Van Arsdell 2015d (Info)
Could Kent Kings Cast Coins?
2: something you don’t normally see.
By Robert Van Arsdell
Derek Allen analyzed the coin above and never mentioned the red spot (info). Commander Mack bought the coin after 1973, but never wrote anything about it. I catalogued the same coin in 1989 and never mentioned it, either (info). I’ve always regretted missing the opportunity to meet Allen and Mack – and wondered what they thought about this enigmatic red spot. You see…
It goes through the coin.
When viewed with a low magnification loupe (7x -10x), it vaguely looks like a spot of corrosion. Red corrosion products of copper and tin do exist, but normally you would expect something green.
However, viewed in stereo at 25x to 90x, the material no longer looks like a corrosion spot – it’s clearly something very different.
It’s now determined to be a piece of hardened clay that’s embedded in the coin. A stray lump of soft clay must have become trapped inside the mould as the two halves were assembled for casting. It became embedded in the coin as the molted metal flowed around it and froze. The clay hardened at some point either as part of the casting process or perhaps after the coin was removed from the mould.
I believe this lump is the evidence of clay moulding material that Allen asked archaeologists to look for in 1968 (info). It provides reasonable evidence that the Cast Bronzes were indeed cast in clay moulds.
The material is trapped inside the coin: the metal was cast around it. On the obverse, the metal rises over the edge of the red spot.
A small piece of the inclusion peeks through the metal at the red arrow. A thin “bridge” of metal covers the embedded clay at the white arrow.
The same is true on the reverse – the metal captures the clay.
It must have started as a lump of soft clay, trapped in the mould when the two halves were put together for casting. When they were pressed together, the lump was squeezed into the image of the bull’s tail on the reverse. The hardened clay completes the image of the tail of the bull.
It’s not a cold-shut filled with clay
One might claim that the hole through the coin is just a “cold-shut”, a place where the molten metal froze before it could completely fill the mould. The hole would then be filled with clay after the coin was buried, producing a “plug” when the clay dried. However, this is not the case, otherwise, the clay would not complete the image of the tail.
Furthermore, cold-shuts have the wrong profile – they wouldn’t produce the kind of inclusion seen on this coin.
This example of V131-01 has a fairly typical cold-shut. It has an hourglass shaped section.
On both the obverse and reverse, the cold-shut tapers inward as you go deeper towards the center of the coin. If such a hole were subsequently filled with clay, the clay would lie above the surface of the metal at the edge of the spot (the clay would fill the hourglass and capture the coin).
Kinds of coins expected to have these inclusions
The V108-03 is an example of an Experimental Period coin. These coins and also the ones produced in the Innovative Period (info) are marked by a multitude of casting mistakes (info). We have now identified a new kind of mistake: capturing some of the moulding material in or on the coins.
This error now offers the evidence Allen wanted. It would be more useful to have the actual moulds, but a sample of the moulding clay is the next best thing.
A petrologic analysis of the red spot could yield the following kinds of information:
- It could determine whether the clay was simply dried or was fired before the mould was used. The copper-tin alloy melts roughly at half the temperature needed to fire clay (it’s a simple kind of silver-less solder) so the casting process wouldn’t likely have fired the clay itself.
- It might identify a general region for the source of the clay (is it Wealden clay, or is it French clay?)
- With luck, the exact source of the clay might be identified (though Wealden clays vary in composition, this is probably asking too much)
- The coin could give additional information about casting techniques. Were the clay moulds fired individually, or were they fired as an assembly? If the inclusion was fired, the moulds were probably fired as an assembly. There is some indication the inclusion may have distorted the mould itself, as it filled the tail of the bull.
- Does the clean separation of the inclusion from the mould indicate the moneyers used a mould-release agent?
At this point the matter must rest for the V108-03 coin. It is currently a unique example and destructive testing is out of the question. A testing program must be formulated to analyze the coin.
I know that this is not the only coin that exists with residues on it or inclusions in it. It may be possible to identify other coins to analyze. To do this, we have to look at the way coin conservators have made the task difficult.
Three Coin Conservation Styles
Though many Cast Bronzes have a glossy brown surface; this is nothing like the original appearance of the coins. The copper/tin alloy (a simple kind of solder) is normally dull gray. The surface becomes corroded over time in the ground and changes colour. Coin conservators are then faced with several choices when cleaning these coins.
The first coin (V108-01) demonstrates the conservation technique least commonly used. The surface of the coin has been chemically stripped to reveal the raw metal underneath. This was then protected with a clear coating (something like clear nail polish). The resulting coin looks roughly as it did when it left the mint.
