Silvered coins - How'd they do it?

[Danny S. Jones]

I just read a thread under "numism" on the topic of silver wash on late Roman coins from back in 2002. There were several theories presented as how the coins were silver coated, but there seemed to be no consensus on the topic. I wonder if there has been any new information revealed in the past seven years to shed some light as to the actual process of silvering AE coins during the late Roman Imperial times... pickling? dipping? chemical electrolysis?

You can view the old thread on this topic here:
https://www.forumancientcoins.com/board/index.php?topic=446.0

[Paul D3]

These coins were made in such large quantity that I am skeptical that any processes such  as electrolisis, pickling etc. could have been used. I am convinced that the silver migrated to the surface on its own, shortly after the strike, through a process called precipitation. I am currently consulting my old college materials science texts as well as other books. If I can come up with some math and chemistry to back up my beliefs, I will post them.

[Will Hooton]

These coins were made in such large quantity that I am skeptical that any processes such  as electrolisis, pickling etc. could have been used. I am convinced that the silver migrated to the surface on its own, shortly after the strike, through a process called precipitation. I am currently consulting my old college materials science texts as well as other books. If I can come up with some math and chemistry to back up my beliefs, I will post them.

Your post reminds me of a test for the presence of aldehydes I used to do in my Organic Chemistry major. Tollens Reagent, when oxidised, forms a precipitate of silver on the surface of the test tube or glass it is in contact with.
Indeed, the Victorians used this process to make mirrors. Was it known to the Romans however?

[cliff_marsland]

This is a good question - I've always wondered that myself.

Why is silvering on Julian I AE1s and Victorinus and Carausius Ants so rare?   It's of the greatest rarity that I see even the trace of silvering on AE1s for sale.

[PeterD]

There are two distinct technologies here.

The first is when coins still contained a reasonable amount of silver -maybe 20% or more. When flans were made, the base-metal on the surface oxidised while the silver didn't. The flans were then cleaned in an acid, such as vinegar. The oxidised base metal dissolved, leaving the surface of the flan silver-rich. This was probably an accidental discovery as flans were probably always cleaned in acid anyway. For a full description and photographs of disected coins see 'The Silver Coinage of Sepimius Severus and his Family (193-211 AD)' by Haim Gitler and Mathew Ponting (the subtitle is ' A Study of the Chemical Composition of the Roman and Eastern Issues').

Clearly when silver levels went even lower, there was insufficient silver to allow for surface enrichment. Silvering had to be added afterwards. It is here that I think no agreement has been reached as to how this was done. On another thread hidden in the archives I described a lecture that I went to where a paper was presented suggesting that mercury amalgam was used. There are plenty of theories out there, though.

[Robert_Brenchley]

A lot of 3rd Century couns are surface-enriched. That becomes obvious when they've got a bit of wear, and what was originally the interior of the coin shows pink by comparison. Silvering, though, was done on coins with so little silver content that it wouldn't work. They have to have made the coins, and then added a layer of silver somehow.

[Danny S. Jones]

They have to have made the coins, and then added a layer of silver somehow.

The "somehow" is what I'd like to know. A research article I found (link below) mentioned silver foil plating of bronze or copper core coins.
http://www.itarp.uiuc.edu/atam/research/archaeometallurgy/romancoins.html

The article cited the book:
Zwicker, U.; Oddy, A.; and La Niece, S. Roman techniques of manufacturing silver-plated coins. In book. Metal plating and patination. La Niece, Susan and Craddock, Paul T., Editors (1993), pp. 223-246, [English]. [ISBN 0750616113].

Does anyone have access to this work? (it is out of print.)

[PeterD]

I have discovered that the article that I referred to in my post above is still available on my web-site:

https://www.forumancientcoins.com/historia/Silvering%20on%20Roman%20coins.pdf

[Danny S. Jones]

Silver-mercury amalgam plating that is presented the paper is a very interesting theory, and a possibility. But as quoted in the paper, it is "possible that mercury was impurity in the silver ore, or it might have been used in the refining process."

I'm beginning to wonder if we'll ever know the actual process used with any degree of certainty.

[Robert_Brenchley]

If mercury was present in the ore, or was used in refining silver, I'd expect to find it in other Roman silver of the period in the same proportion. That would be easy to check.

[Danny S. Jones]

What would not be easy to check would be whether or not any mercury found was:
   1. already present as an impurity in the ore
   2. present because it was used in the refining process
   3. present because of silver-mercury amalgam plating

The only way, I suppose would be to examine the core of several solid silver coins, checking for the presence of mercury. If found in most or all pieces, it could be assumed that the Hg was present because of one of the first two possibilities, and would contaminate the results for any Au-Hg amalgam testing.

