Unless the artist has sprayed a fixative on their charcoal or pastel artwork, little bits of the medium could fall off over time and so special care is taken when framing these types of artwork.  Often the mats will be cut with a reverse bevel and a spacer will be used between the mats so that if any of the medium does fall off, it won't show on the white bevels of the mats. Also, I take extra care when fitting charcoals and pastels in order to not jar any of the medium loose.