Ochres are a group of pigments varying in colour from red to brown to light yellow. Natural ochres are minerals combining clay and iron oxide: the colour of the ochre is determined by the proportion of iron oxide present. They are an important type of pigment as they are chemically inert and therefore completely reliable on all surfaces and in all media. Ochre is listed among the pigments used for colouring stucco marble in the nineteenth and twentieth century. Trench (2000); Wittenburg (1999). [J. Cassar, R. de Angelis]