Never put anything in direct contact with a charcoal or pastel drawing. Actually, NO drawing as there is always the possibility of it smudging or static removing some of the material.
Kimberly in your case, the glass should not have been placed in direct contact with the pastels. And yes, the pastel WILL come off with the glass. If you expect to sell these pastels, you would have to have them re framed properly. That would mean removing the glass from the pastel surface, and retouching the pastels before reframing. I know this from experience. A gallery owner took two of my smaller pastels (for me) to show to someone in their home.
She placed them side by side in her car VERTICALLY so the pastels fell against the glass. I had to take them to my framer to have them re framed and the glass cleaned. I also had to retouch the pastels. She now wants to have a show of mine!
As to attaching them to a harder surface, they have what are called acid- free hinges and tape for that.
www.nycentralart.com .
Use an archival board or foam-core.
And WHY are you showing things you don't much care about?
As to the future value of a mounted piece, I consulted the drawing department of The Rhode Island School of Design Museum. They said if it was properly done. ie. proper archival backing adhesives etc. it was OK. It was important to go to a knowledgeable framer, not your local chop-shop. They had photos that the had had for years that were perfectly fine. I asked them for a recommendation of a local framer and the name they gave me was fortunately the one I was using.