Azurite
The sea of the painting Bacchus and Ariadne, which has a greenish tint, is painted in azurite, a blue copper mineral, known to Pliny under the name caeruleum. There were azurite deposits in several places in Europe, including France, Hungary, Germany and Spain. Imported ultramarine rarely made it all the way to Northern Europe, at least at an affordable price, and so most of the blues in the works of German and Dutch artists in the Renaissance are azurite. In its vivid blue colour azurite contrasts strikingly with the emerald-green malachite, also a basic copper carbonate, but containing rather more water and less carbon dioxide. Being less hydrated than malachite it is itself liable to alteration into this mineral. The transformation into malachite of azurite, is common in fresco works: medieval Italian churches hold many examples of blues turning into greens. Azurite occurs with malachite in the upper portions of deposits of copper ore, and owes its origin to the alteration of the sulphide or of native copper by water containing carbon dioxide and oxygen.
The blue skies of Las Lanzas by Diego Velázquez were created with azurite, although he also used lapis lazuli in his landscapes so as to intensify the tone. Velázquez changed the colours of his skies by mixing azurite with iron oxide, lead-tin yellow, lead white, calcite and large amounts of oil.
