The ambrotype or amphitype, also known as a collodion positive in the UK, is a positive photograph on glass made by a variant of the wet plate collodion process. Like a print on paper, it is viewed by reflected light. Like the daguerreotype, which it replaced, and like the prints produced by a Polaroid camera, each is a unique original that could only be duplicated by using a camera to copy it.
The ambrotype was introduced in the 1850s. During the 1860s it was superseded by the tintype, a similar photograph on thin black-lacquered iron, hard to distinguish from an ambrotype if under glass.
One side of a clean glass plate was coated with a thin layer of iodized collodion, then dipped in a silver nitrate solution. The plate was exposed in the camera while still wet. Exposure times varied from five to sixty seconds or more depending on the brightness of the lighting and the speed of the camera lens. The plate was then developed and fixed. The resulting negative, when viewed by reflected light against a black background, appears to be a positive image: the clear areas look black, and the exposed, opaque areas appear relatively light. This effect was integrated by backing the plate with black velvet; by taking the picture on a plate made of dark reddish-colored glass (the result was called a ruby ambrotype); or by coating one side of the plate with black varnish. Either the emulsion side or the bare side could be coated: if the bare side was blackened, the thickness of the glass added a sense of depth to the image. In either case, another plate of glass was put over the fragile emulsion side to protect it, and the whole was mounted in a metal frame and kept in a protective case. In some instances the protective glass was cemented directly to the emulsion, generally with a balsam resin. This protected the image well but tended to darken it. Ambrotypes were sometimes hand-tinted; untinted ambrotypes are monochrome, gray or tan in their lightest areas.
I was fortunate to meet Mr. Mirosław Miranowicz, who was taking photos in this magical technique at the street fair in the historic district of Katowice - Nikiszowiec in Poland.