I recently bid on and purchased this small quilt top on EBay.
It came with these extra pieces.
I loved the blue setting squares, but when it arrived and I felt them, I suspected they were not 100% cotton. So I decided to test my theory with the flame test. I trimmed a small piece from one of the extra blocks and lit it on fire with a lighter.
This is what it looked like after blowing out the flame.
It is hard and crunchy. This means it has polyester I. It. The flame melts the poly which is essentially plastic.
This is a piece of 100% cotton.
When burned, the ash turns gray and is very soft to the touch.
You can see the soft gray ash on my finger tip.
I recommend doing this over a sink with water running to prevent any disasters! But it is a good test if you are unsure of fabric content.
Have a great day!