[PeterD]

In 'BM Occasional Paper 120, Metal analyses of Roman Coins minted under the Empire', each table has up to 12 columns for each of the elements. None of them are labelled Hg. This covers coins of all types including silver ones. That doesn't really prove very much though.

[Danny S. Jones]

In 'BM Occasional Paper 120, Metal analyses of Roman Coins minted under the Empire', each table has up to 12 columns for each of the elements. None of them are labelled Hg. This covers coins of all types including silver ones. That doesn't really prove very much though.

It may not be proof, but evidence against the Au-Hg amalgam process, at least in the coins examined.

[Robert_Brenchley]

Depends what the coins were!

[Thomas A2]

The process of getting silver up on coins in large batches isn't difficult.You mix copper and tin, add the right percentage of silver and form into disks, heat and drop into fruit acids to quench.  Now what happens is the acids dislodge the impurities and draw the silver to the surface.  Now you reheat them and start hammering.  The heating them again and strile, the silver would come up granular and when struck the silver would spread and meld, giving the coin it's silver apearance and it would last a long time too.  The problem is if you put too little silver in and it doesn't come to the surface.  You can prove it yourself when you take a silvered piece and use tarnex on it, the silver will float away in crystals since it's likely somewhat become horn silver.  Native South American Indians used the same method to create gold pieces that were near 24 KT near the suface, but below was mainly a copper/tin mix, not quite bronze, but not pure copper.  Fruit acids can be created like vinagar, spoilt apple, and berry juices, if mexed in proper proportions can give you the ideal chemical reaction and easy to get in an era where excess was not uncommon and spoilage was high due to a lack of refrigeration.

[PeterD]

There is no great argument that coins were surface enriched in a similar manner to the way you describe. Lawrence Cope described the process back in 1972. However by the time of Gallienus the coinage was increasingly debased so that by the reign of Claudius II the antoninianus contained as little as 2% silver. Even after the reforms of Aurelian there was still only around 5%. Therefore the silvering must have been applied externally. How this was done is not known but there have been a number of theories including the one referred to above.

[dougsmit]

There was no one 'they' to do it.  Coins were silvered in many mint cities for a long period of time.  Technology was not invented in Victorian England despite what you may have learned in school.  As we study silvering, lets keep in mind that what Gallienus did in Rome and what Julian did in Antioch might not be exactly the same. 

[Danny S. Jones]

There has been some speculation that an ancient form of electroplating was used in antiquity at times. Whether or not this is true, it brought a question to mind about modern forgeries.

With the relative simplicity of electroplating techniques, are there any modern forgeries found to be of base metal that have simply been electroplated with a precious metal? And how easy to spot (or how dangerous) would an electroplated forgery be?

I've always wondered about "silvered" LRBs, if some might have been subjected to modern electroplating.

[Romanorum]

In my modest, uneducated opinion, there is a high possibility that all of the above-mentioned silvering methods were used. Silvering appears to be quite different throughout different periods. All of you probably have seen quite radical variations in coins you have. One of the most interesting contrasts I have found among my coins is a Licinius II whose silvering, despite the spottiness, looks quite solid, shiny, and under a microscope looks "thick", if you will; and it's quite different from, say, a Tacitus antoninianus which looks dull in contrast and appears to have a coating of silver paint on it. As Mr. Smith mentioned, presumably the methods varied from mint to mint, from one century to the next, and it is quite possible that different accidents gave way to different experiments being performed on silvering. Whatevere the method, I'm still pretty impressed that the Romans came up with it.

[Danny S. Jones]

So basically, nobody really has any earthly idea as to what method was used in any era of Roman coin making? It sounds like everyone is guessing. Maybe some guesses are better than others. Some may just be reaching, but not finding much. In any case, they are all just guesses. It would be great to see some hard evidence to support the theories given here.

Alas, we may have to settle for ignorance on this matter.

Danny

[Romanorum]

Yes, there's much speculation, and some of us non-experts just share a bit of whatever little we know. However, many studies have been done on different aspects of the subject. Peter D, above, mentioned Cope's work.  There are many others out there. Here's the abstract of an article published in Microchimica Acta, 2006, by Kraft et al., entitled "EPMA Investigation of Roman Coin Silvering Techniques":
"Ancient Roman silver coins, especially forgeries, contain various amounts of silver on the surfaces. The state of preservation of the outer layer can vary strongly among different areas of the same coin, thus an investigation of the entire surface is indispensable. The measurement of the silver distribution is a first step towards uncovering the manufacturing techniques. Element mapping by electron probe micro-analysis (EPMA) with increasing step size allows a survey of the complete coin surface. The suitability of this approach is shown and applied to coins having a dark optical appearance and unknown minting technique."

