I saw this old case today. First of all we cannot be sure there is horn silver anywhere in this coin. The
obverse spots could be horn silver but it is not easy to say. Horn silver is more usually seen on purer Greek and
republican silver, for example. The
reverse green marks are definitely accretions of copper carbonates, because it is mainly the copper in the
alloy that mineralizes in these coins. These are not serious and in this case do not detract much from the appearance or value. As a matter of fact they are a proof of its
antiquity. I wouldn't mess with this coin.
Horn silver can end ruining totally a coin but probably not in your lifetime, or indeed in many lifetimes. In the end it is a matter of taste or of choosing between kinds of ruin. The natural or the artificial. The presence of genuine horn silver implies a loss of original silver, removing it will necessary uncover some degree of corrosion which can prove ugly and contrast horribly with the rest of the coin. If thoroughly cleaned the coin will probably look corroded but unnaturally soft and tin shiny.
