Pastel is a pure pigment, the same pigment used in making all fine art paints.  It is the most permanent of all media, when applied to appropriate sub-media and properly framed. Pastel has no liquid binder that may cause other media to darken, fade yellow, crack, or blister with time.

Pastels from the 16th Century exist today, as fresh as the day they were painted.  Their first creation is attributed to the German Painter Johan Thiele.
 

Some famous artists that used pastels were Chardin, La Tour, Delacroix, Millet, Manet, Reinor.
 

For more about pastels: "ArtLex.Com"