The ambrotype, or collodion positive, first appeared in about 1853. By the 1860s the process had largely disappeared from high street studios, but it remained popular with itinerant open-air photographers until the 1880s, because portraits could be made in a few minutes while sitters waited.

The collodion positive process, which was based on the collodion negative process invented by Fredrick Scott Archer, reversed a negative image by bleaching the silver salts. The dark areas which would normally form the highlights in a printed image turned pale, and the clear areas which would form the shadows in the print appeared to be dark.

When presented against a black background, the dark areas of the original negative, which had been bleached with nitric acid or bichloride of mercury, appeared as highlights. The black backing, visible through the clear areas of the plate that originally formed the highlights, appeared as shadows.

Although the so-called collodion positive was in fact a negative, the emulsions were too thin to make satisfactory prints on paper. When a collodion positive was held to the light without the backing material, the image still looked like a negative, though paler than the standard required to make a satisfactory positive print.

The dark backing material could be a velvet pad held inside a presentation case, or a simple coating of black varnish for those made in lower-class studios and temporary booths erected at holiday resorts.

(source : https://blog.scienceandmediamuseum.org.uk/find-out-when-a-photo-was-taken-identify-collodion-positive-ambrotype/)