Char Cloth, or (Charred Cloth) is one of the best materials for catching a spark. Char cloth is a cloth (linen or cotton) that has been combusted in the absence of oxygen. You can use jeans, t-shirts, etc. Don’t use synthetic fabrics as these materials do not char and have a tendency to melt.
Any method of burning the cloth while excluding oxygen should be adequate. The most common container used is an empty metal can (coffee tin, small paint can, soup can, altoids tin, whatever you can find) with a tight fitting lid.
Build a fire and let it die down until you have a bed of roasting coals/logs. Before beginning, throw the empty container into the fire and let it sit until anything that might have adhered to the container is burned off. When the container is black, take it out of the fire, let it cool, and brush off any ash.
Once your container has been cleaned out and is cooling, you can prepare the cloth. Cut the material into pieces that are about 4×4 in size. Place them loosely into the container. Punch a vent hole in the lid of the can to allow smoke to escape. The hole should be less than 1/8″ diameter. Place the lid onto the can and seal it well. You should have a long twig on hand whittled so as to fit snugly into the vent hole. This can be will be used later to remove the hot container from the fire, as well as plug the vent hole while cooling.
Place the container into the embers of the fire making sure the vent hole isn’t blocked. You should see smoke pour out of the vent hole. Watch that these gasses coming out do not catch and make flames. Don’t let the cloth get overcooked or it will be scorched, crumbly, and basically useless. Once the smoke has ceased to come out of the hole, the cloth should be charred.
Remove the container with the twig leaving it plugged in the hole. If air gets into the tin while it is still hot, the cloth will burn to ashes.
Set the container aside until it is cool enough to handle. Open the container once cooled. Properly charred cloth should be a uniform black. If there is still color left in the fabric and it doesn’t seem quite done, then it was not cooked long enough, or the tin was not hot enough, or air got into the tin either during cooking or cooling. In this case put the cloths back in the container, seal it, and put it back into the fire again until it’s right. The cloth, although weak, should not disintegrate or be ashy. If charred correctly, the cloth requires a gentle force to tear it. Keep your char cloth in a dry container or zip lock bag.
Notes: You can do the same type of thing with the dry, punky wood you find in old stumps. Just take some pieces of it and char it in the can like you did for the cloth.