I have always wanted to know more about the paints that I use daily and was more than slightly disappointed that the topic was generally ignored during my formal studies. I did have one teacher give a helpful couple of lectures on paints and pigments, but I still feel there is so much to learn about the materials I use in my oil painting. So I have decided to perform some research on my own and catalog my findings here for others who are curious. Naturally, I decided to start with materials that I use frequently, then pan out from there. I sort of randomly chose Burnt Sienna to start off, but that could be influenced by the large levels of burnt sienna that I used today...
Burnt Sienna is a staple color. During my first year (give or take) of painting, burnt sienna was one of only three colors in my palette - along with French Ultramarine Blue and Yellow Ochre. I still use more of it than any other reds or browns (combined). I love the warm reddish-brown hue. It is classified as an Earth pigment which pretty much is self-explanatory; sienna is a clay rich in iron oxides, specifically ferric oxides, that was originally mined from the earth near Siena, Italy - hence the name. Natural sienna is actually yellowish brown and referred to as "Raw Sienna". In order to get the warm reddish brown, the raw sienna is, well, burned. Although, that isn't totally accurate. The sienna is actually calcinated. Calcination is much closer to roasting than burning. (I understand opting not to call the resulting pigment "Calcinated Sienna", but I wish they had gone with "Roasted Sienna"...maybe its just too close to Christmas...) The calcination drives off some of the combined water from the raw sienna which results in a color change. Raw sienna powder is heated to at least the thermal decomposition temperature of ferric hydrate. The ferric hydrate converts to ferric oxide and releases water. (Sorry for the technical details, I used to be a chemistry major...) If you have ever described Burnt Sienna as "rusty", you are right on; Ferric oxide actually is rust (well at least the most common form of rust).
So that is how burnt sienna is made, on to properties before I write a full essay tonight.
As with other Earth pigments, burnt sienna is very stable with a ASTM lightfastness rating of I (The American Society of Testing Materials rating scheme goes from I-V, I being the longest lasting and most lightfast, V being the most unstable). That just means that the color doesn't fade or deteriorate over time. Burnt sienna ranges from transparent to semi-transparent. It is often used as a glaze or wash because of the low opacity. Similarly, burnt sienna has a low tinting affect - when mixing with other colors it doesn't have a strong impact. It is kind of a "weak" color - easily over-powered - but I guess it makes up for it in endurance. Plus, contrary to popular thought, not all oil painting materials contain hazardous toxins; burnt sienna is quite safe - another reason I love using it!
