What you are doing when you make charclothe is converting the clothe into "charcoal". You are driving off all the internal and chemically bound moisture and assorted gasses from the fibers - leaving just the carbon. That carbon will then catch your sparks and burn - becoming your ember/coal.
The more used/washed/worn your clothe, the better the charclothe it will make - either cotton or linen. That "use" of the fabric loosens up the fibers in the threads, and "fuzzes" them up more. All this will make it easier for the sparks to catch in the fibers, and then keep burning.
Since nobody has yet to find any documentation for ... tins ... for making charclothe, Karl K came up with a method of making it that did not use a tin.
Take a long strip of your clothe, and roll it up onto the end of a green stick. Make a roll a couple inches thick. Don't roll it too tight. It should look kind of like you were making a "torch". Now put it in your fire. The outside will catch and flame up. Let it burn a while. When the ouside is well blackened, or glowing red, carefully bury it in the dirt to smother the fire. Leave it buried until you are sure it is completely out. Then dig it up, and clip off the extra stick. The stick through the center helps support your new charclothe. Store it in some sort of leather pouch/container to protect it, but also to keep from getting black soot over everything else.
The outside will now be black, with parts crumbling. This crumbly part has had some of the carbon in it burnt as well - leaving less to catch your sparks. As you start to unroll it a bit, the cloth will still be black/charred, but won't crumble on you. There is your "charclothe".
To use, just unroll a section as long as you need, and tear it off.
As you unroll and use it up, the clothe will start to change color from black to gray to brown. When you get to the browned clothe, it will no longer catch sparks well. When it gets to this level, just ... burn/roast ... it in your fire again to "char" some more of it.
Yes, this does "waste" some clothe. But you don't need any "container" to cook your charclothe in. So this method gets around that issue of no documentation for "tins" for making charclothe in.
The almost complete lack of documentation for charclothe itself is ... another discussion.
Just my humble thoughts to share. Take them as such.
Mikey - that grumpy ol' German blacksmith out in the Hinterlands