Red color slowly disappearing from Van Gogh's paintings

Recent scientific research has revealed that the blue irises in Van Gogh's Field with Irises near Arles were once purple. That is because the red is slowly disappearing from Van Gogh's paintings. The discolouration has arisen because the red pigment that Van Gogh used in his paint is strongly deteriorating, conclude the scientists who have investigated Van Gogh's work. They talked about their findings in the Dutch TV programme De Kennis van Nu [The Knowledge of Now] that was broadcast on Sunday