These postcards of France in the last years of the 19th century were created using the Photochrom process, a technique by which black-and-white photos were imbued with vibrant and lifelike color.
Invented in the 1880s by a Swiss printer, the Photochrom process began with coating a tablet of lithographic limestone with a light-sensitive emulsion and exposing it to sunlight under a photo negative for several hours.
The emulsion would then harden in proportion to the tones of the negative, resulting in a fixed lithographic image on the tablet.
Further litho stones would then be prepared for each tint to be used in the final color postcard — a single image could require well over a dozen different stones.
Though a time-consuming and delicate endeavor, the Photochrom process resulted in color images with a rare degree of verisimilitude, especially at a time when true color photography was still in its infancy.