An ambrotype (also known as a collodian positive) was an image produced by a type of photographic process first used in the 1850s, succeeding the more expensive daguerreotype, with the image on glass (as opposed to film, paper or metal).
The resulting image is actually a negative, but when viewed against a dark backing (such as black velvet, or a layer of black varnish) or on dark reddish-coloured glass, appears as a positive. The collodian emulsion is usually protected by a layer of varnish and a glass cover (or sometimes put in the case emulsion-side down), and usually supplied in a presentation case, like the earlier daguerreotype. Ambrotype images were sometimes hand-tinted.
The term ‘ambrotype’ is a particular variant of the collodian positive process (patented by James Ambrose Cutting of Boston in 1854) where Canada balsam is used to seal the collodian plate to the cover glass, but the term is now used to describe all collodian positive photographs.
The ambrotype was popular only from around 1855-1865, and after that the process disappeared from high-street studios. It continued to be used by open-air photographers until the 1880s as the process took a short amount of time. It was superseded by the sturdier tintype process, and also by albumen prints on paper.