The Photochrom is based on the color lithographic printing process, using a monochrome negative. To create the Photochrom, several lithographic stones had to be produced: one stone for each color. This quantity varied depending on the required final image. These stones were coated with bitumen which, once exposed to sunlight through a negative, hardens and makes the areas exposed to light insoluble. They were then etched with acid. The image became imprinted on the stone in bitumen. Each tint was applied to each different stone that bore the retouched image.
The charming character of the Photochrome resided in its enormous chromatic possibilities. We could use up to 14 stones to have detailed nuances. Therefore, the Photochrom left an important part to the creativity of a colorist, real performer of the process. The performer executed a skillful composition between reality and interpretation. Photochrom prints delighted the eye of the people of the era thanks to the subtle alchemy.
The Photochrom Zurich and Detroit Publishing Company produced exclusively photochrom prints. They bought negatives from photographers of the time, such as Félix BONFILS (1831-1885), Francis FRITH (1822-1898), William-Henry JACKSON (1843-1942), Jean-Pascal SÉBAH (1872-1947) and Polycarpe JOAILLIER (1848-1904), Félix- Jacques MOULIN (1802-1879) and so on. Most of them provoked a sensation of their contemporaries through their photographs on the Middle East.