Solubility parameters and the cleaning of paintings: an update and review

Paintings and painted surfaces can acquire a wide variety of coatings or deposits in their lifetime - varnishes, overpaint, surface dirt, etc. - any of which might be considered to impair the object's aesthetic, historical or physical integrity and, therefore, to warrant removal. One of the most common cleaning operations for paintings conservators is the removal of discoloured varnish, usually a spirit varnish composed essentially of natural resin, such as dammar or mastic. Varnishes based on these low molecular weight resins are prone to oxidation, so changing in their solubility, they do not generally form a large polymeric fraction on ageing. So, unless they have other added ingredients like drying oils or polymerisable resins which can form a macromolecular network, such spirit varnishes should remain effectively re-soluble, provided the right liquid to bring about solution can be selected. It is perfectly logical, therefore, to look to organic solvents as a primary tool for the removal of solventbased, low molecular weight resin varnishes.