I was waiting for you to slide in this thread and make that point. The most dastardly example that comes to mind are Bust Halves. Finding original, high grade Bust Halves can be a fool's errand sometimes, but many of them, of course, have been properly cleaned. I've always seen the properly/improperly cleaned dichotomy as similar to market acceptable toning. There is a degree of cleaning that the majority of numismatists will seem to accept, and the same is true for some rainbows here and there. Now as for why we accept 18th and 19th century gold with hairlines running all over the place as "original" is beyond me.
That's an interesting point, seeing as though NGC brands their coins with the "Improperly Cleaned" label.
Unfortunately, this is almost a fools errand. I have seen many comments from reputable individuals who claim all coins, prior to some year, have been cleaned in some fashion because it is what was done in the days of yore.
Unfortunately, this will never happen. Their services would suffer and we would be back to buying coins in two by two's.
I recall several years back a guy I would see at local coin shows that had hundreds of coins were all glimmering white. After seeing him only five or six times he was never seen again. Probably figured out why nobody was buying his stuff.
Interesting that the MS70 bottle says "safe for copper" yet it turns copper coins blue and purple. I guess it depends on your definition of "safe?" I would never use anything like that on a coin that I cared even a little bit about. It might be fun to experiment on worthless coins just to see what the stuff does, but that's as far as I would ever go, and I doubt that I would ever even go that far.
I had a bunch (97) of extremely worn buffalo nickels. I used 'Nic-a-Date' to restore the date and mint mark. I found no, "Gee, I wish they were AU's or MS's" in the lot and no key dates either, but it was fun.
To give a full answer, a polishing cloth contains abrasives that will scratch the bejebers out of a surface. If you don't see it, use side light and tilt the object back and forth to see all the tiny scratch marks.
I've done the same and I found a very worn, obviously to use nic a date, 1916 Buffalo over date. I figure since they're worn and have no date you can't hurt them. Yes it creates damage but what's one to do?