Patented in 1854 by James Ambrose Cutting, an ambrotype is a negative on a glass plate, which, when viewed in transparency, becomes a positive when placed in front of a black background.
 
 The ambrotype was commonly used from 1854 until the 1870s. Less expensive and requiring a much faster exposure time than the daguerreotype, it was widely used by portrait photographers. To obtain the negative of this unique image, a previously cleaned glass plate was coated with a layer of collodion. After drying, a transparent varnish was applied, sometimes with touches of color. Presented on a background of fabric or black paper, it was delivered in a union case or frame, as with daguerreotypes, which often leads to confusion in identifying these two processes.