Smelting and the Furnace

A furnace is crafted from eight cobblestone (or another stone-tier block, like blackstone or cobbled deepslate) arranged around an empty center square. Its interface has an input slot for the item being processed, a fuel slot below it, and an output slot for the result. A furnace turns raw ore into ingots, cooks food, and does both at a fixed pace: one item finishes every 10 seconds, regardless of which fuel is burning. A lit furnace also emits a light level of 13, bright enough to keep the area right around it clear of hostile spawns.

Fuel choice decides how many items one smelting session actually finishes. A single piece of coal or charcoal burns for 80 seconds, enough for eight items; a full block of coal burns for 800 seconds, enough for eighty. Once a fuel item starts burning, it keeps going for its entire duration even if the input slot runs dry partway through, so starting a big batch of fuel for a handful of items just wastes the rest of it.

Most ores drop a single raw or finished item when smelted, which is exactly what makes a handful of ores an exception worth knowing. Copper ore mined directly, with an ordinary pickaxe, drops 2 to 5 raw copper; redstone ore drops 4 to 5 dust; lapis lazuli ore drops 4 to 9 lapis — all without a furnace involved at all. Smelting always produces exactly 1 unit of output no matter what mining directly would have given, so using Silk Touch on one of these ores and then smelting the block afterward actually nets you less than just mining it normally in the first place. Save Silk Touch for ores you want to keep as a whole block, not these multi-drop ones.