Since you think there is a possibility that the fabric is not cotton, have someone familiar with older fabrics check it out. You do not necessarily need to do the burn test to determine fabric content. The feel of the fabric and size of the threads used in the weaving are also clues. You may be concerned over nothing. My mom made me a quilt from fabric that is cotton, poly cotton, and poly. It is beautiful, but the quilt police probably wouldn't think so.