“ The original colour of these ochres can be modified and varied into browns and reds of more or less intensity by calcination. The nature of the associated earth also influences the colour assumed by an ochre under calcination, aluminous ochres developing red and violet tints, while the calcareous varieties take brownish-red and dark-brown hues. The well-known Terra da Sienna which in its raw state is a dull-coloured ochre, becomes when burnt a fine warm mahogany brown hue highly valued for artistic purposes. ”