I grew up calling it Ash Cloth, but it seems that most of the modern world calls it Char Cloth, so I’ll go with that. My main blog, One Hundred Ways to Make Fire without Matches, explores the necessity of some type of pre-tinder for some types of fire starting. Flint and Steel is one of the methods that require such stuff–for the purpose of catching a spark that can in turn be nursed into full flames.
There are natural tinders, such as a fungal residue that grows mostly on Birch trees, and various shelf fungi and a preparation of these called Amadou (French for tinder), parts of dried Milkweed pods, dried Mullein Stem velvet, and pitch-pine scrapings to name a few of those better known. These can be hard to find in a pinch. Char Cloth has become the standard, but this is a relatively new development.
Ancient peoples, even those living in relatively recent times, seldom had the luxury of burning modern textiles for this use. On the other hand, the use of charred wood and other plant materials is likely as old as is the use of fire by man. Nonetheless, char cloth is a good material to use for the purpose of catching sparks in absence of natural tenders that require no preparation. It is also superior–with very few exceptions.
Char Cloth is easy enough to prepare, given natural fabrics such as cotton, linen, and silk. Rayon, which is a synthetic fabric made from wood pulp can also work in some cases. Most other synthetics just melt instead of properly charring. Wood, mushrooms, leaves, and other natural plant materials also can be charred well enough to use to catch a spark and transfer it to a larger tinder bundle.
Following is a simple down-and-dirty how-to for making a small quantity of char cloth. This amount will last an individual user through the making of many fires using flint and steel, parabolic reflectors, and magnifying glasses. For charring the materials I use the same old metal shoe polish can I began with long ago. Altoids tins are often cited as good for the same purpose.
When making a large amount of char cloth or other materials for group use, I follow the same steps but use a bigger can. The main requirement is to have a means of starving the material of air within the can while it is burning, so that the burn leaves mostly carbon-rich material–which is easily ignitable at low temperatures.
Old, but stain-free and otherwise clean denim blue-jeans or cotton Tee-Shirts make good materials for charring. I cut it in squares small enough to fit flat in the shoe-polish can with the can about two-thirds full, not too tightly packed. The top of the can has a small nail hole in it. With the lid tightly secured, I place the can of cloth on a bed of coals, in a fireplace, or on a gass grill. I have even used a blow-torch as the heat source. I suppose my preference is a bed of good grilling coals, just before or after I have grilled some charcoal burgers.
As the can heats up, smoke will blow out of the hole in the top, like a steamboat. when the smoke quits coming out, I toss the can around and shake it a couple of times while still on the heat and make sure it has really stopped smoking. If it has, it is done. You don’t want to burn longer than that, or it will burn up and will be useless, so pull the can out and let it cool.
Don’t open the can up until it has cooled or it may burst into flames. Once cooled, open and inspect the materials. If it didn’t burn long enough, some of the cloth may appear partly brown. If it is just a couple of pieces, you may just want to pull them out and forget it, but it is easy enough to remove those completely charred black and return the brown ones to the fire to finish charring. It may take a couple of times to find the best procedure for you–given the materials, can, and heat source you’re using.
I will say that not all char cloth batches are created equal. If it is made correctly, it will catch the slightest spark and burn. If it doesn’t do that, something is wrong. It5 is so easy to do, I would recommend tossing that batch and starting over.
The links to my bigger blog below shows both a video and more details about char materials than you will likely ever need or want.