Tutorial:Furnace

MineTweaker also allows modifying furnace recipes and furnace fuels using a few simple commands.

Do you believe smelting ores in a furnace is too easy? You can fix it as such:

furnace.remove(<minecraft:gold_ingot>);

// alternatively, you can specify the input item
furnace.remove(<*>, <minecraft:iron_ore>);

Found it sad you can't turn charcoal into real coal? Easy fix:

furnace.addRecipe(<minecraft:coal:0>, <minecraft:coal:1>);

Want to make items burnable? Set their fuel value wit the setFuel function:

furnace.setFuel(<minecraft:rotten_flesh>, 100);

The same method can be used to set existing fuel - EXCEPT for vanilla fuels since those are, sadly, hard-coded. Additionally (with the exception of vanilla fuels) you can remove fuels by setting their fuel value to 0.