Three basic elements I’ve found out tinkering with Zink paper:
Blue comes from heat, thus reversing the common tought that considers it a “cold” colour: various means ended with mixed results, and after two tries I avoided hot liquids because they tend to soak the paper and to cause the layers to peel off without any benefit. Not surprising Magenta has a more psychedelic nouance that comes from applying to paper different quantities of Acetic acid (the one commonly used in darkroom stop-bats) : at first I was considering LSD but that’s becoming scarce nowadays…; Yellow comes from a more physical approach, scratching: I’ve tried first with a very sharp ceramic kitchen knife, but for more even results I’ve found that #400 sandpaper does it better. The only colour I’ve been able to obtain “mixing” two layers is Red, due to a more rhythmic approach with heath.

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