Pastels are an art medium composed of the same pigments that are in all art media such as oils, watercolors and acrylics. Because it has no liquid binder it does not darken over time. It is a pure dry pigment with a small amount of binder. Pastels from the 16th century still exist today without restoration.
Pastel paintings should be framed under glass with either mats or frame spacers to keep the painting from touching the glass. All of them are "painted" on archival papers.
Pastels are my favorite medium. I love the rich colors and the fact that the pastel particles are like little prisms when the light strikes them.