3Na2O.3Al2O3.6SiO2.2Na2S - A pure, deep blue, fine particle, synthetic ultramarine commercially developed in 1828 in France by Jean-Baptiste Guimet. Synthetic ultramarine is prepared be slowly heating anhydrous sodium sulphate or carbonate, clay, silica, sulphur, rosin and charcoal in a reducing atmosphere to 750º C (1,380º F). Variations in the mixture proportions give various shades of blues, greens, violets and reds. Synthetic ultramarine blue has rounded particles that are finer and more regular in size and shape than the natural pigment. It is inexpensive and is used as a permanent artist pigment in oil and watercolour paints. Synonyms: artificial ultramarine blue; French ultramarine; French blue; new blue; permanent blue; Oriental blue; Gmelins blue; Guimet's blue; royal blue; Pigment Blue 29; CI 77007; Ultramarinblau (Ger.); outremer (Fr.); oltremare (It.); ultramar (Sp.). Museum of Fine Arts, Boston (2000). [J. Cassar, R. de Angelis]