The Nether Furnace is a cool new block added with Nether X. It is crafted the same way as a normal furnace, except with netherrack. A cool thing about the nether furnace is that it does not require any fuel. Because of this ability and that they are so easy to make, they have a one out of eight chance to turn back into a piece of netherrack, spewing all the items inside it on the ground, (so at least you don't lose them). It can be very useful, but also very risky.