Smelter

The smelter are made for crafting things. Upon opening the GUI, there is a slot for lava buckets, which is the only fuel you can use for this. There is a slot with a coal shape in it, this is where you usually put coal. Then a slot next to that which has an outline of an ingot.

Usage
Used for smelting things.

Upgrades
The smelter has modes, or upgrades that you can add. You place these in the fuel slot of the smelter. Note, you still need to have lava in the smelter.

Trivia

 * The process of making steel is similar, you add carbon to iron.
 * Even though it's an entirely new block with it's functionality and GUI, you still can get the achievements for steel.
 * The area for the fuel was overlapping the "Inventory" text, which then had to be moved over some.
 * There used to be a recipe for the smelter that used coal and steel to get titanium. This was removed and titanium ore was added instead.