Hard to find much information on the process of silvering late antoniniani and folles and from what I understand the process is unknown. I did come across this video of the striking of Roman coins, which is pretty instructive in general. One of my thoughts on the silvering process is this: whether or not the silvering was applied before striking the coin or after. To me it seems that if applied before, the process of striking the blank would lead to an uneven "silvered" surface on the coin, with perhaps some unsightly gaps in the silvering due to the impression of the dies. So it seems more logical that the silvering was applied after striking in the form of a bath or wash that adhered to the surface.
I have a page on metallurgy of Constantinian bronzes with a bit about the silvering process including sources for lots more information-- http://www.constantinethegreatcoins.com/metallurgy/ some of the articles are available on my page here-- http://www.constantinethegreatcoins.com/articles/
It appears Mercury was involved... The element (Hg), not the god. Experimental investigation of silvering in late Roman coinage C. Vlachou, J.G. McDonnell, R.C. Janaway (cited in Victor's metallurgy page and accessible via his articles page)
you can download the Vlachou article (along with others) from the page I already linked to, without signing up or logging into anything. http://www.constantinethegreatcoins.com/articles/
