Furnace

A Furnace is a special block currently only available in the Indev, Infdev, Alpha and Beta versions of Minecraft. 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 smelting menu where smelting 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, coal, charcoal and lava buckets (plus chests, workbenches, 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 9.25 seconds. Furnaces resemble dispensers, but the two blocks have different uses and crafting recipes.

A pickaxe is required to pick up the furnace.

Smelting will work when the smelting menu is closed, as long as there are still objects to smelt 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. 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.

Items such as gold, and iron blocks can be smelted into ingots.

In Beta 1.8, a future update, you can cook up to 64 pieces of food at a time, as stated by Jeb.

As a building material
Furnaces take more time to destroy than other blocks. However, since they take eight blocks of cobblestone to create, they are an inefficient building material. They are also weaker against TNT than ordinary stone or cobblestone.

Tips and tricks

 * 8 coals will smelt an entire stack of 64 blocks. 43 wooden planks are the next best thing - 43 wooden planks 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 planks, then take the first two smelted blocks out, and put two extra unsmelted blocks in later.


 * Remember that pork (like other food items) won't stack. This means that 1 coal will fuel 7 units of pork, depending on how fast you are with re-loading the furnace. Sticks come in handy, as one stick should be enough to cook an eighth unit of pork if you did not waste too much fuel on the transactions, or one can use sticks or other inefficient fuel to burn food items one at a time.
 * 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. Charcoal is the most efficient form of wood burning, but Wooden Planks are second best and can be made quickly.


 * 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 10 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.


 * 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 it is required for the "Acquire hardware" and "Delicous fish" achievements, because you need to smelt iron and bake fish for those achievements.

Trivia

 * In the first version of the game, you can have the same effect as a furnace by dropping items on the ground and then burning them with the flint and steel. This was the only way to smelt ores.
 * As of the Halloween update, furnaces will face towards the player when placed.


 * As of the January 13th Update, the top of furnaces have a texture similar to cobblestone. They used to have a texture exactly the same as stone.


 * 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 and low temperature.
 * Lava is one time use when using as fuel. It is usually used to smelt a large amount of ores at once.


 * There is a bug in the 1.8 Pre-release that if you use a furnace, your game will sometimes crash, making it difficult for players to obtain ingots.

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