Colour Story: Sienna

Sienna is an earth colour made up of iron oxide and manganese minerals. It can come in a variation of hues, depending on its chemical composition, ranging from a yellow-brown to a red-orange. Sourcing and producing sienna became a flourishing industry during the Renaissance period, but when demand for the colour outsourced the available natural supply, artists and manufacturers had to seek out a synthetic alternative.

Sienna: A timeless earth pigment from Tuscany

The term ‘earth colour’ is given to naturally occurring colours that come from the earth, for example iron oxides from clay. They include Yellow Ochre, Raw Sienna, Burnt Sienna, Raw Umber, Burnt Umber and Terre Verte. In its natural state, yellow-brown Raw Sienna comes from iron ore or ferric oxide found in clays. Unlike Yellow Ochre, which is generally opaque, Sienna is more transparent. When heated, it becomes a reddish brown and is called Burnt Sienna.

Like other earth colours, Sienna was one of the first pigments used for painting and can be found in prehistoric cave art dating back 40,000 years. Examples remain bright to this day, showcasing the durability of Sienna. Though the term sienna was first recorded in English as recently as 1760, the name of the pigment derives from ‘terra di Siena’, which means ‘Siena earth’, referring to the city of Siena in Tuscany where it was produced. Mineralogist Alessandro Fei described the clays of this region as ‘colouring earths’. Extraction of earth pigments in Tuscany have been occurring since antiquity, but the first documented records date back to the early 18^th century, and it became a flourishing economy in the 19^th century all the way through to World War II. There are many extraction sites across Tuscany, including the slopes of extinct volcano Monte Amiata.

Sienna in Renaissance art: Raw and Burnt variations

It was not until the Renaissance period that Sienna was developed for artistic use. Earth tones were widely used by Renaissance artists – in fact, every one of Rembrandt’s paintings contains an earth pigment. During this period the Italians enhanced the range of earth colours by roasting Sienna, leading to the creation of Raw Sienna and Burnt Sienna, which both feature heavily in Renaissance painting techniques by artists such as Vasari, Caravaggio and, of course, Rembrandt. Raw Sienna is a great glazing paint or underpainting base tone, perfect for adding thin layers of bright brown due to its superb transparency. However, it’s important not to layer Raw Sienna thickly as it might leave an uneven finish. Burnt Sienna has an equally thin glaze, but it is a deep red-brown colour often harnessed in oil painting.

The evolution of Sienna pigments in the modern age

Towards the close of the 20^th century, Tuscany’s resources of natural clays became scarce. Until 1988, Winsor & Newton bought Sienna pigments with a beautiful bright undertone from a mine south of Siena. But the Tuscan deposits of Raw Sienna became depleted, and so when the mine was closed, Winsor & Newton bought the remaining stocks, which lasted until 1991. After this, there were no potential suppliers of Sienna that carried the same bright undertone. Italian Sienna pigments increasingly came from other locations, such as Sicily and Sardinia, and small quantities of Sienna have also been mined in Germany’s Harz Mountains. But these alternative ores were not always of the same quality, leading colourmen to look to synthetic pigments. Transparent synthetic iron oxides had already been on the market for some time. When these were evaluated and found to have the same transparent undertone that closely matched old standards, they were widely produced, and formed the Sienna pigments we use today.