“Bridge With Mountains” is a watercolor from J.M.W. Turner’s Cyfarthfa sketchbook.
It was on exhibition until 1905, when someone noticed that everything but the part covered by the frame had turned brown.
What happened to the blue? The same indigo that dyes blue jeans was used as a pigment, and it either faded or shifted under strong light.
from Turner's Early Sketchbooks, by Gerald Wilkinson