6.49%, 4.65%, 7.44% and 3.4%, for an
average of 5.81%.
Some coins with full
silvering left on them were showing silver contents half as high as some completely denuded of all visible surface silver. The highest mean
average silver content was shown in coins with half their surface visible silver denuded. The mean
average silver content was almost the same for fully
silvered and barely
silvered coins. On balance, there is not much discernable co-relation between the visible
silvering on the surface of the coins and their silver content. That content differed widely, but not directly proportionate to visible
silvering. The amount of silver in a
tetrarchy coin is not related to how 'silvery' it looks, so whatever method was being used to give the surface
silvering, it was not a layer, however thin, of pure silver.
Next I looked at the data by date of striking. Some of the coins are difficult to tie down to a specific year, so I have divided them into three broad categories. Seven coins were struck after 294 ce ( obviously, because that is when the reform happened and they started to be produced) but before 300 ce. Seven coins were definitely struck in 300 or 301 ce, and six coins were struck between 300 ce and 307ce ( it is possible some of the last group were struck in 300 or 301 ce). The proportions of fully
silvered, half
silvered, barely
silvered and no longer
silvered coins in each of these groups are well distributed (See chart 1), making it unlikely that the results are a reflection of visible or coin
quality.
CHART 1.
296-299 ce 300-301 ce 300-306 ce
Fully
silvered 1 3 1
Half
silvered 2 1 3
Trace
silvering 2 1 1
No visible
silvering 2 2 1
The
average silver content in the early group (296-299ce) is 8.42%, with a high of 16.82% and a low of 4.47%.
Average copper content is 80.216%. The mid group (300-301ce) has an
average silver content of 8.99% with a high of 13.25% and a low of 6.12%.
Average copper is 82.49. The late group ( 300-306ce) has an
average silver content of 6.15% with a high of 9.61% and a low of 3.4%.
Average copper for this group was 79.92.
The copper content is consistent over all the groups, with the mean for all three groups within 3% of each other in copper content at around 80%. The silver content varied by much more. The coins struck in the late period
had on
average 30% less silver in them than the earlier groups.
There does seem to have been a general debasement of the silver content in the coins struck after 300 ce. Six of the seven coins in the middle group ( 300-301ce) come from one
mint,
Alexandria, which as I discuss below, may be a more important factor than date of striking, but the debasement applies equally when compared to the earliest group, which
had a
good spread of mints.
The last arrangement of the data I made was to look at metal content by
mint. Many of the mints are represented by just a single sample, but it seemed worthwhile to at least try and separate by
geography as well as condition and date of striking. Two mints are well represented, with eight coins from
Alexandria and five from
Treveri. There are two coins each from
Cyzicus and
Antioch and one from
Londinium,
Heraclea and
Thessalonika.
Two of the mints (
Treveri and
Londinium) are in the
Roman west, Maximian's territory rather than Diocletian's. Six of the coins were struck in those two mints, the remaining 14 at mints in the Eastern Empire governed by
Diocletian. The six western coins
average silver content is 4.74%, the eastern 14 coins
average is 9.31% , more than double. The difference cannot be explained by coin
quality, amount of visible surface silver, or the dates the coins were struck, as there is a
good spread of
quality and striking dates in both Western and Eastern mints. ( see charts 2 and 3)
CHART 2.
Western mints Eastern mints
Fully
silvered 2 3
Half
silvered 6
Trace
silvering 2 2
No visible
silvering 2 3
There is also a
good spread of dates in both groups, ( see chart 3)
CHART 3
Western mints Eastern mints
296-299 ce 3 4
300-301 ce 7
300-306 ce 3 3
The five coins with the lowest silver content of the entire sample are all from the Western mints. Two of those are fully
silvered EF coins, so Maximian's mints were striking inferior coins to Diocletian's, even though they looked identical. Not all coins
had mintmarks on them and there must have been practical problems in having identical looking coins with such large differences in intrinsic value. The literature predicted a lower silver content in Western minted coins, but the difference is startling and occurred from the very beginning of the minting of the new reformed coinage.
The Western mints also seem to be using more tin in their alloys. The mean tin content
per coin from Western mints is 3.4%, while in the eastern mints it is only 2.09%. The five coins with the highest tin content were all from Western mints, perhaps lending some indirect evidence for the importance of the Cornish tin mines, which would have been much more accessible to a Western
mint than an Eastern one. It would be really interesting to test the tin content of various coins during the earlier ten years of
Carausius and
Allectus Gallo-British revolt.
The lead content of the coins varies more than most of the other metals. There is a mean content of 5.6% lead
per coin, but it varies from a low of 0.97% to a high of 17.18%, and there is no apparent correlation based on coin
quality, date of striking, or the
part of the Empire the coin was struck in. There is one very clear pattern. The
Alexandria mint used much less lead in its alloys than any other
mint, Western or Eastern. The mean lead content from the eight Alexandrian coins is 2.11%, while for the other 12 coins from six different mints the mean is 7.97%, close to four times the lead content. No coin from any other
mint has a lead content of less than 3.6%, and the seven coins with the lowest lead content ( all below 2.7%) are all in the Alexandrian sample.
Alexandrian coins also
had a comparatively high silver content, at 9.14%, well above the mean of the coins from all the other mints of 7.14%. It appears from the data that
Alexandria was using a completely different recipe for their
billon alloy than anywhere else. They
had only just stopped minting their own currency based on a
billon tetradrachma and started using
Roman denominations. It would be interesting to analyse some
Egyptian Tetradrachmas from the late third century ce to see if they also
had a very low lead content.
The sole coin from
Thessalonika, with about half it's surface silver
still visible, has a decent silver percentage at 8.68%, but a completely different
alloy mixture with only 66% copper and a lead content of over 17%. Unfortunately a sample of one has no statistical validity but it would be worthwhile to test a few more
Thessalonika Follis if I can get my
hands on them.
2) The 20
Argenteus coins.
These coins do not have a surface coating, and are supposed to be silver all the way down, so in theory spectroscopy should give a more accurate picture of the metal content than for the
follis. The coins analysed range in date from 294 ce to 302 ce. Nine mints are represented.
Rome has seven coins,
Treveri and
Antioch have three each, and
Siscia has two.
Alexandria,
Cyzicus,
Thessalonika, Aquilea and
Nicomedia are represented by one coin each. Unlike the 20
alloy coins, all the
Argenteus show very similar characteristics. The mean silver content is 96.65% which is higher than the literature predicted. There is very little deviation from that norm, whether the coin was struck in 294 or 302 ce, and whatever
mint it was truck in. The coin with the least silver content ( struck in
Treveri between 295 and 297 ce) has 95.33% silver, and the coin with the highest ( struck in 302 ce at
Thessalonika) has 98.69%. Fifteen of the coins fall within one percentage point of the mean. The one Western
mint,
Treveri still has the two coins with the lowest silver content, and three of the lowest five, but the debasement is far less, at barely 1%.
The other metals in the coins, at least from this surface analysis, are comparatively insignificant, with no other metal in any of the coins at more than 1.6%. There are trace amounts of Tungsten, Iron and copper in all of the coins, and trace amounts of lead, Alumina and sulphur in most of them. For most intents and purposes, the
Argenteus was a pure silver bullion coin, minted at a carefully controlled
weight of 96 to the
Roman pound advertised (at first) on the coins
reverse.
If anyone looking at this has any suggestions, criticisms or corrections I would be delighted to hear them. I am very much feeling my way into this. My email is hposner@bigpond.net.au
Howard Posner. August 2014.