When framing chalk pastels some consideration should be paid to the fact that the painting will shed dust over time. This is because artist do not like using much fixative, as fixative can dull the vibrancy of the colours, and the fact that despite fixative, dust will shred even if the pastel is not knocked at all.
Thus dust accumulates along the bottom edge of the mount in a pile and can cause streak marks between the mount and the glass.It should be added that the purpose of a mount is to prevent the glass touching the artwork. Other techniques can be used, but a mount looks nicer.
The Inspirations solution is to pack out behind the mount (usually with foam centred board), about a centimetre away from the aperture, creating a gap between the artwork and the mount for the chalk dust to fall into. This keeps the mount cleaner and the unsightly pile of chalk dust hidden behind the mount.
In this recent example (see below), you can see the shadow created by the mount along the top of the image.