(Science: chemistry) clay that is coloured red or brown by the oxides of iron or manganese, and used as a pigment. It is used either in the raw state or burnt. Burnt sienna, sienna made of a much redder colour by the action of fire. Raw sienna, sienna in its natural state, of a transparent yellowish brown colour.
Origin: It. Terra di Siena, fr. Siena in Italy.