Jack Hanson is an undergraduate at Queen Mary, University of London, about to start his dissertation on the Third Crusade. He has been volunteering at the Museum on a British Numismatic Society Bursary.
When the coins minted by Philibert de Naillac (r.1396 – 1421) were brought out from the expansive coin collection in the Museum of the Order of St John, I noticed that a number had become rather tarnished. Unlike other silver coins held by the Museum, the coins display a range of bright colours, rather than the black expected from usual silver tarnishing. The impressive array of colours exhibited by these coins range from a hint of blue or yellow, to purple and green.
Philibert de Naillac served as Grand Master of the Order of St John, whilst the Order was residing in Rhodes. It is a mystery as to why some of the coins minted by him are presenting such a spectrum of colours, and why some have not tarnished at all. This may be linked to their materials, or to how they have been treated, cleaned and stored.
Tarnishing
Silver tarnishing tends to be black, particularly after an extended period. Given that these coins are nearly six hundred years old, it would be expected that the process of tarnishing would be complete by now. However, some of the coins are in pristine condition, or as pristine as you can hope for after six centuries.
The origins of the coin are easy to pin down, being minted in Rhodes. Yet, as to what happened between their minting and arriving in the Museum several centuries later is unknown. The coins of the Museum are now all kept together in the same environment, and whilst some of the coins minted by other Grand Masters have tarnished to black, many similar coins show little sign of exhibiting such colours.
Nor do the coins all display the same colours; some are just blue, others range on a spectrum of blue to yellow and green. The most spectacular of the coins is a combination of yellow, blue and purple. If they were all made of the same material, it would be expected that the colour of the tarnish would be consistent across the collection.
It is not easy to tell why some of the coins have tarnished, and some have not. At least two-thirds of the coins are exhibiting the tarnish. This may reflect their origin, being debased with different metals at different times, or how they may have been treated, cleaned and stored.
Other Materials?
According to D.M. Metcalf, the coins minted by the Order of St John in this period were made of silver. Whilst silver does occasionally go blue, it does not go all the way to explaining the vast array of colours demonstrated by these particular coins. It is possible to Naillac used other materials in his coins, in order to make the silver content stretch further. Other metals in the mix, and the possible impurity of the silver, could explain why some of these coins have reacted so strangely.
The methods used to test metals can be destructive or non destructive, but to test such a large number of coins would require very careful thought, especially considering the array of colours demonstrated.
Cleaning process?
Some of the colours created by the tarnishing look rather like an oil spill. This suggests that the tarnishing could have been caused by something used to clean them in the past. It is possible that the different chemicals used to polish the coins over the centuries have had an effect on the colouring of the metal. This would also explain why some coins exhibit colouring and others do not. They may also have been cleaned differently depending on who owned them and at what time they came into the Museum collection.
[R9/4 This coin’s tarnishing is almost completely green. The colour appears most in places where fluid might linger.]
Some of the coins also have different colours on either side. As they must be the same metal throughout, it suggests that other factors have influenced the colouring. The colours are also not uniform across the coin, some parts are more blue or yellow than others. The colours tend to conglomerate in areas of deeper relief.
Conclusion
It is difficult to determine with certainty how these coins have ended up so colourful. Whether the metallic rainbows are due to a particular material used in the minting process or afterwards, the colouring is extremely unusual!
If you can tell us how these coins have ended up this way, and what may be causing the unusual colouring, please tweet us @StJohnsGate, or email as at museum@sja.org.uk