Indanthrone blue, also called indanthrene, is an organic dye made from 2-aminoanthraquinone treated with potassium hydroxide in the presence of a potassium salt. It is a pigment that can be used in the following mediums: acrylic, alkalyd, casein, encaustic, fresco, gouache, linseed oil, tempera, pastel, and watercolor painting. It is used to dye unmordanted cotton and as a pigment in quality paints and enamels. As a food dye, it has E number E130, but it is not approved for use in either the United States or the European Union. Indanthrene Blue RS was patented in 1901 by Rene Bohn as the first anthraquinone vat dye, one of the dyes with very good fastness to light and washing.