Photochrom

The process was invented in the 1880s by Hans Jakob Schmid (1856–1924), an employee of the Swiss company Orell Gessner Füssli—a printing firm whose history began in the 16th century.

[7] The Detroit Photographic Company reportedly produced as many as seven million photochrom prints in some years, and ten to thirty thousand different views were offered.

After World War I, which ended the craze for collecting photochrom postcards, the chief use of the process was for posters and art reproductions.

[8] A tablet of lithographic limestone called a "litho stone" was coated with a light-sensitive surface composed of a thin layer of purified bitumen dissolved in benzene.

A reversed halftone[dubious – discuss] negative was hand colored according to the sketch and notes taken at the scene, then pressed against the coating and exposed to daylight through gel filters, causing the bitumen to harden in proportion to the amount of light passing through each portion of the negative.