my experience?
wife caught me kindling a tiny, tiny fire in the middle of the garage floor, and took my char cloth away. silly her! it wouldn't have worked anymore anyway with all of that white stuff that comes out of a fire extinguisher on it.
but before Red Adair of the north swooped in i had a respectable lick of fire going. used the spine of my patch knife for the steel and the brand new charcloth i made of 1000 count cotton sheet. most of the commercial cloth i have used was too loose of a weave.
when i throw a sparck on the tight stuff the ring of fire travel slow but steady.
i even have time to put tiny slivers of pitch pine on it and catch these a fire.
then i snuff the char cloth for further use.
i wonder if that white stuff would wash out?