Iron Furnace

The Iron Furnace is much slower but more fuel efficient than a Furnace. For example, it can smelt 10 items per piece of Coal compared to only 8 in a regular Furnace.

Two possible recipes are available. Using a stone furnace reduces the cost by 3 Iron.

Usage
Iron Furnaces can be converted into Generators. This method requires less smelting than the other Generator recipe calling for a Machine Block.

The Iron Furnace can also be upgraded to an Electric Furnace.

Energy Values
Each operation costs 160 units of heat at a speed of 40 units every 9 second, resulting in 36 seconds per smelting operation. Other Wood Items = Any block with its material set to "Wood" (includes blocks added by other mods if material defined properly). Includes all items that can be burned in a regular Furnace.

Video tutorial
http://www.youtube.com/watch?v=1QdT-7wnVas&feature=player_detailpage#t=96s