Pastels are like chalk. If you touch the surface of art made with pastels it can smear, whether it is new or old. To prevent this, pastels are placed behind glass. When a pastel is matted,
the hard edge of the mat can leave a permanent impression in the art. To prevent this pastels are often floted on top of a mat. If the framed pastel is bumped, dust can migrate from the art onto the
glass or can be trapped between the glass and mat. You can overcome this problem by adding a second mat with a window opening and a spacer between it and the art. This will provide a space for the
dust to fail behind the mat where it will not be visible.
Tip: Avoid using acrylic coverings on pastels. Acrylic is more susceptible to static electricity and the static can "pull" dust particles from the pastel, resulting in an unsighty mess. Pastel should be conservation framed.