History   (Wikipédia)

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.^[1] Füssli founded the stock company Photochrom Zürich (later Photoglob Zürich AG) as the business vehicle for the commercial exploitation of the process and both Füssli^[1] and Photoglob^[2] continue to exist today. From the mid-1890s the process was licensed by other companies, including the Detroit Photographic Company in the US (making it the basis of their "phostint" process)^[3], and the Photochrom Company of London.

The photochrom process was most popular in the 1890s, when true color photography was first developed but was still commercially impractical.

In 1898 the US Congress passed the Private Mailing Card Act which let private publishers produce postcards. These could be mailed for one cent each, while the letter rate was two cents. Publishers created thousands of photochrom prints, usually of cities or landscapes, and sold them as postcards. In this format, photochrom reproductions became popular.^[4] 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 One, which ended the craze for collecting Photochrom postcards, the chief use of the process was for posters and art reproductions. The last Photochrom printer operated up to 1970.^[5]

Process

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 worker then pressed a reversed half-tone negative against the coating and exposed it to daylight for 10 to 30 minutes in summer, or up to several hours in winter. The image on the negative caused varying amounts of light to fall on different areas of the coating, causing the bitumen to harden in proportion to the amount of light. The worker then used a solvent such as turpentine to remove the unhardened bitumen, and retouched the tonal scale of the chosen color to strengthen or soften tones as required. This resulted in an image being imprinted on the stone in bitumen. Each tint was applied using a separate stone that bore the appropriate retouched image. The finished print was produced using at least six, but more commonly 10 to 15, tint stones