its either a wet or dry plate ambrotype ...
they were made from the time of the civil war (wet plate ) until maybe the 20s-30s ( dry plate )
the wet plate ones are made with collodion ( spiked with other stuff ) and silver nitrate and a portable darkroom
the dry plate ones are made by using silver gelatin emulsion on glass ...
sometimes instead of glass metal was used ( ferrotype / tin type )
there are still people making images like this even today ( and even here on apug )