Tintype
A wet collodion positive made on a thin sheet of blackened iron, producing a cheap, durable, unique image.
In depth
Also called a ferrotype, it uses the same chemistry as wet plate but on a dark metal support, so the thin negative image reads as a positive against the black backing. Tintypes were tough, quick and cheap, which made them the photograph of the street fair and the battlefield rather than the studio.
Examples in practice
A Civil War soldier's tintype could be made in minutes, survive a pocket, and be posted home without a frame.