The Stone Furnace in Permafrost turns raw Copper and Iron ore into ingots, the metal that recipes such as Wire call for. It works differently from most survival-game furnaces: there is no separate slot to load fuel, no button to light it, and nothing to turn on. You pick a recipe, set an amount, and the furnace runs a queue while you walk away.
Quick answer: Press F to bring up the Stone Furnace. Select either the Copper Ingot or Iron Ingot entry, choose a quantity with + / – or Max, and press Craft. Coal is an ingredient, not a separate fuel you load.
What you need before smelting
Before you can queue an ingot, you need the furnace built and the raw materials in your bag. The Stone Furnace is a build piece under Building blueprints, tier 3, and costs 2 blueprint points to unlock. Placing it takes 40 Stone, and it needs no foundation or roof.
- A Stone Furnace, built for 40 Stone.
- Copper, mined from a Copper Boulder, for Copper Ingots.
- Iron, mined from an Iron Boulder, for Iron Ingots.
- Coal, obtained by mining a Coal Boulder or by processing Wood in the furnace.
- A pickaxe that can break those boulders.
Pickaxes that break Copper, Iron and Coal boulders
A boulder displays its name, a small pickaxe symbol, and a prompt to break it. The symbol’s color indicates whether your current tool can damage it: white marks effective swings, while red means every hit is ineffective, no matter how long you try. A Stone Pickaxe or Flint Pickaxe appears red against an Iron Boulder, so neither can mine Iron.
For Iron, Copper, and Coal, the Scrap Pickaxe handles all three. Unlock it in Blueprints under Scrap Tools Set by spending 2 points, then build it using the Workbench.
Copper Boulders and Coal Boulders sit together inside caves, Copper usually deeper past the entrance. Walk up to a deposit and read its label rather than judging it by colour.
Iron Boulders also stand out in the open in the Woods, clustered with dark rocks next to Moonrock Boulders. One Iron Boulder gave 12 Iron, which is enough for four Iron Ingots once you also have the Coal.
Stone Furnace recipe costs
Each craft takes about 15 seconds, and Coal is burned inside the ingot recipe itself rather than loaded as fuel. The furnace lists three rows in A–Z order: Coal (x2), Copper Ingot and Iron Ingot.
| Result | Ingredients | Yield | Craft time |
|---|---|---|---|
 Coal (x2) | 5 Wood | 2 Coal | about 15 s |
 Copper Ingot | 3 Copper + 5 Coal | 1 | about 15 s |
 Iron Ingot | 3 Iron + 3 Coal | 1 | about 15 s |
The Iron and Wood costs do not read the same in every world. Iron Ingot can show 2 Iron instead of 3 (always with 3 Coal), and Coal (x2) can show 3 Wood instead of 5. Copper Ingot reads 3 Copper + 5 Coal in both. Read the “have/need” numbers on your own panel; a number in red means you are short of that ingredient.
Smelt ingots at the Stone Furnace
Collecting your ingots off the floor
Finished metal does not enter your inventory, and the panel has no output slot. Coal, Copper Ingots, and Iron Ingots fall onto the ground at the furnace mouth. Look down to see each item’s name and pickup prompt. Press F to collect them individually; your total updates as you go, for example, +1 Coal, +2 Coal, then +1 Iron Ingot. Items left on the floor remain until you return, so smelting is done once they appear at the furnace.
Make Coal from Wood when you run short
Coal is an ingredient rather than fuel you load, so you can start with an empty furnace and no Coal. Put the Coal (x2) recipe in the queue before the ingots; each craft converts Wood into 2 Coal. Collect the Coal from the floor, then queue your ingots. With 50 Wood and a 5 Wood cost per craft, choosing Max sets Craft x10 and produces 20 Coal.
Coal Boulders in caves give Coal directly, so you can skip this step if you already mined some.
Turn Copper Ingots into Wire
Your first Copper Ingots open up the Workbench recipe for Wire, where 1 Copper Ingot makes 1 Wire, one craft at a time. Four Copper Ingots give four Wire across four crafts.
