I've been building a house made of both wood and stone, and inside it I built a lava furnace to burn unwanted items. Problem is, every so often things begin catching fire. :S The lava furnace is built completely out of cobblestone and stone bricks, which are not flammable, unless I'm mistaken. Moreover, the fire never starts immediately after completing the furnace and setting down the lava. It ALWAYS starts only much later, e.g. 40 minutes later. Am I missing something here?