A collodion positive on glass, popular from the mid-1850s to the 1860s. The image is a thin negative, which looks positive when the glass is backed with a dark material such as black varnish or cloth.
Ambrotypes were cheaper than daguerreotypes and were usually sold in small cases. James Ambrose Cutting of Boston patented improvements to the process in 1854. See also wet plate collodion.
