Smelting

Smelting is the way you cook, melt, and burn items in Minecraft. Before the furnace appeared, smelting was accomplished by creating a fire and dropping the ores into it. The furnace, when right-clicked, has its own smelting menu where smelting operations can be done. It consists of one field for the object that will be smelted, one field for the fuel and one field for the output object. Each smelting operation takes 9.25 seconds. Smelting will work when the smelting menu is closed, as long as there are still objects to smelt and there is enough fuel. The fire icon diminishes to represent the fuel burn time. When the fire icon diminishes fully, another fuel item is consumed and the heat is refilled. If all objects are smelted, the furnace will stop using additional fuel. If there is no fuel left, the furnace will become inactive. If a smelting process was running, it will be cancelled and must be redone.

Fuel efficiency
Each operation costs 200 units of heat.

Trivia

 * If you sleep in a bed while a furnace is smelting, the furnace will smelt as if no time has passed. The reason for this is because when you sleep in a bed, no time actually goes by, but instead the game sets the time of day to morning.
 * Smelting Wood with Wooden Planks, making Charcoal, is over 5 times more efficient than using the wood itself as fuel. It is just over 1 1/4 times as efficient as using the planks that that wood would be crafted into as fuel.