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article on the process of photochrom: en.wikipedia.org/wiki/Photochrom "A litho stone is coated with a thin layer of purified bitumen dissolved in benzene. A reversed half-tone negative is 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 hardens and becomes resistant to normal solvents in proportion to the light. The coating is then washed in turpentine solutions, removing the unhardened bitumen. It is then retouched in the tonal scale of the chosen color to strengthen or soften the tones as required. Each tint needs a separate stone bearing the appropriate retouched image, and prints are usually produced by at least six, and more commonly from 10 to 15 tint stones.[4]"

article on the process of photochrom: en.wikipedia.org/wiki/Photochrom "A litho stone is coated with a thin layer of purified bitumen dissolved in benzene. A reversed half-tone negative is 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 hardens and becomes resistant to normal solvents in proportion to the light. The coating is then washed in turpentine solutions, removing the unhardened bitumen. It is then retouched in the tonal scale of the chosen color to strengthen or soften the tones as required. Each tint needs a separate stone bearing the appropriate retouched image, and prints are usually produced by at least six, and more commonly from 10 to 15 tint stones.[4]"

article on the process of photochrom: en.wikipedia.org/wiki/Photochrom "A litho stone is coated with a thin layer of purified bitumen dissolved in benzene. A reversed half-tone negative is 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 hardens and becomes resistant to normal solvents in proportion to the light. The coating is then washed in turpentine solutions, removing the unhardened bitumen. It is then retouched in the tonal scale of the chosen color to strengthen or soften the tones as required. Each tint needs a separate stone bearing the appropriate retouched image, and prints are usually produced by at least six, and more commonly from 10 to 15 tint stones.[4]"