When framing your finished pieces of crewel, cross stitch, etc., mount them on white posterboard (foam core board) only.  Do not use cardboard as the resins in it will eventually discolor your work.

Lisa from Arizona has this to add:
.....sometimes foam core board can be just as bad, because some is very acidic. I keep a php testing pen in my purse at all times, so that wherever I am, I can check the acidity of papers (I also scrapbook and decoupage.) That's the only true way I think it can be tested.