Highly debased "Silver Antoninianus" and cleaning.

[L.C.Sulla]

Hello - didn't find any past discussion covering this exact aspect:

I personally favor my Silver coins to be as clean as possible. This offcourse as long as they have any SOLID silver look (i.e- not for the coins that are SILVERED/having their surface looking like the typical late Roman silvered coins).

One of my main collection themes are the 3rd century Denarius or Antoninianus and especially the very last Denarius and first Antoninianus. I noticed and read that the Antoninianus types of Valerian I and Gallienus mark a main transition stage in the history of Roman silver coins. I have quite a few, some I bought uncleaned or partly uncleaned. While some Antoninian's have the solid "Silver look" (=the look typical for Gordian III coins), others, once cleaned, have the spongy/grainy surface looks. Strong feeling as if the coin will disintegrate in reaction to further cleaning or pressure. Other coins of the same emperors (like the Animals series of Gallienus) get the typical late Antoninian's full Bronze look and obviously haven't removed any patina of it.  

My question: is there a source discussing any consistent pattern of metal structure of these transition coins? any way knowing, when a coin is covered with patina and dirt how debased it is? what are the changes we get a nice Silver looking coin in contrast to the grainy looking coins you can see in abundance in Forums "Under 40$" category? I noticed some dealers sell Valerian I coins partly covered by the "deserd patina" type of cover, while other sell them cleaned. Same for Gallienus, where many coins the seem to be 40-50% silver are actually sold uncleaned.  

Is my feeling that the Valerian I coins are "disintegrating" gradually correct?

* I placed this question in this forum as it is related to cleaning the coins under discussion.

[PeterD]

Silver coins as they became debased were treated by a process known as 'surface enrichment' which left them silver on the outside but with a silver/base metal interior.

Once debasement had become severe, say an arbitrary 10% silver, the method no longer worked. Thereafter coins were 'silvered'; that is to say they were plated with silver (or silver looking metal) after they were minted. One theory is that it was done with mercury amalgam. Some surviving coins made with this method show a good silver surface; others seem to be left with little or no silver surface.

Certainly that 10% figure would have been reached in the reigns of Valerian and Gallienus but it did vary between mints. I'm not sure if silvering started then or later.

There has been plenty of discussion. Search for 'surface enrichment'  and 'silvering'.

[L.C.Sulla]

Peter thank you for your response. I am aware of the various stages, and different techniques, defining the transition from the Silver coin and ending with the bronze coin washed with Silver. My question was how can we know, or suspect, which uncleaned Valerian I/ Gallienus coin will be "Silver" (or bullion core coin with thick outer layer of Silver, thus stable one),which would have the spongy looking debased coin where, as I read, the Silver content is about 20-30% and which would already belonging to the final stage = bronze coin washed with silver.

From what I've read Valerians coins could belong to either of the 2 first stages while Gallienus's could be either of the 3.

Looking thorough the various selling sites one can usually see there is a certain pattern in that, but I wonder if there are sources that are more specific in defining which types/mints/years define what technique. Specifically for the Valerian and and his family coins which are a certain series I tent to collect and also like sometimes to buy them uncleaned just for the fun of cleaning.

Thanks.

[PeterD]

You need to get hold of a copy of 'British Museum Occasional Paper Number 120 - Metal Analyses of Roman Coins under the Empire'. It has graphs of silver content for different periods and mints, as well as pages of actual measurements of metallic content of coins. For example, the silver content at Rome under the joint reigns of Valerian and Gallienus (according to the graphs) goes in four stages from 35% to 15% and under the sole reign of Gallienus from 12% to 2% in six uneven stages.

However, these are just averages. If you consult the tables of analyses of individual coins you find enormous variations in silver content, even for the same issues. There is also a varied mixture of base metals as well as copper, including tin and lead. Also surprisingly large traces of gold!

[L.C.Sulla]

Peter thank you very much for the detailed answer. Will get my hand on a copy as soon as able to get to an academic library over here.

Seems like a remarkably interesting topic for further reading - the Valerian/Gallienus (and family) coinage silver content (and historical linkage). I recently bought an uncleaned "Silvered" Gallienus "LIBERALITAS" Anton. that turned to be a spectacular Silver-Debased coin after cleaning it (first with a careful cleaning, then using lemon juice...). On the other hand a Valerian coin, of exactly the same period, was practically ruined after I've cleaned it (assuming results will be the same...). Guess you learn the hard way.   

[Mayadigger]

Ave!

Post pix.
Best,
Kevin