A clear blue to blue-green pigment. Cobalt blue is composed of a double oxide of cobalt and aluminium that is also called cobaltous aluminate. Cobalt blue is resistant to weathering, sunlight, acids and alkalis. Though possibly discovered earlier, the process for making cobalt blue by heating cobalt phosphate with alumina was first published by L. J.Thénard in France in 1802. Leithner discovered a process for making cobalt blue from cobalt arsenate and alumina. By the early 19th century, cobalt blue was sold as an artists pigment as a replacement for smalt. In recent years, the cobalt blue hue has been imitated using mixtures of ultramarine and phthalocyanine. Synonyms: Thenard's blue; Pigment Blue 28; CI 77346; cobalt ultramarine; cobaltous aluminate; king's blue; Olympia blue; Vienna blue; Vienna ultramarine; Leyden blue; Hungary blue; azure cobalt; Gahn's blue; Leithner blue; new blue; Kobaltblau (Ger.); bleu de Thénard (Fr.). Museum of Fine Arts, Boston (2000). [J. Cassar, R. de Angelis]