As posted on Incredible hand-tinted postcards capture 1890s Ireland in vivid color

These postcards of the sweeping hills, cliffs, and towns of Ireland were created using the Photochrom process, a complex method of imbuing black-and-white photographs with relatively realistic color.

The closely-guarded process was invented in the 1880s by an employee of a Swiss printing company. It entailed coating a tablet of lithographic limestone with a light-sensitive emulsion, then exposing it to sunlight under a photo negative.

After several hours, the emulsion would harden in proportion to the tones of the negative, leaving a fixed lithographic image on the stone.

Additional litho stones would then be produced for each tint to be used in the final color composite — a single image could require well over a dozen different tint stones.

Though delicate and time-consuming, the Photochrom process resulted in color images of striking verisimilitude for a time when true color photography was in the earliest stages of development.