From Roger Handley
I found it strange that your article about the darkening of some of van Gogh’s yellows suggested that the cause of chrome-based paints darkening over time was unknown (19 February, p 5).
Having worked with a major manufacturer of chrome pigments for 20 years, I can tell you that the overwhelming cause is exposure to atmospheric sulphide gases. Over time, exposure causes the lead chromate present in the pigment to be converted into lead sulphide, which is black.
Van Gogh’s paintings must have had many years of exposure to a polluted atmosphere – they certainly started their life long before clean air acts!
Llandre, Ceredigion, UK
