"Hot enough?"--- Thaumonomicon Entry
The Infernal Furnace is a larger, more efficient method of smelting items. It is classified as minor Forbidden Knowledge, and will give the player a small amount of Warp when researching, or when activating one.
Items to be smelted can be tossed in the top of the furnace. An item grate on the top will keep you from falling in, and adding a hopper and chest atop that will make feeding items in somewhat easier. Smelted items and experience are tossed out the front of the furnace. It's wise to put a Hungry Chest directly below the output grate, and a Brain in a Jar to one side of that.
Attaching up to three arcane bellows to the furnace can increase the speed of smelting by a huge amount and produce a bonus when smelting ores by producing nuggets. Note that there are four "ports" on the sides and bottom of the Furnace; a fourth bellows will give no extra benefit, but....
Fueling the furnace with Ignis will greatly increase the speed at which the Infernal Furnace smelts items, and the chances of nuggets. Formerly, it was sufficient to place a jar of Ignis essentia nearby, but in current versions, the essentia does need to be piped into that fourth port. Alternatively, the Funace can be powered with Ignis centivis, from an Energized Node.
The Furnace also produces nuggets when smelting meat. These meat nuggets are necessary to produce the wondrous Triple Meat Treat.
Despite the Thaumonomicon entry, the Infernal Furnace no longer produces flux during use. However, if any block of it is broken, not only will the construct revert to its blocks (the lava is lost), but a Blaze will be spawned inside the furnace.
By harnessing raw Ignis within a furnace crafted from obsidian and nether brick you have created a highly efficient and fuel free furnace. So efficient in fact, that occasionally it will produce bonus materials in the form of nuggets or other items.
While this is essentially free, it is also very slow unless the furnace gets a new supply of Ignis essentia every now and again. This can be done by placing a jar of Ignis essentia within line of sight of the front of the furnace, somewhere within 10 blocks of the front of the furnace. Luckily, a little essentia goes a long way.
It should also be noted that the occasional bit of Flux escapes into the environment.
Items you wish to smelt need to be dropped into the top of the furnace and will be spit out the front.