Печь

Stones, Wood and Low Grade Fuel — no workbench, no blueprint, buildable from the moment you spawn. It takes 15 seconds to craft and doesn't need to be placed inside a base to work, though it will decay and can be stolen from if it's left outside your Tool Cupboard's range.

Why the Low Grade Fuel matters

Most starter recipes don't ask for fuel at all, so it's easy to queue a furnace craft and get stuck short — make sure you've skinned a few animals for fat (see the low grade fuel guide) before you try to place one, not after.

One furnace isn't enough for long

A single furnace smelts slowly, and ore piles up fast once you're mining seriously. Most bases run two or three furnaces side by side within the first hour, and a Large Furnace (unlocked separately) later on for real bulk smelting — don't treat the first one as a permanent setup.

What it's actually for

The furnace is what turns Metal Ore into Metal Fragments and High Quality Metal, and Sulfur Ore into Sulfur — it's the single most load-bearing structure in your early base. Placing one is usually the first thing worth doing after a sleeping bag.