AMBROTYPE is validvalid

an early photographic technique using black varnish on a glass plate negative [n AMBROTYPES]

89,627 of 196,601 in NWL2023·also valid in CSW24

a picture taken on a plate of prepared glass, in which the lights are represented in silver, and the shades are produced by a dark background visible through the un-silvered portions of the glass [n -S]