Pbk31 turns pink when scratched?

17 August 2021 0 5K Report

The pigment perylene green (Pbk31), CAS 67075-37-0 (perylene tetracarbonic acid), is used in watercolors and is a blackish green when applied to paper. When dry and scratched however it turns pinkish. The green colour is reconstituted once the scratched spot is rewetted with water.

What causes the colour change?

See the chemical structure attached and also the observed phenomenon.

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