I went through my mom's fabric too - and picked out only the cotton.
My sister and I were going through boxes of fabric -
We lit a candle - then put the candle and an ash tray in/on a large metal cake pan - I had a pitcher of water handy, too - just in case.
I snipped off a piece of fabric about 1/4 inch wide and 2 inches long - held it with tweezers over the candle until it flamed
Most man-made fibers (I think rayon might be an exception) burn with black smoke. Like others have said, they will either make very hot globs or leave a hard or crispy ash.
THE RESIDUE FROM MAN-MADE FIBERS ALSO STAYS HOT LONGER THAN THE COTTON RESIDUE after the flame goes out.
I know this because I just had to feel the ash before it cooled down. And got sore fingers.
Cotton always leaves an ash that literally powders in one's fingers. So does wool. I don't know anything about silk or linen.
Wool has a different smell from cotton.
Nothing wrong with using whatever works in a piece.
My personal preference is to keep the weights and fiber contents about the same, but other than that - - -
Some cotton poly blends are lovely and stay nice for years. (There are some that are not so nice, too)