you can't really do a burn test because there's nothing you can spare to cut off. however, if you use it as a large center panel, it doesn't matter what it is as long as it's washable. i would still make the borders out of cotton that has been preshrunk, as the panel should be. in fact if there's a doubt in your mind that there's any rayon in there. shrink it 2x.
it's really a great find and would be a beautiful and striking quilt, if not for yourself, then as a gift.