One of our favorite sites, 50watts, has compiled a wonderful collection of Photochromes they found in the Beineke Rare Book library. Photochrome is a photographic technique that was popular between 1890 and 1910 with prints being turned into postcards. The process involved pressing half-tone negatives against special litho stones and retouching the tonal scales of colors to strengthen or soften them.
A litho stone was coated with a thin layer of purified bitumen dissolved in benzene. A reversed half-tone negative was then pressed against this light-sensitive coating and an exposure in daylight made (taking from 10-30 minutes in summer, to several hours in winter). The bitumen hardened and became resistant to normal solvents in proportion to the light. The coating was then washed in turpentine solutions, removing the unhardened bitumen. It was then retouched in the tonal scale of the chosen color to strengthen or soften the tones as required. Each tint needed a separate stone bearing the appropriate retouched image, and prints were usually produced by at least six, and more commonly from 10 to 15 tint stones.