A key question that is being examined is whether the varnish of the Amsterdam painting could safely be removed. Van Gogh did not varnish his pictures, preferring a matt finish, and this coating was probably added in 1927, to help protect the surface. The varnish has aged since then, leaving a brownish tinge and dulling Van Gogh’s sparkling colours.

A further problem causing concern is the deterioration of Van Gogh’s chrome yellow pigments, which have darkened and become slightly greenish-brown. This is due to a photochemical reaction that takes place when chrome yellows are exposed to light. The effect on the Amsterdam picture is to make the flowers appear flatter than originally intended. Later tiny retouchings, also probably added in 1927, now appear more orange than Van Gogh’s original ochre-looking flowers. These retouches probably reflect the colour that was present nearly 90 years ago, revealing how the picture has changed since then.