"Non glare" glass is used in contact printing frames to eliminate newton's rings. Get the finest pattern your picture framing supplier has; single-sided is fine. Make sure they cut a defect-free section and remember to put the rough side toward your negative.

If you've never struggled with newton's rings, congratulations! Keep using plain glass when contact printing. If you have been frustrated by this problem, using "non glare" glass will bring great satisfaction.