These postcards of Belle Époque Venice were printed by the Detroit Publishing Company using the Photochrom process, a time-consuming and exacting technique by which convincing layers of artificial color are applied to black and white photos and reproduced.
Invented in the 1880s by Hans Jakob Schmid, an employee of a Swiss printing company, the closely guarded process begins with coating a tablet of lithographic limestone with a light-sensitive emulsion, then exposing it to sunlight under a photo negative for up to several hours.
The emulsion hardens in proportion to the level of exposure and the less hardened portions are removed with a solvent, forming a fixed lithographic image on the stone.
Successive litho stones are then prepared for each individual tint to be used in the final color image. A single Photochrom postcard might require over a dozen different tint stones.
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Though the process was especially lengthy and painstaking back in Schmid's day, the end result was surprisingly lifelike color at a time when true color photography was just starting to be developed.