Cooked items are food items which give more food points than their raw variants. They can be obtained through methods such as cooking them in a furnace or on a campfire, trading or killing mobs with Fire. In Minecraft, cooking food increases the amount of food points when consumed and removes the risk of getting hunger from raw chicken.
Smelting is a method of obtaining refined goods from raw materials by heating in a furnace, blast furnace, or smoker. To smelt, an input item and fuel must be placed into the input and fuel slots, respectively. The furnace then begins to smelt and will continue even after the player closes the interface. The furnace burns one fuel item at a time, with the fuel gauge indicating how much of that item's burn time remains. As each fuel item is fully consumed, another one is taken from the fuel slot and the gauge starts over.
See the below articles to learn more about smelting.
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