Missing paint layer
Image showing an example of the area examined under the microscope, top edge
In normal viewing conditions, the top 2cm of the painting is a different colour from the main body of the painting. The area under the rebate is a dark blue and the area not protected by the rebate in this area is brown.
It was assumed that this was connected to a faded area, but microscope examination seems to show that the blue layer in the sky is missing the brown layer which seems to have peeled off.
This is the top edge of the painting where the top paint layer appears to have peeled away to reveal the blue layer below. This image shows an area of the painting about 4mm across.
This suggests that most of the sky, up to the top edge would have been covered in this brown layer. What is not clear is whether or not this layer would have always been brown or whether light or solvents have caused it to go from a blue to brown.