How Smelting Works
A furnace runs on ticks: each tick it burns a little fuel, processes the ore in every occupied slot, and drops the refined output. The smelt times are fixed across furnace types:
- Metal ore -> 2.5 seconds each (1:1 to Metal Fragments)
- Sulfur ore -> 2.5 seconds each (1:1 to Sulfur)
- High Quality Metal ore -> 10 seconds each (4x slower, 1:1 to HQM)
HQM is the perpetual bottleneck -- four times slower than everything else. That's why serious bases run banks of furnaces split by ore type rather than mixing loads. And always fill EVERY ore slot: a furnace with one ore slot used smelts at a third the rate of one with all three.
Furnace Types
Four cookers, each with a role:
- Small (standard) Furnace: 1 fuel + 3 ore slots, burns ~1 wood per 10s. The early-game default; a 1000-wood load clears ~600 metal ore in ~33 minutes.
- Large Furnace: 18 slots and far more fuel-efficient -- roughly 40% the wood for metal, ~80% for sulfur, ~33% for HQM versus the equivalent small furnaces. Catch: it can't go on a foundation or in caves (open terrain only), and its smoke column is a loud raid beacon.
- Electric Furnace: 5 slots, needs 3 power, smelts ~2x a small furnace -- but produces NO charcoal. Endgame automation for a stable grid.
The Large Furnace also costs less to build than three small ones (100 less stone, half the low grade fuel).
Charcoal & the Parallel-Furnace Rule
Every wood-fueled smelt is also a charcoal factory: each piece of wood burned has roughly a 75% chance to yield charcoal, so a 5,000-wood smelt banks ~3,750 charcoal. Charcoal feeds gunpowder, so keep it -- and never farm wood twice by chopping a second pile just for charcoal.
The single most important efficiency rule: more furnaces never cost more wood. Splitting 10,000 sulfur across 5 furnaces uses the same total wood as running it through 1 -- but finishes in a fifth of the time, which also means a fifth of the smoke-column exposure. Build wide, not tall: a row of small furnaces beats one over-loaded furnace every time.
Making Low Grade Fuel
Low Grade Fuel (LGF) runs every vehicle and the oil refinery. Two ways to make it:
- Craft it: 3 Animal Fat + 1 Cloth -> 4 LGF, no workbench needed. Balance your farming near 1:1 fat-to-cloth (cloth is usually the bottleneck -- farm hemp hard). Deer drop 20-40 fat, boar 15-30; a bone knife boosts fat yield.
- Refine it: a Small Oil Refinery turns 1 Crude Oil + ~7 Wood -> 3 LGF. Higher yield once you're set up, but there's a bootstrap catch -- you need fuel (or wood) to prime it, so craft from fat first. Get the refinery from Outpost vending for 125 scrap (rare as a crate drop).
Other sources: the Pump Jack at Power Plant / Trainyard / Water Treatment (~170 LGF + 60 crude per cycle), oil barrels (densest under Dome), Outpost (20 LGF for 10 scrap), and the Trainyard fuel wagon (100-500 LGF). Refining makes noise -- others can hear it.
Tips & Common Mistakes
A handful of habits separate efficient smelting from wasted wood:
- Don't over-fuel. Add only the wood the ore needs, or the fire keeps burning empty and wastes wood. Sulfur is the classic trap: a 1000-ore sulfur load only needs ~835 wood, not a full 1000.
- Don't bulk-smelt sulfur in small furnaces -- the Large Furnace cuts the wood to ~80%.
- Don't smelt HQM in wood furnaces if you have a grid -- the Electric Furnace is faster and frees the bottleneck.
- Keep ONE wood furnace running even after going electric, so you never run dry on charcoal for gunpowder.
- Wall-protect your furnaces. The smoke column is visible across the map; place furnaces inside walls so the smoke is the only tell, not the loot.