Smelting

Smelting is a method of producing refined goods in Minecraft, also known as melting, baking, cooking, burning, and producing. It has the same idea as crafting; one must put in acceptable ingredients, and a corresponding output will be given. However, smelting utilizes furnace blocks, which have a unique interface: one "input" field for the object that will be heated, one field for the fuel, and one "output" field for the final smelted product. For example, two saplings (fuel) could be used to smelt one wood (input) into one charcoal (output).

To smelt something, an input material and a fuel must be loaded into the furnace. It will begin to smelt on its own and will continue to work if the menu is closed and the player leaves. When the furnace begins to smelt, it will consume one piece of fuel and the fire gauge will fill up. As the input smelts, the fire gauge will slowly recede until it is gone, and then the next piece of fuel will be used. If there is no more fuel left, the furnace will be interrupted and the current item being smelted will be left raw. However, if fuel is burning and runs out of input, the fire gauge will continue to burn down, wasting the burn time left. Once the fire gauge is out, no more fuel will be used.

As things smelt, an arrow icon represents the cooking process. Each smelting operation takes 10 seconds and the progress will be shown on the arrow. If the furnace runs out of fuel before the arrow is filled up, then the input will not be smelted and the process will have to be repeated. When the arrow fills up completely, one input item will be put into the output field as an output item.

If the player leaves a furnace while it is smelting and travels so far that the chunks unload, the smelting process will halt until the player returns. Smelting will also pause if one leaves the dimension the furnace is located in. If the player sleeps in a bed while a furnace is smelting, the furnace's progress will be the same as if the bed had not been used and no additional time had passed. This is because when a player sleeps in a bed, no time actually passes. Instead, the game sets the time of day to morning.

Wasting ores
The following ores can be smelted, but in these cases this is unnecessary and wasteful. All these ores will yield their product freely when mined with an appropriate pickaxe. The ore blocks themselves can only be obtained with the Silk Touch enchantment. Also, these ores either produce multiple drops when mined, or may do so when mined with a Fortune-enchanted pickaxe. Smelting, however, will always give only a single unit of the product. Mining the ore blocks will also give far more experience.

Fuel efficiency
Sleeping while smelting/cooking items will not speed up the process.

Note that after a piece of fuel has started burning, you cannot stop it from burning, though you can remove any unburnt pieces of fuel. If there are fewer items that the fuel can burn, some of the fuel will be wasted. However, a single bucket of lava (or a coal block in 1.6) can burn more items than will fit in the furnace -- both input and output are limited to a stack of 64, and lava can burn 100. To use lava efficiently, keep the remaining load of 36 items handy, and put it in during the middle third of the burn (after it has done that many, but before it exhausts the original stack), alternately, using the hopper on the top of the furnace will funnel smeltable items into it. Remember to both refill the unsmelted stuff, and remove the smelted product! (A despawn timer on a note block may help.)

Also note that a wooden tool such as a pickaxe still smelts 1 item even if the durability is drained. so for example if you just began a survival world and you have a little remainder left on your wooden pickaxe you can toss it into a furnace to burn a piece of wood to get charcoal...

Trivia

 * It takes 10 minutes and 40 seconds to smelt a stack of 64 items.
 * Burning Wood with Wood 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.
 * When making charcoal, it's most efficient to convert 1/7 of the logs into planks (for fuel)
 * Jeb has stated he likes the idea of smelting Gravel to create "Stone Chunks", which could be crafted into Stone Bricks in a 2x2 grid.