Quote:
...I was told by a framing shop that there is "museum glass" that offers 99.9% UV protection...
They were blowing smoke. "Museum glass" is actually a registered trademark of a specific manufacturer. It is 99% effective at reflecting UV light. I know of no glass which is greater than 99%.
Over time mounting any stamp in a frame and hanging it on the wall will cause the stamp to fade over time. Using "museum glass" or equivalent, keeping it out of any sunlight, should keep fading to a minimum. Under those conditions it may take years but make no mistake about it, the stamp will undergo color changes.
Don