So this Septimius Severus denarius had some dark spots, which I ID'd as a combination of horn silver (silver chloride/bromide) and copper. After soaking the coin for 5 minutes in a solution of distilled water sodium thiosulphate, followed by 5 minutes in a solution of distilled water and purified citric acid, the coin looks as follows: Both photo's were taken with exactly the same photosettings (ISO 100, F22, 2 sec), which is my default setting, with no post-editing apart from cropping. I was pleasantly surprised to see that the detail of the lettering improved as well, likely because of a very thin layer of horn silver on the entire coin. The mild yellow tone that is visible is due to using the same solution of sodium thiosulphate for a bit to long, causing the sodiumthiosulphate to break down in sulphate, which is yellow. This can easily be removed, but 'in hand', it's not visible. Please note that removing copper (or iron) depositions requires the coin to be of relatively high silver purity, else pitting may occur. Sodium thiosulphate is inert to silver.
I always considered horn silver to have bits of silver from another coin attached to it. What would you call this condition, particularly around the mouth?
Great cleaning effort @Roerbakmix What effect does sodium thiosulfate have on elemental copper? Can it be used to clean bronze coins or treat bronze disease? John
Hi DavidPCC, though I'm unable to zoom in, I'm convinced this is horn silver indeed. Using the sodium thiosulphate method, this coin would likely have a good cleaning result (I would definitely do it if it was mine). Sodium thiosulphate anhydrate (not pentahydrate) is cheap and safe to use. This morning, I've cleaned this hadrian ar Denarius: which turned out quite well This is another type of horn silver: a more diffuse rough layer, which usually shows a bit of pitting underneath. In short, there are (in my experience) four types of horn silver: 1) a metal-like type, as present on the coin by David 2) a black patchy type, as on the first coin 3) a diffuse layer, as in this hadrian drachm 4) a thick encrustation of a dirt, debris, other elements (iron, calcium, etc) with horn silver in it