page_title: Saponification
section_title: Oil paints
hierarchical_section_title: Saponification / Oil paints
caption_attribution_description: English: Detail of Madame X (Madame Pierre Gautreau), John Singer Sargent, 1884, showing saponation in the black dress. Detail of this painting
context_page_description: Saponification is a process that involves conversion of fat, oil or lipid into soap and alcohol by the action of heat in the presence of aqueous alkali. Soaps are salts of fatty acids and fatty acids are monocarboxylic acids that have long carbon chains e.g. sodium palmitate.
context_section_description: Saponification can occur in oil paintings over time, causing visible damage and deformation.  Oil paints are composed of pigment molecules suspended in an oil binding medium. Heavy metal salts are often used as pigment molecules, such as in lead white, red lead, and zinc white.  If those heavy metal salts react with free fatty acids in the oil medium, metal  soaps may form in a paint layer that can then migrate outward to the painting's surface.
  Saponification in oil paintings was described as early as 1912.  It is believed to be widespread, having been observed in many works dating from the fifteenth through the twentieth centuries; works of different geographic origin; and works painted on various supports, such as canvas, paper, wood, and copper.  Chemical analysis may reveal saponification occurring in a painting’s deeper layers before any signs are visible on the surface, even in paintings centuries old.
  The saponified regions may deform the painting's surface through the formation of visible lumps or protrusions that can scatter light.  These soap lumps may be prominent only on certain regions of the painting rather than throughout.  In John Singer Sargent's famous Portrait of Madame X, for example, the lumps only appear on the blackest areas, which may be because of the artist’s use of more medium in those areas to compensate for the tendency of black pigments to soak it up.  The process can also form chalky white deposits on a painting's surface, a deformation often described as "blooming" or "efflorescence", and may also contribute to the increased transparency of certain paint layers within an oil painting over time.
  Saponification does not occur in all oil paintings and many details are unresolved.  At present, retouching is the only known restoration method.
caption: Detail of Madame X (Madame Pierre Gautreau), John Singer Sargent, 1884, showing saponification in the black dress.
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