I believe
Robert has it correct. The rough surface ansd the
pitting were there, just filled in with the dirt and copper oxide. A true silver coin vs a slivered broze coin are two different things. A true silver coin is rarely bothered by acids or ammonia. But a
silvered broncze coin is very different. There is offten a corrosion layer, oxidized copper, thet runs under the silver, and when the coin is cleaned, that layer is disloved, there is then nothing to bond the silver to the bronze or copper base, so it flakes away. There may be other reations also, so you must take into account the possible metal vs cleaner reactions. That is one reason for not using most acids on bronze, but you can use the same acid on silver.
Bruce