Hmmmm, did you char it enough?
I use denim most of the time. I've tried t-shirts, but denim is tougher. Not sure how you charred it. I put 2"x2" squares of old denim cut from worn jeans in a small metal can with a tight fitting lid (with a small hole in the lid). Throw the thing on the fire and watch it. When it starts putting out a lot of smoke, remove it from the fire, plug the hole so no air can get inside and let it cool.
So far, it hasn't failed me.