You can tell if it is cotton by how it burns. Snip a small piece off and hold by your tongs over the kitchen sink...with the water running. As soon as it flames, stick it under the water. If it is soft and ashy, it is cotton. If it has a hard ridge like burnt spot, it has some other type of additive. Polyester melts and makes the hard ridge. Want to know how I know to do it over the sink with the water running?