ULTRAMARINE BLUE

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Formula of ultramarine blue is Na_8-10Al₆Si₆O₂₄S_2-4.

The natural variety is a very costly blue pigment with exceptional chemical stability except in the presence of acids. 

It is two types of ultramarine blue:  natural ultramarine and synthetic ultramarine.

Natural Ultramarine  is famous for having been the most expensive pigment. It was more expensive than gold during the Renaissance. First used in 6th century Afghanistan, the pigment found its most extensive use in 14th and 15th century illuminated manuscripts and Italian panel paintings, often reserved for the cloaks of Christ and the Virgin. Genuine blue pigment is from the semi-precious stone, lapis lazuli, which is a mixture of the blue mineral, lazurite, with calcspar and iron pyrites. Natural ultramarine is unaffected by red heat or by alkalis but is decomposed by dilute acids, even acetic acid, with complete loss of color and the discharge of hydrogen sulphide gas. The blue is stable, however, in strong light and many specimens which are several hundred years old show no apparent change in color.

Synthetic ultramarine is one of the best-documented pigments of the nineteenth century probably because its invention was requested of chemists and not the result of their independent research. Ultramarine, genuine made from the semi-precious gem lapis lazuli was so costly in the nineteenth century that artists infrequently used it. The hue is a necessary component in a balanced palette of warm and cool colors; without it a cool, deep blue is lacking. In chemical composition and structure it is identical with the natural ultramarine. This blue is stable under all conditions, except in the present of acids. It is permanent to light and is unaffected by high temperatures. Since it is unaffected by alkalis, it is stable in fresco. Synthetic ultramarine is today quite widely used as an artist´s pigment and is known to many as "French ultramarine" because of its discovery (1830) and long production in France.