I love this discusion , lets say you find something that looks like a coin and you pay 1 euro , you clean it and find out it is a very rare coin and you can sell it for 40.000 euro , but the guy from the auction house tells you that since it is cleaned you can only get 40.000 uncleaned it would have been 100.000. Then in theory your cleaning would have cost you 60.000 euro , on the other hand if you would not have cleaned the coin , it would have been a lump of metal value 1 euro , so the cleaning gave you a profit of 39.999 euro .
I have known privatly some coin dealers , all of them clean coins , specialy zinc coins and a few others would all be cleaned , they are good at it , so you can not see it.
I have never cleaned a coin , but my collection is not that old , so i can clearly see what it is. If i would collect ancient coins I would clean the once that where to much gone to indentify
scm