Ive recently been experimenting with different types of 100% cotton cloth to catch sparks. Not to scientific standards though. :-?
I found that there were tremendous differences between the cloths. The worst I used was a cotton dress shirt which was thin but good quality cotton. It would catch a spark but was irregular and not reliable. :-(
The best was a cotton tea towel. It seems that the more fibrous the cotton the better. It is regular as clock work. :-D
Another tip is that when you make your charred cloth don't cut and throw the brown patches away. When you have made your ember you can wrap the brown cloth around this giving you a bigger and hotter ember.