Pastel is not colored chalk, which is a limestone substance. Pastel is pure pigment - the same pigment used in making all fine art paints. The color effect of pastels is closer to the natural dry pigments whan that of any other process. It is the most permanent of all media when applied to a permanent ground and properly framed. There is no oil to cause darkening or cracking, nor any other substance or medium to cause fading or blistering. Pastels from the 16th century exist today, as fresh and alive as the day they were painted!
Historically, Pastel can be traced back to the 16th century. Its invention is attributed to the German painter Johaim Thiele. Edgar Degas was the most prolific user of Pastel, and its champion. His protege, Mary Cassatt, introduced the impressionist style and Pastel to her friends in Philadelphia and Washington, and thus to the United States. In the spring of 1983, Sotheby Parke Bernet sold at auction two Degas Pastels for more than $3,000,000 each! Both Pastels were painted about 1880.
In the United States, initially pastles only had occasional use in portraiture. However in the late 19th century, Pastel became more popular. Today, Pastel paintings have the stature of oil and watercolor as a major fine art medium.
Pastel is an exciting and unique medium. An artwork is created by stroking the sticks of dry pigment across an abrasive ground embedding the color in the "tooth" of the surface. Light reflects from the jewel-like particles that form the surface of the painting. The colors are mixed on the ground by overlaying or blending, rather than on a palette as with oil paint. If the ground is completely covered with Pastel, the work is considered a Pastel painting; leaving much of the ground exposed produces a Pastel sketch or drawing.
Some artists protect their finished pieces by spraying them with a faxative. A pastel fixative is an aerosol varnish which can be used to help stablize the small Pastel particles on a painting or drawing. It cannot prevent smearing entirely without dulling and darkening the bright and fresh colors of Pastel.
From "About Pastels" by the Connecticut Pastel Society