This conservation technique was used for a short time in the mid 1950s (info). It is the best one of the three because it shows the surface of the coin and reveals any anomalies. The other two techniques mask the imperfections on the surface and prevent proper inspection.
The V117-01 coin demonstrates the second technique. It’s been cleaned to remove the dirt and then lightly lacquered to protect the surface. The surface oxidation hasn’t been chemically stripped. Although this technique produces a pleasant appearance, you can’t really see the metal, and surface imperfections can’t be spotted.
The V104-01 coin has suffered the worst of the conservation techniques. The surface has been cleaned, but the oxidation layer hasn’t been stripped. The coin was given a thick coating of lacquer to hide problems on the surface. This shiny, thick coating hides everything – you can’t see the surface of the metal at all.
When viewing the V104-01 coin closely, one can see places where the lacquer is breaking away. Something lurks beneath, but there’s no possibility of determining what it is. Analyzing this coin with a stereomicroscope is a waste of time.
One wonders what the conservators were tying to hide. The lacquer should be removed and the coin re-conserved to reveal the metal surface.
Finding coins to analyze
In general, if we want to identify inclusions in Cast Bronzes, we need to look at coins that have been stripped and clear-coated. Lacquered coins will reveal nothing.
It’s best to look at coins from the Experimental and Innovative Periods, because those coins exhibit the greatest number of casting problems. Optimization Period coins don’t have inclusions in the metal. Most of the casting problems they do have involve cold-shuts.
Thus the best rule to follow to find coins with inclusions is: analyze Experimental and Innovative Period coins that show the dull grey of the original metal. The coins should be inspected in stereo at magnifications over 25x.
And remember…. YOU CAN’T WORK FROM PHOTOGRAPHS!
An Experimental Period coin with possible inclusions
The V108-01 coin appears to have areas of red encrustation when viewed at low magnification (7x – 10x). However, when viewed at 25x – 90x in stereo, the material shows significant differences.
On the reverse, near the bull’s rear leg, most of the encrustations lie on the surface of the coin. These could be residues of the casting mould, or they may be nothing more than clay that attached itself to the coin in the ground. The blob at the bottom left looks suspiciously imbedded in the metal, however.
To the right, blobs of material above and below the bull’s neck may be embedded clay. This V108-01 coin warrants further study.
An Innovative Period coin with possible inclusions
This V114-01 coin has many suspicious spots, but they are very small. Any lacquering would have hidden them entirely.
On the obverse, near the neck, there are several red spots. The larger one on the left may be embedded clay.
On the reverse of the V114-01 coin, a tiny red blob on the bull’s neck could be embedded clay.
On the bull’s tail, a larger red blob possibly retains metal above the clay. This is another coin that warrants further study.
The next steps
Clearly, non-destructive techniques will have to be employed for analyzing these small residues. We need to know the kind of clay involved. We will also need analyses of clays with known sources for comparison.
While these techniques are being sought, a search for other coins can be conducted. A stereomicroscopic analysis of the Sunbury Hoard would likely repay the time invested. Other examples could easily lie undiscovered in museum and private collections.
The current problem is that all the coins identified so far are located far from the experts that study British and French clays. This is not an insurmountable problem, but it’s not as simple as it once was. In days past, the coins could simply be mailed to the experts for analysis, but this has been made much more problematic by export/import restrictions today.
One more detail
When looking at coins that have been clear coated, it’s important to keep in mind that the coating will make any clay look darker and more deeply coloured than natural clay.
The photo shows a Roman floor tessera made of clay. The tessera comes from Hampshire and was presumably made of clay sourced in southern Britain. The arrow points to a darker inclusion in the clay. One half of the inclusion and its surrounding area have been coated with clear nail polish. The tessera has been lighted so the shiny surface of the coating appears as a white reflection, just beyond the point of the arrow.
Upon magnification, and lighting the coated surface to eliminate the surface reflection, the effect of the coating becomes visible. The coated area appears above the arrow as a rectangular dark band running diagonally. The dark inclusion in the tessera is the circular patch just to the upper right of the arrow’s point. Both this inclusion, and the overall surface of the tessera appear darker and more deeply coloured as a result of the coating.
The caveat is that any clay material on a Cast Bronze that has been clear coated will appear somewhat different from normal clay (so don’t compare a candidate to a brick and reject it!). Consider any reddish or orange material on a Cast Bronze as suspicious, and worthy of further study.
This may be one study in which many hands make light work.