Task: Question Generation
Input (English): Bitumen was the nemesis of many artists during the 19th century. Although widely used for a time, it ultimately proved unstable for use in oil painting, especially when mixed with the most common diluents, such as linseed oil, varnish and turpentine. Unless thoroughly diluted, bitumen never fully solidifies and will in time corrupt the other pigments with which it comes into contact. The use of bitumen as a glaze to set in shadow or mixed with other colors to render a darker tone resulted in the eventual deterioration of many paintings, for instance those of Delacroix. Perhaps the most famous example of the destructiveness of bitumen is Théodore Géricault's Raft of the Medusa (1818–1819), where his use of bitumen caused the brilliant colors to degenerate into dark greens and blacks and the paint and canvas to buckle.
Output (English): What pigment did 19th century artists use that was deemed unfavorably?