Why Paintings Turn Yellow

Varnishes are often made from tree resins and are intended to make the colours of a painting appear more intense, to give a shiny surface and to protect the paint layer. However, the layer of varnish gradually changes from colourless to a yellowing, shrinking crust with the characteristic craquelé of old paintings, creating colours which may differ greatly from the original paints used by the artist -blue may look like a kind of green, for example.

Research shows that the aged varnish has an entirely different chemical composition compared to when it was new, having been changed by oxidation and decomposition into smaller, or by polymerisation into larger molecules. When old masters are restored using various alcohols or acetone, for example, the small molecules in the varnish layer -which include oxidised stages of the original triterpenoids- are easily dissolved, whereas the larger reaction products in the old varnish are much more difficult to remove and may become concentrated on the surface of the paint.

The Dutch researchers assume that it is precisely this group of harder to dissolve molecules which contribute to the typical yellow discolouration. They found indications for this in experiments in which they exposed pure triterpenoids or triterpenoid varnish in a suitable solvent to light in order to artificially induce accelerated ageing. Within a few weeks, the molecules were found to have undergone a cross-linking reaction producing larger complexes with the characteristic yellow colour. The team is still investigating whether these larger complexes have the same chemical structure as the substances which cause the yellow colour in aged varnish.