Build a smelting furnace

Stack a clay chimney-furnace that holds charcoal and ore while the bellows drive the heat higher than any campfire.

hard⏳ a weektradition — not yet hand-tested

A campfire spends its heat on the sky. A smelting furnace is a clay chimney about chest height that traps the heat in a tall column of burning charcoal, with your bellows blowing in near the bottom. Inside that column, temperatures climb to 1100–1300°C — hot enough to pull metal out of rock. This design is called a bloomery, after the "bloom," the rough lump of metal it can make. Build it well and slow; the same furnace smelts copper now and iron later.

You need

• Clay, a lot of it — plan on two or three big basketfuls (roughly 100 kg). Dig it where your brick clay came from.
• Sand and chopped dry straw or dried dung — mixed into the clay as temper. Temper is filler that lets clay dry and heat without tearing itself apart.
• Fired bricks or flat stones — a dry, level base. Ground moisture is the enemy.
• Your bellows, plus a clay tuyere — a fireproof clay tube, thumb-thick hole, forearm long, that carries the air through the wall. Make and dry two spares; tuyeres crack.
• Charcoal for the drying fires.
• Water, and a dry week or a simple rain roof on posts — rain on a half-built furnace undoes days. A roof, never walls: this furnace lives outdoors for good, for the reason in Watch out.

Steps

1. Pick the spot: dry ground, sheltered from rain, nothing burnable hanging overhead, room to work the bellows on one side.
2. Lay the base: one flat layer of fired bricks or stones, about 70 cm across, truly level.
3. Mix the body clay: about 2 parts clay to 1 part sand, plus a generous double handful of chopped straw per basket. Knead with a little water into a stiff dough that holds a thumbprint.
4. Build the shaft in rings, like a giant coil pot:
   • Inner width about 25–30 cm — two hand-spans. Walls 10–15 cm thick.
   • Add no more than about 15 cm of height per day, so each ring firms up before carrying the next. Smooth each joint with wet fingers.
   • Stop at about 1 to 1.2 m tall — chest height. Taller pulls better but cracks easier; this is the honest beginner size.
5. Leave two openings as you build:
   • The tuyere hole, 15–20 cm above the floor, sized so the clay tube fits through pointing slightly downward into the furnace.
   • A front arch at the base, roughly 15 cm wide and 20 cm tall — your door for raking out ash, slag, and metal. During a smelt it gets plugged with bricks and clay.
6. Set the tuyere in its hole and seal around it with clay. The bellows nozzle meets the tuyere's outer end — wood stays outside, clay takes the heat.
7. Dry it slowly. Air-dry as you build, then let the finished furnace stand two or three days. Then light a small fire inside and keep it small for half a day before letting it grow. Slow drying is most of this week.
8. Test-fire before any ore: fill with charcoal, plug the arch, and pump. Within an hour the wall glows warm and the air-blast spot inside shines yellow-white. Now it is proven.

Watch out

• Carbon monoxide, from the first fire onward. Even the drying and proving fires make the chimney breathe an invisible, odourless poison gas out of its top. Build and fire this furnace in open air only — a roof against rain is fine, walls are not; a furnace inside a lean-to or shed is a trap. Stand upwind, and never lean over the mouth of a burning furnace to look in. Headache or dizziness while you work means bad air, not tiredness: walk away into clean air.
• Steam bursts. Water trapped in clay turns to steam when heated fast, and steam can pop hot shards off the wall — or blow a chunk out. Dry slowly; always start any firing gentle. This is the one rule that hurts if you skip it.
• Cracks will appear. Hairline cracks are normal — patch them with soft tempered clay. A wide crack that goes right through means too little temper or too much hurry: patch it, and mix more straw next time.
• Tuyere height matters. Too low and it drowns in ash and slag; too high and the bottom stays cold and your metal freezes into the waste. Keep it 15–20 cm above the floor.
• Building too fast makes the wet wall slump under its own weight. Fifteen centimeters a day feels slow. It is faster than rebuilding.
• Fire above your head. The chimney top spits sparks. Clear branches and roofing above it, keep water and a pile of sand within reach, and never lean over the top of a working furnace — it can belch flame.

How it works

The chimney holds a deep bed of charcoal, and the bellows feed air in at the bottom, so the fiercest burning happens low down and all the rising heat must pass through everything above it. Whatever you drop in the top sinks slowly through hotter and hotter zones. The thick clay walls do the rest: they bounce heat back inward instead of spending it on the wind, which is exactly what an open fire cannot do.