Any pastel artist who frames their work will tell you that pastel dust is an issue. Over time pastel dust falls from the work and dirties the bottom edge of the matting. When framing and matting a pastel painting, before mounting the matting into the frame fix a thin piece of mat board (about an inch wide) around the back of the last mat, an inch or so from the window opening, on all four sides. This will give a space for that dust to fall and not smear the bottom edge and will keep the artwork just a little farther from the glass.