An artificial product, which takes fire upon exposure to the air, and hence called air-tinder. It is prepared from alum by the addition of various inflammable substances. The simplest mode of preparing it is to mix three parts of alum with one of wheat flour, and calcine them in a phial or mattrass, until the blue flame disappears, then keep it in the same phial till cold, well stopped with a good cork. In this powder be exposed to the atmosphere, the sulphuret attracts moisture from the air, and generates sufficient heat the carbonaceous matter mingled with it.