The medieval wall paintings in Undløse Church, considered to be among the most beautiful in Denmark, display an unusual pattern of dirt deposit: the figures are dirtier than other parts of the decoration. Illumination with UV light produced strong fluorescence by the figures, sharply delineating them from the background. Gas Chromatography-Mass Spectroscopy (GC/MS) analysis identified the components of a consolidant or surface treatment consisting of oil, resin and casein. This substance had been previously known only through anecdotes and superficial references as the Carlsberg Preparation, used in the first decades of the 20th century. Dust seems to have an affinity to the treated surface, which is not tacky. Other soiling problems, located primarily on the non-treated areas are a result of an incorrect cleaning procedure during a later restoration. The examination has indicated that the future cleaning will be a particularly difficult problem.