About the medium of pastels
Despite the fragility of its surface, a pastel can last as long as work in any other medium and will preserve its color and distinctive matte surface with the same freshness as when first applied. Pastel is made using the same powdered pigment found in every medium, rolled into round or square sticks and held together with minimal binder and therefore contain greater concentrations of pigment than other media. It can be either blended with finger and stump or left with visible strokes and lines. I use a textured/sanded paper support. When protected by glass, pastel is the most permanent of all media because it never cracks, darkens, or yellows.
If possible, display pastels in hallways without windows. Avoid direct sunlight and fluorescent lighting. Both are rich in ultraviolet light and provoke fading.
For glazing, use only anti-static materials to avoid having the powder lifted off the surface of the composition. I recommend non-reflective glass that incorporates an ultraviolet barrier (such as Tru-Vu Museum Glass). Matting materials should be 100% ragboard.
~ from Pastel Society of America and IAPS (International Association of Pastel Societies)