if you burn the end of polyester it will 'melt' become a hard ball of melted plastic. cotton and other natural fibers simply burn, turn to ash, fall away. I do not understand why you would not use polyester thread- since it's pretty commonly used in quilting- but what ever- your choice. personally I would do a strength test & if the thread is strong I would use it- although I doubt that I would use it for piecing since it is such a heavy thread. (most of us use somewhere between 40wt and 60wt thread for piecing) threads that heavy I would use for big stitch quilting, decorative stitching or for other applications (like sewing on buttons- toy construction, pillows-- sewing outside of quilting)