The darkening of oil paintings may not be due solely to the effects of dirt. Near the end of the 19th century, writers on oil painting materials and practices in the fine arts repeatedly expressed concern about the darkening of oil paintings due to the presence of sulfuretted hydrogen in the air. This darkening, thought to be caused by the formation of the dark brown compound lead sulfide, was held to occur wherever lead was present in the painting whether as a pigment or as a drier in both ground and oil paint layers, or as a drier in a final oil varnish. The darkened tacking margin of an unvarnished oil sketch by J.M.W. Turner, and a darkening 19th-century primed canvas, were examined, and analyzed by chemical microscopy, gas-chromatography, and fluorescence microscopy. Chemical tests of both and x-ray photo electron spectroscopy (XPS) of the latter sample indicated the presence of lead sulfide, perhaps partly converted to lead sulfate. A hydrogen peroxide treatment to produce lead sulfate, recommended in the 19th century, worked successfuly on both samples.