Naumann’s patina

[Nikko]

I think we all have a big issue with the so called Naumann patinas.
I won the coin attached in a recent auction (august 2021); a pleasant coin with dark patina and some silvering but....

[Nikko]

...this is the coin now, after a brief soaking in distilled water and a few hits with tooth brush.
I think I will no more join their auctions.

[Victor]

definitely something strange going on. I have all my NN coins in the same tray so that I can keep an eye on them.

I'd like to say I will not buy from them again, but they list so many coins that interest me. it's like gambling...I hope these don't turn green. 

[marandnumiz]

The coin is amazing in any case. I like fully cleaned version the most, it now looks almost as struck (minus silvering).

[six2ten]

Sigh .... Another one. But not sure it's bronze disease. It didn't brush off at all, and only came away once treated wit a sodium carbonate / bicarbonate mix