Magnesium

Magnesium was an element that "burns". When science teacher Ian Chesterton taught this to his Coal Hill School class, Time Lord student Susan Foreman, bored with this rather obvious "revelation", had in mind to make him burn too with one of her grandfather's matches. (PROSE: Time and Relative)

On unnamed planet, the First Doctor saved John from a hostile native tribe by throwing magnesium on their fire, frightening them off with his "magic". (COMIC: The Didus Expedition)