Coin restoration: cleaned an unidentified Merovingian denier

[Roerbakmix]

Part of my hobby is cleaning silver coins. I've posted several threads on CT on my methods. 

A few weeks ago, I bought this Merovingian denier:

This type of encrustation is the most gratifying to remove. I've described it previously as 'flaky metalic horn silver'. My hypothesis is that this type of encrustation occurs in coin hoards. Usually, the metalic deposits are sharply demarcated, on coins of high silver purity. I've employed my usual method of baths in boiling hot sodium thiosulphate solutions, followed by gentle mechanic removal of the deposits. I'm quite happy with how the coin turned out:

The coin is slightly darker than it was prior to cleaning, which is a (positive) effect of this restoration method. Basically, silver chloride (AgCl) reacts with sodium thiosulphate (NaS2O3) and forms a thin and stable layer of silverthiosulphate (Ag2S2O3):

2AgCl + Na2S2O3 → Ag2S2O3 + 2NaCl 

In my experience, this thin layer can fill the thin cleaning scratches, as was the case on the large central cross on the reverse: the scratches on the reverse where recent and shiny, but are now evenly colored with the rest of the coin. 

I'm however still stuck with identification. Phillipe Schiesser, the leading Merovingian Numismatist (he's auctioning his collection right now at CGB by the way) considers it similar to coin 185 which was part of the Nice-Cimez hoard:

... but I'm not sure I'd agree. I've scrolled through 3 of the 4 volumes of Belfort looking for a positive match, but alas, none. 

 

[expat]

Good job! Looks much better with an overall darker appearance. Hope you are successful with an attribution eventually.

[AETHER]

Incredible results, some of best! 

[CPK]

That turned out beautifully!

[JeandAcre]

Bravo!

[JayAg47]

I bet it was satisfying to watch the layers shed off revealing the details! 

[Theodosius]

Great result! Your technique has added a valuable tool for horn silver cleaning that I have never seen described by anyone else.

John

[John Conduitt]

17 hours ago, Roerbakmix said:

Phillipe Schiesser, the leading Merovingian Numismatist (he's auctioning his collection right now at CGB by the way) considers it similar to coin 185 which was part of the Nice-Cimez hoard

It looks almost nothing like 185 to me 😂

What about it tells you it is Merovingian?

[Roerbakmix]

Thanks all for the positive comments. 

14 hours ago, JayAg47 said:

I bet it was satisfying to watch the layers shed off revealing the details! 

It really is! Poking a small metal rod under the deposits, until they crumble, revealing intact and smooth surfaces ... 

For fun, here is a GIF (5 seconds or so) which I posted previously on Cointalk, showing a timelapse of a severus alexander denarius in the solution. It took about 10 minutes. 

2 hours ago, Theodosius said:

Great result! Your technique has added a valuable tool for horn silver cleaning that I have never seen described by anyone else.

John

Thank you @Theodosius. It is however a commonly employed method. Here is an interesting article that compares different cleaning methods:

https://www.researchgate.net/publication/276192158_The_Conservation_of_Early_Post-Medieval_Period_Coins_Found_in_Estonia/figures?lo=1

 

2 hours ago, John Conduitt said:

It looks almost nothing like 185 to me 😂

What about it tells you it is Merovingian?

Allegedly, the coin was found near Etaples, France (West, coastal region). It's the size and thickness of a sceatta / merovingian denier. I'm very much open for alternative suggestions! It doesn't help that the legend is illegible.