Smelting

Smelting (or Melting, Baking, Cooking, Burning) is the way you smelt or heat items in Minecraft. The furnace, when right-clicked, has its own menu where heating operations can be done. It consists of one field for the object that will be heated, one field for the fuel and one field for the output object. Each heating operation takes 9.25 seconds. Smelting will work when the smelting menu is closed, as long as there are still objects to heat 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 flames are replenished. If all objects are heated, the furnace will stop using additional fuel. If there is no fuel left, the furnace will become inactive. If a heating process was running, it will be cancelled and must be redone.

Fuel efficiency
Each operation costs 200 units of heat.

History
Before the furnace appeared, smelting was accomplished by creating a fire and dropping the ore into it. During the Survival Test versions of Minecraft you started with a bunch of TNT and flint and steel which allowed you to smelt any ores before furnace was released.

Trivia

 * It takes 9 minutes and 52 seconds to smelt a stack of 64 items
 * 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.
 * Burning Wood with Wooden Planks to make Charcoal is over 5 times more efficient than using the wood itself as fuel. It is just over $1 1⁄4$ times more efficient than using planks.
 * Note that when using a bucket of lava, the bucket vanishes when used. Jeb tweeted a while back that he would like to change this so it does not vanish.
 * As of the latest twitter tweets, Jeb has stated he wishes to allow smelting of gravel to create stone bricks, which would then make stone brick blocks in a 2x2 grid.