[5] Like the other earth colors, such as yellow ochre and umber, sienna is a clay which is partially composed of iron oxides.

In the case of sienna, the most prevalent iron oxides are limonite (which in its natural state has a yellowish color), and goethite.

[citation needed] In the Middle Ages the sienna pigments were used by artists such as Duccio di Buoninsegna and other painters who lived and worked in and around the Republic of Siena.

Along with umber and yellow ochre, sienna became one of the standard browns used by artists from the 16th to 19th centuries, including Caravaggio (1571–1610) and Rembrandt (1606–1669), who used all three earth colors in his palette.

[10][11] Cross sections of Rembrandt's works, analyzed by X-Ray and infrared lenses, reveal that he used variations of sienna to prime his paintings.

[12] Although these artists are known to have used sienna and its variations in their works, scholars have pointed out that the pigment was not commonly referenced by name in European sources until the mid-eighteenth century.

[citation needed] In the 20th century, pigments began to be produced using synthetic iron oxide rather than natural deposits.

Historically, the pigment was prepared by taking lumps of earth and placing them into a fire either using a crucible or shovel, in order to induce the necessary chemical reaction.

[15] Sienna varies slightly in shade and hue based on the chemical composition of the soil and the temperature and length of time in which it is prepared.

[17] Raw sienna is a yellowish-brown natural earth pigment,[2] composed primarily of iron oxide hydroxide.

It is made by heating raw sienna, which dehydrates the iron oxide, changing it partially to hematite, giving it rich reddish-brown color.