I often wonder about Rayon too!
I am also looking for other materials that close the loop! All 100% cotton used undies (possibly too much TMI, sorry everyone!) gets tossed in the worm bin, even my husband's dyed shirts, and they all disappear within weeks. Yes, even if something is labeled 100% cotton, the thread is usually polyester, so I never compost these materials in the bin outside only because I can't be bothered with it later.
Anyone have any experience with Modal, another material they claim is plant-derived?
