Last year I was given the opportunity to acquire attached painting of bulb fields made by Ben Viegers. This painting was in the family for over 70 years. It was exposed to much smoke. Hence cleaning was required to remove the dirt.
The attached picture shows the cleaning process half way. The right half is cleaned, the left half still dirty. This is particularly visible in the blue sky.
Following cleaning the colours are again as intended by Ben Viegers some 100 years ago.