also clay pigments rich in iron oxide, which were mined during the Renaissance around the city of Siena in Tuscany. Sienna contains less than five percent manganese. The natural sienna earth is a dark yellow ochre color; when roasted it becomes a rich reddish brown called burnt sienna. Mummy brown was a pigment used in oil paints made from ground Egyptian mummies. Caput mortuum is a haematite iron oxide pigment, used in painting. The name is also used in reference to mummy brown. Van Dyck brown, known in Europe as Cologne earth or Cassel earth, is another natural earth pigment, that was made up largely of decayed vegetal matter. It made a rich dark brown, and was widely used during the Renaissance to the 19th century It takes its name from the painter Anthony van Dyck, but it was used by many other artists before him. It was highly unstable and unreliable, so its use was abandoned by the 20th century, though the name continues to be used for modern synthetic pigments. The color of Van Dyck brown can be recreated by mixing ivory black with mauve or with Venetian red, or mixing cadmium red with cobalt blue. Mars brown. The names of the earth colors are still used, but very few modern pigments with these names actually contain natural earths; most of their ingredients today are synthetic. Mars brown is typical of these new colors, made with synthetic iron oxide pigments. The new colors have a superior coloring power and opacity, but not the delicate hue as their namesakes. Walnuts have been used to make a brown dye since antiquity. The Roman writer Ovid, in the first century BC described how the Gauls used the juice of the hull or husk inside the shell of the walnut to make a brown dye for wool, or a reddish dye for their hair. The chestnut tree has also been used since ancient times as a source brown dye. The bark of the tree, the leaves and the husk of the nuts have all been used to make dye. The leaves were used to make a beige or yellowish-brown dye, and in the Ottoman Empire the yellow-brown from chestnut leaves was combined with indigo blue to make shades of green. Brown eyes With few exceptions, all mammals have brown or darkly-pigmented irises. In humans, brown eyes result from a relatively high concentration