What is that black paper on the outside of the black boss firecracker box?

The black paper on the outside of the black boss firecracker box is the ignition paper. According to the relevant public information query: black boss firecrackers head is a kind of phosphorus, ignition point is relatively low, and then the box next to that piece of paper is sandpaper, friction is relatively large, so that produces more heat, it is easy to ignite the low ignition point of phosphorus.