The article by Vlachou et al. mentioned in the Forum back in 2007, is quite interesting. Here's the link for the pdf:
www.springerlink.com/index/QHH1074266775418.pdf

So there are new research techniques still being developed to clarify the matter, and the quest continues. But, as I said earlier, there are obviously many different ways of silvering coins, and the final chapter is yet to be written.

[Genio popvli romani]

I am very interested in the technical issues concerning the late Roman coinage and I am particularly obsessed by the issue of  silvering process of Constantinian bronzes or more exactly post-Diocletian reform bronzes. In fact, this is one of the first successful examples of industrial metallurgical process never reached to such an extent and will not be used until very recently with such mastery.
The silvering process by using amalgam sounds convincing, especially when you consider the amount of coins to produce to try to halt the recurrent lack of “bronze” currencies.
But, the 1/5 ratio Ag / Hg amalgam would tend to consider the use of significant quantities of mercury that we should trace (not very bio-degradable) near mints since only a small amount remains on coins. It is also likely that the evaporated portion was recovered and recycled, if only by economic worries which begs the question: Was mercury cheaper than silver that such use was economically viable?
Another question arises and that might allow to advance in the hypotheses : Can we clearly identify if silvering was filed before or after the stamping?
A clue could be given by noting the presence of silver in the flans cracks caused by the deformation due to the stamping. But those gaps are the most conducive location to corrosion.
Or, we should find some of these coins which would have suffered an non perfect workmanship that gives us clues.
I had in my possession such coins that were obviously defective. So I decided to remove the patina to see more clearly what was going on down there. The result is indisputable, it is clear that the  silver deposit was filed after the stamping but was not spread properly over the coin so that on the VLPP, the silvering "submerged" reliefs and the on Constantine II coin, the deposit has accumulated in places. On this coin, it is clear that if the strike had taken place after the silvering, reliefs would appear on the silver which is not the case, on the contrary, the deposit covers the reliefs.

Constantine I nummus, Trier mint, AD319, RIC321


Constantine II  nummus, Trier mint AD321-323, RIC312 or 353 or 382

I am curious to know your opinion and hypotheses about these coins.

[Genio popvli romani]

First, a few details. During this period, it is not impossible that about 100 million nummi were struck a year, nothing to compare with non-official or official sauced coins struck from Republic to Diocletian. Those coins were minted from London to Alexandria and were used everywhere in the whole Empire. So, they had to be nearly identical from a mint to an other and for evident economical reasons the amount of metals (silver and copper) had to be a very accurate thing. The word "industrial" appears to be the one to use to depict the silvered bronze coinage producing process.

Then a few pictures. I tried the dipping method, you can see the result below. I agree that I am not a metallurgist expert and an expert can have better results but what is clear is that this method "softens" reliefs and looks like a  thick coating.

Magnified reverse, please have a look at the crack which is very softened by the coating.

Two coins coated with 1700 years old silvering, one from Trier mint and one from Alexandria. You can see how thin is the silvering and how we can see accurate details compared to the previous coin.

What is interesting with this coin is to compare the crack to Helena's coin, keeping in mind that in the two cases, silvering had been done after stamping.

 I took a better picture of the coin I had previously post that shows the "post-stamping" silvering process.

[Genio popvli romani]

Just a detail I forgot, what would have been the interest of introducing up to 5% Ag in the copper core if there was no technical issue? And as far as I know, dipping process don't require such a rate to operate.

[daverino]

I recall a study using x-ray elemental analysis of coins in which the silvering always correlated with the presence of mercury so I have assumed that these were made by use of an amalgam. If there is enough silvering left to see it was likely done this way. This is not a simple process but with slave labor and not having to worry much about mercury toxicity I suppose it was practical on the large scale. Still, many antoniniani seem not to have ever been silvered. Were these less valued than the silvered types? Or were these also silvered but by a more fugitive method such as by immersing a copper coin in a solution of a silver salt. Initially they would have a silver surface but it would be too thin to last over the centuries.

It seems probable that the Romans would have been aware of this passive plating method. On the other hand I think it unlikely that they used  electrochemical plating which can build up thick and durable layers of metal but requires a technology that didn't come about until the 18th century.