Furnace

A Furnace is a special block currently available in all Minecraft versions except for Classic. Furnaces can naturally occur in NPC Villages. 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 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, Wooden Planks, Wooden Pressure Plate, Coal, Charcoal, Blaze Rods and Lava Buckets (plus Chests, Crafting Tables, Bookshelves, Sticks, Saplings, Jukeboxes, Note Blocks, Locked Chests, Wooden Stairs, Trapdoors and Fences, none of which are very efficient) are all fuels. Each smelting operation takes 10 seconds. Furnaces resemble Dispensers, but the two blocks have different uses and crafting recipes.

A pickaxe is required to collect the furnace.

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 be cancelled and must be redone.

Gold and Iron ores can be smelted into ingots. Furnaces can also be used to smelt Redstone Ore, Lapis Lazuli Ore, Coal Ore, and Diamond Ore (obtained with the Silk Touch enchantment) and returns 1 Redstone, Lapis lazuli, Coal, or Diamond respectively.

Natural Occurrence
Furnaces can be found inside NPC Villages. These naturally occurring furnaces are found in the blacksmith workshops, which contain two furnaces.

Fuel efficiency
Internally, each smelting operation takes 200 ticks to complete. When a suitable fuel source is added, the furnace checks the "heat value" of that item, which is given in ticks.

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

History
Since the Halloween Update, furnaces will face towards the player when placed. Before the update, they faced away when placed.

Since the Beta 1.2 January 13th update, the top of furnaces have a texture similar to cobblestone. They used to have a texture exactly the same as stone.

Since Beta 1.8, you can cook up to 64 pieces of food at a time. Since meats can stack, it's possible to cook them like any other item. Before 1.8, the player could only cook 1 piece of food at a time (e.g. a single Porkchop).

Before the Beta 1.9 Pre-releases, the furnace took more time to destroy than other blocks.

Furnaces also seem to be brighter when smelting an item and dimmer when not smelting anything

As a building material
Since the Beta 1.9 Pre-releases, Furnaces no longer take as long to destroy. Since they take eight blocks of cobblestone to create, they are an inefficient building material. Placing furnaces adjacent of another furnace is more difficult as well, as right-clicking the existing furnace will pop-up the furnace menu rather than allowing to build the furnace. The player always has to build against another non-furnace block. Furnaces are weaker against TNT than ordinary stone or cobblestone.

Tips and tricks

 * 8 coals will smelt an entire stack of 64 blocks. 43 wooden slabs are the next best thing - 43 wooden slabs are capable of generating 64.5 smelted blocks. If you want to be really efficient and not lose that half item to nothing, put in 44 wooden slabs, then take the first two smelted blocks out, and put two extra unsmelted blocks in later.
 * Note: Charcoal is now less useful than wooden slabs comparing to the amount of logs required, use coal from ores or wooden slabs.


 * The bonus of using charcoal or other forms of wood is that they are a renewable resource. While there is an abundance of coal in many cave systems, it is still necessary to search out new deposits. Trees, however, can be regrown (see tree farming). Wooden slabs are, as of 12w17a the most efficient form of wood burning.


 * Blaze Rods are another renewable source of fuel, as they last 50% longer than coal, although the player has to actively farm them in the Nether Fortresses, which aren't easy to find to begin with.


 * A single bucket of lava will allow for smelting of 100 blocks. The largest stack of blocks is 64. Considering that each smelting operation takes 8 seconds, to maximize the efficiency of a lava bucket, place a lava bucket with 64 blocks of unsmelted material (such as sand, cobblestone, etc.) and return between 6 minutes and 10 minutes 40 seconds later to remove 36 smelted blocks and insert 36 more unsmelted blocks.


 * Furnaces 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.


 * For a large project, you can use more than one furnace, it is not uncommon to see a wall of furnaces near a large project, not used all the time, but to reduce the amount of time it takes to smelt a large number of items. When working at this scale, the most efficient fuel would be Coal (or Charcoal).  The most efficient method is to use about 16-32 Furnaces and use 4 Coal/Charcoal for fuel and 32 of the blocks that need to be smelted.  This can convert an entire inventory's worth of resources in the timespan of around 10 minutes.


 * 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.


 * Since all types of logs burn for the same amount of time as wooden planks, converting logs to planks is a quick and easy way to quadruple the burning time (instead of 16 logs burning for 240 seconds, converting them to 64 planks would burn for 960 seconds).
 * In addition, converting planks to wooden slabs is a quick and easy way to double the burning time (in stead of 63* planks burning for 945 seconds, converting them to 126 wooden slabs would burn for 1890 seconds). *63 was used in this example due to the nature of the slab recipe.

Trivia

 * Furnaces function as multiple devices combined into one block. Furnaces are really blast furnaces (for smelting ores), an oven (for baking and cooking) and a kiln (for baking of clay).
 * In an early version of the game, you can have the same effect as a furnace by dropping items on the ground and then burning them with Flint and Steel. This was the only way to smelt ores.


 * Despite paper being a burnable resource in the real world, you cannot use it as fuel. This might be the case because it doesn't burn for very long, and it only creates a small flame at a low temperature.
 * Lava has an one time use when used as fuel. It is usually used to smelt a large amount of ores at once.
 * A torch can be placed onto the front of the furnace by placing a stairs block one spot forward and to the left of the furnace and using the torch on the stairs.
 * Furnaces can hold items in the boxes, however crafting tables cannot.

Bugs

 * In Minecraft 1.0.0, there is a rare bug in which a bucket of lava burns forever in a furnace, without stopping. The fire sprite doesn't change and it can smelt more than 100 items. It will keep on burning unless you destroy that furnace.
 * In Minecraft 1.0.0, there is a rare bug where if you log out while the furnace is burning, on logging back in the fire will burn until the next log out.
 * In Minecraft 1.0.0, if there is a layer of snow on top of a furnace, it cannot be removed except by destroying the furnace.
 * In early Alpha, if you hold a bucket of lava and opened a furnace the lava would be placed.
 * In Minecraft 1.2.5, it is possible to switch the item being smelted at the last instant and replace it with another item which will instantly be smelted.
 * In Minecraft 1.2.5, if you start smelting in English and then switch to Klingon, it will burn for an extended period of time.