Saturday, August 31, 2019

BURNT SIENNA: Colour of the month #10

The earth colour pigments known as Raw and Burnt Sienna are traditional colours, which consist mostly of iron oxides, hematite and goethite. One pigment is yellowish-brown and the other is reddish-brown, which differentiate in the thermal processing of the raw material.

Photo: Raw Sienna. Studio Laura Daza

Raw Sienna: Siena, an Italian city historically known for the production of the pigment and the origin of the name. This pigment and material was widely collected and produced during the Renaissance in Italy and sold in Europe. Raw Sienna is clay, which contains the iron oxide ‘goethite’ giving the yellow shade.

Burnt Sienna, is also a traditional colour pigment and is the result of roasting Raw Sienna or yellow ochre, which becomes a reddish-brown. Burnt Sienna is obtained by dehydrating the mineral 'goethite' or Raw Sienna at 270 degree celsius approx.

Image: Left. Technisches Museum Wein. Right. Studio Laura Daza

The phenomenon that yellow ochre when heated converts to red ochre has been known since the Paleolithic, which not only makes it the first synthetic pigment but also the earliest form of pyrotechnology.

Photo: Gudy Herder. Work by Studio Laura Daza

Pyrotechnology stands for ‘pyros’ fire and ‘technology’ the use of fire through alchemic transformation. Fire was used to alter the chemistry of the naturally occurring material 'iron oxides'. Yellow ochre is rich in goethite minerals that when heated changes its colour to a reddish-brown. 

This phenomenon of colour transition is caused by a process of dehydration of the goethite molecule which reassembles and changes to a hematite molecule, therefore the colour changes to red.

Photo: Studio Laura Daza

This heating process was also used to increase the painter’s palette because their colours were limited to few pigments. Old masters and alchemists were keen and resourceful in creating new hues. Raw and Burnt Sienna pigments were quite used for its excellent coating power and tinting strength. 

Photos: Studio Laura Daza

Throughout history, many alchemists, artists, chemists have documented different technological processes for making colours, names such as: Cennino Cennini, Pliny the Elder, Vitruvius, Vannoccio Biringuccio “De La Pirotechnia". In traditional manuscripts, we may find different concoction recipes and methods for creating new hues.  

Photo: Studio Laura Daza

In Italy, clay-earth deposits are found in the ochreous sediment region of Tuscany, where Raw Sienna and Burnt Sienna were exploited. These names denote a geographical origin but today are used for other pigments with similar tinting properties. 

These pigments were widely extracted during the 19th and 20th centuries and used by many Tuscan artists in the Middle Ages and Renaissance, Duccio di Buoninsegna and Ambrogio Lorenzetti. Then it expanded to other Italian and European artists such as Caravaggio and Rembrandt. There is evidence that these deposits were formed in that area in Italy in the Pleistocene geological era.

Image: Rembrandt

When moving to Italy, I became quite interested in their traditional shades, earths and authentic techniques for colour making during the Renaissance time such as roasting siennas and umbers in order to obtain more colours.

I decided to hunt and search these traditional colours. As a colour alchemist and with the guidance of a geologist, I learnt how to identify earth pigments, collect, extract, ground, purify, mix, sieve and roast.

Photo: Studio Laura Daza

In the experimentation phase, yellow ochre specimens were collected, studied and then step-heating experiments were done. We used 5 specimens that were treated at different temperatures starting from 200, 350, 500, 750, 900 degree celsius. They started to change colour at 250 – 300 degree celsius approx.

Photo: Studio Laura Daza

The result is a beautiful colour transition palette of different shades going from yellowish-brown to deep orange,  red to reddish-brown. I love the versatility of earths, its aroma and the most important property is its non-toxicity.

Photo: Studio Laura Daza

--> --> -->