An ambrotype is a negative photograph on a glass plate. When
black paper or velvet is placed behind the image, or when the back of the
image is painted black, the negative appears as a positive.
The sensitizing agent of an ambrotype was silver nitrate, the developing
agent was pyrogallic acid.
Invented in Britain by Frederick Scott, it was introduced in America in
1854. Its years of greatest popularity were 1857 to 1859. Interest slackened
in 1861 and, with a few holdouts, the ambrotype was last made in 1865.