Furnace
Link to 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:
ZenScript Copyfurnace.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:
ZenScript Copyfurnace.addRecipe(<minecraft:coal:0>, <minecraft:coal:1>);
// or, if you want to generate 0.5 XP by doing so
furnace.addRecipe(<minecraft:coal:0>, <minecraft:coal:1>, 0.5);
Want to make items burnable? Set their fuel value wit the setFuel function:
ZenScript Copyfurnace.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.