Coal

Machines in the Land of Mists have long functioned via kinetic energy supplied by their users, slaves, domesticated animals, or even the undead. Kinetic energy supplied by wind or water is the rule in mills, although its supply by the undead is not unknown. Steam engines can make use of the chemical energy of wood but operate far more efficiently exploiting the greater chemical energy contained in coal. Blast furnances would be imposible without the greater energy content of coal.