This work was recovered in 2016. It had been stolen from the museum 14 years earlier. When it came home, the conservator was relieved to discover that it had suffered little damage. She did however decide it needed a clean. She removed the old, yellowed layer of varnish.
Before the painting was treated experts first examined it thoroughly. A tiny sample of paint and a special scan of the surface revealed a lot of information about the structure of the paint and varnish layers. It was also possible to analyse what materials they are made of. With this knowledge, the conservator knew which solvent she could safely use to remove the discoloured layer of varnish.
A layer of varnish is applied to protect the paint from dust and dirt. Varnish also gives the colours depth and contrast. Van Gogh preferred to leave his paintings unvarnished, but the owners of this painting decided to varnish it for practical or aesthetic reasons.
Under the stereomicroscope the conservator discovered that Van Gogh also applied a kind of varnish.
It is in fact a thin layer of egg white. Van Gogh probably applied it for temporary protection. It also allowed him to assess the effect of the colours more easily.