Burnt sienna, also known as Caledonian brown and terra di Siena, comes in a variety of shades, depending on its source. It gets its rich dark brown coffee color from a heating process.
In order to get burnt sienna, you must first heat raw sienna in a process called calcination which removes the water from the clay and turns it from a yellowish to a red color. According to wikipedia, calcination in a thermal treatment process applied to ores and other solid materials in order to bring about a thermal decomposition, phase transition, or removal of a volatile fraction. The Dutch were famous for this technique and made a rich earthy color they marketed as English Red.
One of the earliest known pigments, it has excellent drying and glazing qualities. I find it useful for wood tones, the less lighted side of red objects, fabrics, to warm blacks (and speed up the drying time too) and many other applications. It also has a very pronounced tooth.
Kurt Wehlte states in his book, The Materials and Techniques of Painting, "when stored for long periods, powder pigments sometimes increase their color saturation by continued oxidation." Saturation is the intensity of a color and is a critical component of a pigment for an artist. Many colors/hues have what is known as a "low tinting power" or a "high tinting power" meaning it takes a little or a lot of paint to achieve the desired color.
So, what Wehlte is saying is over time powdered pigments can become more intense. I certainly found this true recently, quite by accident. While gathering thoughts and doing research for this project, I recalled I had a few pigments in storage from many many years ago, perhaps 20 or 25. One of the pigments suitable was burnt sienna, stored in a tin all these years.
After making some paint, and using it in a painting, I wondered why it was so intense. Now, after stumbling on this research information in the Wehlte book, I know why. Continued oxidation! Whodathunkit?