Natural earths (Raw Sienna, Burnt Sienna, Raw Umber, Burnt Umber, Green Earth) All the siennas, umbers, and green earths are entirely natural and come from Italian soil. They are all natural iron oxides. The natural earth colours are completely lightfast and stable in mixtures. Colours referred to as “burnt” come from calcified native earth. Siennas and umbers require a great amount of oil in grinding. Since earths naturally have excellent drying properties, avoid adding drying agents. Earth pigments are suitable for all techniques, and are recommended for fresco.