Pastels contain pigments (dry color that is capable of a range of effects),  that  create brilliant color that does not yellow with time while creating a velvety matte surface unlike any other medium.  Distinctive to pastel, these  characteristics result from the medium’s composition, a blend of finely ground pigment and white extender coalesced with a minimal amount of binder—the latter merely enough to enable the artist to grasp the stick of color between the fingers yet crumble when stroked across a support. It is this powdery property that accounts for the delicate surface of works in pastel and underlies the issues that must be considered in their care and preservation.
Technically, pastel is powdered pigment rolled into round or square sticks and held together with methylcellulose, a non-greasy binder. It can be either blended with finger and stump or left with visible strokes and lines. Generally, the ground is toned paper, but sanded boards and canvas are also popular. If the ground is covered completely with pastel, the work is considered a pastel painting; a pastel sketch shows much of the ground. When protected by fixative and glass, pastel is the most permanent of all media because it never cracks, darkens, or yellows.

Historically, the origin of pastels can be traced back to the sixteenth century, when Guido Reni, Jacopo Bassano, and Federigo Barocci were notable practitioners.  During the Renaissance artists such as Chardin, Delacroix, Manet, Renoir and Quentin de la Tour experimented with the medium.  Thereafter, a galaxy of artists, including Mengs, Nattier, Copley, Delacroix, Toulouse-Lautrec, Redon, Vuillard, Bonnard, Glackens, Whistler, Chase, and Hassam, just to list the more familiar names, began using pastels as finished work rather than merely for preliminary sketches.

Degas was the most prolific user and champion of pastel, raising it to the full brilliance of oil. His protégé, Mary Cassatt, introduced the Impressionists and pastel to her wealthy friends in Philadelphia and Washington, and thus to the United States. Today, many of our most renowned living artists distinguish themselves in pastels and enrich the world with this glorious medium.

The greatest glory and the greatest weakness of pastel is its powdery composition. These opposing factors have provoked countless debates among artists as to how to stabilize these works, and which substances to use. While some materials and methods may suit a particular artist’s goals, there is no ideal fixative. In theory, any liquid applied to pastel will penetrate the spaces between the fine particles of powder and cause certain colors or the overall composition to become dull or darken, thereby diminishing its characteristic light-scattering property. Fixatives can also alter the color of exposed paper. These factors should be kept in mind if you choose to fix your pastel, and if so, it is best to experiment in advance with the materials you plan to use.  Some artists choose to use no fixative at all.

Information provided with permission by www.pastelsocietyofameria.org.