So after my last cleaning project of a Nerva denarius (https://www.cointalk.com/threads/how-a-dirty-ugly-duckling-became-a-clean-ugly-duckling.338255/), I decided to clean yet another silver coin. First, this is how it looked before: Someone had a go at it, as can be seen at the scratches on the portrait. The encrustations looked like horn silver to me, which is quite difficult to remove. Then the final result (proves very difficult to photograph; the coin looks nicer in hand, e.g. the eyes, nose and mouth can be seen easily) I tried various cleaning methods. First, I boiled the coin for approx. 30 minutes in a solution of purified citric acid and water. This resulted in the following: As can be seen (by zooming in), the encrustations are of metal, which solidified my hypothesis of horn silver. Under these encrustations, the details were surprisingly good. Using mechanical cleaning and more baths with the citric acid solution, the coin showed a bit more detail (take for example a look at the monogram at the reverse: However, this is where I got stuck. The remainder of the encrustations were suprisingly hard and did not give up easily. Also, a blackish goo (I presume a layer of silver-sulphilte) showed up at the right side of the portrait, and the left side of the reverse (which is the same side, flipping the coin over). I tried various methods: first, an ammonia solution (4%) which didn't do anything really. Second, using a higher concentration of citric acid, which helped a bit. Finally (after approx. 5 hours of cleaning), I tried backing soda and aluminium foil which did wonders.
