if it were a chemical reaction, then why would it be lighter than a normal one?
If the chemical reaction consumed the copper in the alloy on the surface of the coin, proportionally more tin/zinc will be left, giving the coin a more silvery/gray appearance. I thought it possible
, given the appearance of the grain on the obverse. "Pickling" was also done long ago to give low silver content coins (billon) a more silvery surface, by chemically removing the copper.