Sienna, Italian: Terra di Siena, "Siena ground", is an earth pigment containing iron oxide and manganese oxide. In its natural state, it is yellow-brown and is called raw sienna. When heated, it becomes a reddish brown and is called burnt sienna. Like the other earth colours, such as yellow ochre and umber, sienna is clay containing iron oxide, called limonite, which in its natural state has a yellowish colour. In addition to iron oxide, natural or raw sienna also contains about five percent of manganese oxide, which makes it darker than ochre. When heated, the iron oxide is dehydrated and turns partially to haematite, which gives it a reddish-brown colour. Sienna is lighter in shade than raw umber, which is also clay with iron oxide but which has a higher content of manganese (5 to 20 percent) which makes it greenish brown or dark brown. When heated, raw umber becomes burnt umber and gets a very dark brown colour. Raw sienna is a yellowish brown natural earth pigment, composed primarily of iron oxide hydroxide. It contains a large quantity of iron oxide and a small quantity (about five percent) of manganese oxide.

The colour of Sienna pigment in its natural or raw state.

This kind of pigment is known as yellow ochre, yellow earth, limonite, or terra gialla.

Variations of sienna

There is no single agreed standard for the colour of Sienna, and the name is used today for a wide variety of hues and shades. They vary by country and colour.

Burnt sienna contains a large proportion of anhydrous iron oxide. It is made by heating raw sienna, which dehydrates the iron oxide, changing it partially to haematite, giving it rich reddish brown colour.

The pigment is also known as red earth, red ochre, and terra rossa.

Burnt Sienna:

How it is made: Burnt sienna is prepared by calcining raw sienna which in the process undergoes a great change in hue and depth of colour. In going from ferric hydrate of raw earth to ferric oxide, it turns to a warm, reddish brown.

Chemical Properties: Fe₂O₃ * nH₂O + Al₂O₃ (60%) Manganese dioxide (1%), calcined natural iron oxide. Microscopically, heating makes the pigment more even in colour and the grains are reddish brown by transmitted light.