Sure, you can remove the oxidation which will result in a pitted surface. If the surface isn't pitted, it wasn't oxidation hence not horn silver.
I may be wrong, but to me it just looks like a denarius which has tarnished/toned to black. It is quite common for silver coins to tarnish unevenly.
I certainly wasn't expecting such a strong reaction, or language. What makes you sure it isn't it a tarnished coin? I don't know what "burnt paint patina" is, but I'm pretty sure this coin hasn't been in a fire if that's what you're saying.
Horn silver (AgCl) comes in a variety of textures and colors. Sometimes it looks a bit like tar, as on this coin.
I wrote a thing or two about horn silver and restoration of coins with horn silver. The patina of this coin is very characteristic of horn silver. Please find my elaboration on horn silver here: https://www.cointalk.com/threads/wh...-admission-49-532-49-533.362851/#post-4607392 I would characterize this type of horn silver as type I: By the way: I would not treat this coin. I expect severe pitting underneath the horn silver.
I am under the impression that horn silver is normally seen on purer silver coins like Greek tetradrachms or Roman Republican issues. Reference from this old Forvm thread here: http://www.forumancientcoins.com/board/index.php?topic=56738.0 But I admit I could be wrong.
Basically, horn silver can form on any silver coin, if exposed to high concentrations of Chloride or Bromium (AgCl or AgBr) for a sufficient period of time. I've seen it on medieval coins (this is the same type of HS as the coin of OP): ... Roman (this is the metallic type): Greek (still have to clean this one): ... Carolingian (didn't clean it, as it was too unstable): ... Anglo-Saxon (again the metallic type): etc. Restoration of horn silver can be lucrative, if you know what you're doing. Uncleaned coins usually sell for way lower prices.
I think it is quite possible that this could be a former antique patina burned in a fire. Some decades ago I was able to acquire a cast or bronze denarius (Limes denarius), which could be recovered from the fire debris of a ruin years after the firestorm of the allied bombing raids on Hamburg in 1944/45. The similarity seems obvious to me.
There doesn't really have to be a difference between 'burnt patina' and horn silver: the chemical reaction leading to the formation of AgCl or AgBr could have been formed during a fire, or during centuries in favorable conditions: the results are the same (horn silver)
Thanks for sharing Roer, great stuff. Off and read all your other posts on this topic after that. I knew the metallic deposits were horn silver, but I always thought that black waxy stuff was just dirt/clay/muck. Question re: sodium thiosulphate, does this have any effect on the stubborn green and red gunk found on bronzes?
@Co1ns, thanks! Appreciate the compliment. No, I would not think so. These deposits are something different than AgCl and/or AgBr, so it's not 'one treatment fits all'. I must say, I find bronze very hard to treat well and usually refrain from doing so. The only caveat of silver coin restoration is the often very soft patina (and underlying metal) in combination with the usually very hard deposits.
For cleaning purposes I would say there are two kinds of coins with horn silver deposits A) Ones where the silver derives from the host coin itself - which means there will be pitting underneath proportional to the encrustation on top. B) Ones where a different coin in the ground - close by - corroded and the deposit is composed of material transferred onto the host coin Obviously cleaning type B is a good idea, type A - less so. So the key question is not how to clean - but whether to clean. Obviously too - if horn silver is "chipping off" - it points towards at least some of the deposit being transferred..... R
@EWC3 classifies it in the same way as I see it. If horn silver is deposited on the coin (e.g. part of a coin hoard where the outer coins degrade), the results are usually fine. In another thread, I just posted the cleaning process of a coin which I suspect was part of a hoard: before: after: This one would fit in the first category: the horn silver is upon the coin, and the surface below is smooth. This one however is different: before: after: Note that on some places (e.g. on the obverse at 7 o clock; on the reverse ate 9 - 11 o'clock), there is pitting, while on other places (e.g. on obverse at 3-5 'o clock; reverse in the field and legend) the metal is smooth. I expect degradation of the places with pitting, and deposition of the AgCl on the other parts of the coin. Same with this one: before: after: a pitted obverse, with a very sharp reverse.
A rare event and cause for celebration Aside from cleaning (very many) such coins I have been paid by trade to clean their own (4 figure value) coins but also paid to merely give an opinion on whether to clean. But I find this track record counts for little in our new web based form of democracy R