"A pastel is an art medium in the form of a stick, consisting of powdered pigment and a binder. The pigments used in pastels are the same as those used to produce all colored art media, including oil paints; the binder is of a neutral hue and low saturation. The color effect of pastels is closer to the natural dry pigments than that of any other process.
Pastels have been used by artists since the Renaissance, and gained considerable popularity in the 18th century, when a number of notable artists made pastel their primary medium.
The manufacture of pastels originated in the 15th Century. The pastel medium was mentioned by Leonardo da Vinci, who learned it from the French artist Jean Perreal after that artist's arrival in Milan in 1499." -Wikipedia
Mary Cassatt was one of the most influential artists of 20th century American art. Her friend Edgar Degas was the one who first inspired her to work in pastels and painting pastel portraits was eventually how she made a living in Paris. Pastels could be manipulated with greater speed and ease, had no odor, and allowed for frequent interruptions. For Cassatt, who did many portraits of children, that was essential. Her pastels are most noted for their bold linear strokes that create sensitive, substantive forms.
Edgar Degas is considered the artist most responsible for the transformation of pastel into a major medium. When he began working in pastel they were primarily used for portraits, and often as precursors to complete oil paintings. Degas, however, created a very large body of finished pastel drawings. He worked in pastel throughout his career and in 1880 it became his primary medium. The artist often combined pastel with other media such as watercolor, oil, and monotype, creating rich surfaces with a variety of paint qualities applied in complex layers."
(Source: www.artistdaily.com "Pastel Drawings by the Most Famous Artists of All Time" by Courtney Jordan)