Gum bichromate or, as some call it, gumprint is one of the essential permanent and manipulative processes, created to give photographs a more artistic, painterly look. Due to the character of the process based on the sensitivity of gum arabic and bichromate, each of the images is unique and can hardly be repeated without introducing variations. Gumprints are characterized by unrivalled softness and painterly quality achieved through manual manipulation. Their permanence is virtually unrivalled.
A short film about gumprinting: https://www.youtube.com/watch?v=JNi6Qgbhvt8