Smelters are a machine that is included with Nether Metals. They are Furnaces that are fueled by Lava and smelt metals extremely fast. You load a Smelter by right clicking on it with a Lava Bucket, which loads lava into it and fills its fuel meter.

In order, from slowest to fastest, the ranks of smelters are:

Ignatius Smelter -> Shadow Iron Smelter -> Shadow Steel Smelter-> Vyroxeres Smelter -> Inolashite Smelter