Museum Glass made by Tru Vue is a terrific product, if the cost doesn’t scare you away. I’ve been wanting to try it for some time. You have to be very careful when handling and cutting, as it scratches and picks up finger prints easily. With almost no reflection and 99% UV protection, a pastel painting under glass can almost look like an oil painting.
Besides being used over mats or with a spacer in framing, Museum Glass is and has been used directly on art work, even pastels. It’s a French method of framing use for years called Passe Partout. With this framing technique, the art is sandwiched between backing board and glass, then the edges are sealed with pH neutral tape, like FrameTac. In theory, humidity is better eliminated since there’s no space between art and glass. It’s back to the future as far as I can see, since a small but growing number of pastel artists are using the Passe Partout Method. I’ve tried it and noticed it does make paintings look amazingly clear.
Concerning this and other framing methods, you may also reference Maggie Price’s article titled “A Clear-cut Guide to Framing” from the December 2004 issue of the The Pastel Journal magazine.