Furnace

A Furnace (sometimes referred to as an oven) is used to heat blocks and turn them into other items. Furnaces are considered one of the fundamental blocks required to advance in Survival mode. Furnaces can naturally occur in villages.

Natural occurrence
Furnaces can be found inside villages. These naturally occurring furnaces are found in the blacksmith workshops, which contain two furnaces. They contain no fuel or smeltable materials.

Usage
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 smelted, one field for the fuel and one field for the output object.

Wood, Wood Planks, coal, block of coal, charcoal, blaze rods and lava buckets are all fuels. Chests, crafting tables, bookshelves, sticks, saplings, jukeboxes, note blocks, locked chests, wooden pressure plates, wooden stairs, trapdoors and fences are also fuels, though none of them are very efficient. Each smelting operation takes 10 seconds.

Smelting will continue to 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 heat is refilled. If all objects are smelted, the furnace will stop using additional fuel. The furnace will also stop if the output field contains a full stack, or if it contains a different item (e.g., trying to smelt gold ore with iron ingots in the output). If there is no fuel left, the furnace will become inactive. If a smelting process was running, it will rewind at twice the normal smelting speed until new fuel is added.

Gold and iron ores can be smelted into ingots. Furnaces can also be used to smelt raw beef, porkchops, mutton, rabbit, chicken, fish, salmon and potatoes into their cooked counterparts.

When the items smelted in a furnace are collected, they drop experience orbs proportional to the number of items in the stack. This makes furnaces a reliable (if slow) early source of experience.

Furnaces when active have a luminance of 13 and can provide light if you've run out of torches, and they can make charcoal for use in torches. Through this they can essentially be used as fireplaces (since there is no specific fireplace block currently in the game) by placing lots of long-lasting fuel items in it (lava buckets for an example), and then "lighting" the fireplace by placing a burnable item into it.

When holding, the player can place blocks like redstone, repeaters, levers and torches directly onto furnaces.

Trivia

 * Furnaces function as multiple devices combined into one block. Furnaces are really Bloomeries and blast furnaces (for smelting ores), an oven (for baking and cooking), a charcoal pit (for the charring of wood) and a kiln (for baking of clay).
 * Furnaces can be used as an early-game XP farm, but on the one hand cobblestone, food, etc. don't give much XP, on the other, it will take a while to produce the materials for an Enchantment Table. Later, smelting accumulated ores etc. becomes a way to controllably gain experience in fair-sized chunks; a stack of iron gives 45 XP, while a stack of gold gives 64 XP.
 * Furnaces can be used for BUD Switches by placing a fuel and burnable item, making it become an on-state furnace.
 * Furnaces can also be used as a long timer using a BUD switch because each item burns for 10 seconds with a max of 64 items you can make a timer for 10 minutes and 40 seconds depending on the fuel source used and if you use lava you can get 16 minutes and 40 seconds and with a hopper you can make it much longer.
 * The furnace is featured in the "Hot Topic" achievement and is required for the "Acquire Hardware" and "Delicious Fish" achievements, because you need to smelt iron and cook fish for those achievements.
 * Before the furnace appeared, smelting was accomplished by creating a fire and dropping the ores into it. Furnaces are currently available in all versions of Minecraft except for Classic.
 * You can use furnaces to power redstone. A change in a block powers redstone so you can use furnaces as secret doors.