It was a pleasure to visit the newly reopened show of works in Derby this weekend. Joseph Wright is probably most well known for his paintings depicting demonstrations of science (see last image of the orrery) during the enlightenment and his use of strong light-dark effects (chiaroscuro).
This image of a rainbow drew my attention for the strange rendering of the colours which missed the red from the usual rainbow sequence and replaced it with more of a violet outer edge. It left me wondering if the paint had changed during the last two hundred years or if he'd chosen to weave his own rainbow and pay less attention to nature to possibly make a better picture.
My restorer, conservationist friend, HGW has a perspective on the missing red of the rainbow....
"Joseph did have problems with his pigments, like many 18thC British artists; and quite a few of his paintings, particularly the darker ones where he mixed bituminous substances and employed drying additives, have resulted in shrinkage of the paint layer. 18th C pigments were not as stable as 19th and later. Reds, for instance, reacted to sunlight and faded. No doubt he added the red over his sky and the fading of the red, coupled with a cooler hue beneath will give a more violet colour. If, for instance, the blue of the sky contained a degree of 'smalt', a blue glass pigment which reacts with the oil in the medium and fades to grey, this also might give a violet hue.
Just a couple of thoughts; would need to check old Joseph's paint box to be absolutely sure!"