Re: i don't know
The button you are asking about is neither silver nor platinum. It is a metal called "White-Tombac" ...which is an alloy containing three metals ...consisting mostly of copper, with some zinc, and arsenic. Perhaps because of the arsenic in it, White-Tombac buttons are often dug with very nearly no verdigris/patina on them, even still showing a grey-silver lustre (shine, gleam) after 200 years in the ground. The lustre typically shows best on a White Tombac button's back, because unlike the front side, its back side was usually "turned" smooth in a lathe.