Many unfinished 19th-century paintings were not varnished. The types of dirt and pollutant likely to have been encountered by 19th-century paintings are discussed, as is dirt adhesion; underbound scumbles bind dirt less strongly than glazes. Cleaning can lead to a reversal of tonal values of dark and light areas in consequence. Never-varnished sketches, especially those by J.M.W. Turner, are discussed. Mars Plastic Erasers proved effective and safe, and produce better results than saliva or white spirit. Ammonium citrate chelating agents are also mentioned, as are protective coatings and other measures for the cleaned surface.