Rag (cotton) matt is better for its purity, though it requires an extremely sharp and accurate cut. No framer would ever use mattboard on artwork that discolours. Immediately after matting, frame-up: the longer the material is exposed to atmospheric oxides, the greater the chance of discolouration. That's why all matted work is usually wrapped in acid-free covering to protect it if frame-up is not going to occur in the very near future.