Minecraft wiki entry for Furnace, associated text: Block entity data
Tags common to all block entities[show]
 BurnTime: Number of ticks left before the current fuel runs out.
 CookTime: Number of ticks the item has been smelting for. The item finishes smelting when this value reaches 200 (10 seconds). Is reset to 0 if BurnTime reaches 0.
 CookTimeTotal: Number of ticks It takes for the item to be smelted.
 CustomName: Optional. The name of this container in JSON text component, which appears in its GUI where the default name ordinarily appears.
 Items: List of items in this container.
: An item in the furnace, including the slot tag:
Slot 0: The item(s) being smelted.
Slot 1: The item(s) to use as the next fuel source.
Slot 2: The item(s) in the result slot.
Tags common to all items[show]
 Lock: Optional. When not blank, prevents the container from being opened unless the opener is holding an item whose name matches this string.
 RecipesUsed: Which recipes have been used since the last time a recipe result item was manually removed from the GUI. Used to calculate experience given to the player when taking out the resulting item.
 recipe ID: How many times this specific recipe has been used. The recipe ID is the namespaced ID of the smelting recipe, as used in the /recipe command.