The carbon process is a historic contact printing technique introduced in 1866. It uses tissue coated with pigmented gelatin, which is then sensitized with potassium bichromate and placed in contact with a negative. Exposure to sunlight hardens the gelatin in proportion to the light received, creating a highly detailed, permanent print. The carbon process was valued for its rich tonal range, permanence, and resistance to fading, making it a popular method in 19th-century fine art and photographic printing.