Smelting

Smelting is a method of producing refined goods in Minecraft, also known as melting, baking, cooking, burning, drying, or 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).

Usage and Mechanics
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. You can tell whether a furnace is working or not by seeing if the furnace is lit or not and if the fire particle effects are appearing or not. 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 item will be uncooked. 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 rewind at 2x speed. 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 they 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.

Other smeltable items
A note about fractional experience values: For fractional values, first multiply this value by the number of smelted items removed from the furnace, then award the player the whole-number part, and if there is a fractional part remaining, this represents the chance of an additional experience point.
 * For example, when smelting 1 coal ore and removing the coal, the value is 0.1, so this grants a 10% chance of getting 1 experience point.
 * Or, when smelting 6 cactus and removing all 6 cactus green, the value is 0.2 * 6 = 1.2, so this grants 1 point, plus a 20% chance of an additional point.

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 than the fuel can burn, some of the fuel will be wasted. Accordingly, you will usually want to smelt things in multiples of 8 (when burning lumps of coal or charcoal), or occasionally 3 (2 wooden planks). For individual smelts, you can use sticks or saplings, 2 of which smelt 1 item.

When starting a new world, burning your wooden tools (once you have stone) is a quick way to make your first lump or two of charcoal. They each still smelt a complete item even if almost used up.

For larger jobs, a single lava bucket or a block of coal can burn more items than will fit in the furnace—both input and output are limited to a stack of 64, but a block of coal burns 80, and lava can burn 100. To use lava efficiently, keep the remaining load of 36 or 16 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). Remember to both refill the un-smelted items, and remove the smelted product. A despawn timer on a note block may help.

Alternately, you can use hoppers on top and bottom of the furnace. This will automatically feed and empty the furnace, but loses the experience gained by manually unloading the furnace. For larger smelting jobs, a third hopper can feed in fuel (empty buckets will go in the output hopper).

Video
Note: this video is outdated, as it doesn't talk about blast furnaces, smokers or campfires cause this video is taked before 1.14 version.

Trivia

 * It takes 10 minutes and 40 seconds to smelt a stack of 64 items (that is, 10 seconds per item) in a single furnace, although this time can be reduced by splitting the load between multiple furnaces. This can be reduced even further by using redstone mechanics.
 * Burning wood with wood planks to make charcoal is over 4 times (×4.57) more efficient than using the wood itself as fuel. It is just over 1¼ times more efficient than using planks.
 * When making charcoal, it's most efficient to convert $1⁄7$ of the logs into planks (for fuel)
 * If you want to use wooden tools as fuel, the most efficient way is to use it until its durability reaches 0, then use it as a fuel afterwards.
 * "Smelting" is a pretty broad term in the context of Minecraft while in the real world, smelting has a more precise definition.