Bloomery furnace
A clay shaft that turns rock into iron — the first machine that makes every other machine possible.
Build
Tier 2 · Bronze and Iron Age
Materials
Hard — expect several failed smelts
4–6 person-days plus 2 weeks drying
Crew 4–6 for a smelt (the bellows cannot stop)
Home: Metals, Level 2 · Forge, level 1 of 4 in its track. Serves 3 steps across 3 subjects.
Every build on this site has hazards that injure or kill — quicklime, carbon monoxide, molten metal, collapsing ground. Read the safety notice before starting, and read this build's own failure modes below.
What it is and why this shape
A bloomery does not melt iron. That single sentence prevents more failed smelts than anything else on this page.
Iron melts at 1,538 °C, which a charcoal shaft furnace cannot reach. What it does reach — about 1,200–1,300 °C in a narrow zone just above the air inlet — is enough to strip oxygen from iron oxide while the iron stays solid. The metal never flows. It forms as a spongy, slag-soaked mass that gathers at the bottom of the shaft, and the slag, which does melt, drains away from it. That mass is the bloom, and it looks nothing like metal when you pull it out.
The furnace is therefore a very specific machine: a chimney that holds a column of burning charcoal, with air forced in low down, so that a descending charge passes through a reducing atmosphere hot enough to do this. Every dimension below serves that.
The three things that decide success, in order: enough continuous air, enough charcoal, and ore that was roasted and crushed. Ore quality matters far less than most people expect — bog iron works. Stopping the bellows for ten minutes ruins the smelt.
Vertical section through the shaft
Fig 1
Earth / subsoilMasonry / brickLoose fill / grainGravel / aggregateFire / hot zoneVoid / air
Plan at tuyère level
Fig 2
Masonry / brickVoid / air
Keyed parts
| 1 | Shaft, clay and straw, 110 mm wall | Holds the column and insulates it. Straw or dung temper stops the clay cracking as it dries and fires. |
| 2 | Charge column — alternating charcoal and ore | Roughly equal by weight, charged in layers. Keep the shaft full: a slumped column lets air short-circuit. |
| 3 | Tuyère, 25–30 mm bore, 15–20° down | Clay pipe carrying blast from the bellows. Angled down so the blast reaches the hearth rather than the wall. |
| 4 | Hot zone, ~1,200–1,300 °C | A small region just in front of and above the tuyère. This is the only part of the furnace doing the reduction. |
| 5 | Bloom, forming on the hearth | Spongy iron and trapped slag. Grows through the smelt; it is not a pour. |
| 6 | Slag bath and tap hole | Molten slag drains below the bloom. Tap it periodically or it drowns the tuyère. |
| 7 | Hearth floor, rammed clay over sand | Sacrificial. Expect to rebuild it every smelt or two. |
| 8 | Slag pit | Dug below the tap so tapped slag runs clear of the working area. |
| 9 | Charging mouth | Kept full throughout. It is also the flame and spark exit — stand clear. |
Specifications
| Internal bore | 280 mm | 250–350 mm is the workable range. Smaller chills; larger needs far more air than hand bellows can supply |
| Shaft height above hearth | 1.4 m | Gives the charge time to preheat and reduce on the way down |
| Wall thickness | 110 mm | Clay with straw or dung temper. Thinner cracks through; thicker never dries |
| Tuyère height above floor | 200 mm | The single most important dimension in the furnace |
| Tuyère bore | 25–30 mm | Sized to the bellows. Too large and the blast is slow and will not penetrate |
| Tuyère angle | 15–20° below horizontal | Aimed just past the centreline, not at the far wall |
| Air supply | ~600 L/min, continuous | Double-acting bellows, or two bag bellows worked alternately so there is no gap |
| Hot-zone temperature | 1,200–1,300 °C | Bright yellow-white at the tuyère. Iron does not melt; slag does |
| Charge ratio | ≈ 1 : 1 charcoal : ore by weight | Adjust after the first smelt based on what you find |
| Ore size | < 10 mm | Roasted first, then crushed. Lumps pass through unreduced |
| Charcoal size | 20–40 mm | Screened. Fines choke the column and stall the blast |
| Smelt duration | 4–8 hours | Plus 2–3 hours preheating the shaft |
| Typical yield | 15–25% of ore weight | A first smelt returning anything at all is a success |
Bill of materials
| Material | Quantity | Notes and substitutes |
|---|---|---|
| Refractory clay | ≈ 150 kg | High-alumina or fireclay if you have it; ordinary brick clay works but erodes faster. |
| Straw, chaff or dung | ≈ 15 kg | Temper. Burns out in the first firing leaving a porous, crack-resistant body. |
| Sand | ≈ 30 kg | Hearth bed under the rammed clay floor. |
| Charcoal | 30–50 kg per smelt | Hardwood, screened to 20–40 mm. Have three times what you think you need — running out mid-smelt loses everything. |
| Iron ore | ≈ 20 kg per smelt | Bog iron, limonite, haematite, magnetite. Roasted then crushed. |
| Limestone or shell | 1–2 kg | Flux, to make the slag run freely. Optional on a self-fluxing ore. |
| Tuyère pipe | 1 × 400 mm | Thick-walled clay, fired hard. Consumable — make several, they burn back. |
| Bellows | 1 double-acting or 2 bag | See the separate bellows build. The furnace is worthless without this. |
| Green wood poles | 6–8 | Temporary former for the shaft, burnt out on first firing. Optional but makes a truer bore. |
Tools: Long-handled tongs · Sledge and sledging block · Iron bar for the tap hole · Rake for the charge · Leather apron, gloves and face shield · Water for burns, not for the furnace
Construction
Prepare and test the clay
Mix clay with roughly a third by volume of straw, chaff or dung, and enough water to make a stiff, workable paste. Test it first: make a small slab, dry it, and fire it in a hot fire. If it cracks apart or slumps, add more temper or find better clay. Doing this test costs a day and saves a fortnight.
✓ Check: A test slab survives a hot fire without cracking through or softening.
Build the hearth and lay out the base
Dig in slightly so the hearth is at a comfortable working height with the tap running downhill to a pit. Bed sand, then ram a clay floor over it. Mark the bore circle and the tuyère bearing before you build up.
✓ Check: Hearth floor level, tap direction clearly downhill to a pit clear of where people stand.
Raise the shaft in courses
Build up in courses of 150–200 mm, letting each stiffen before adding the next, or the whole thing slumps under its own weight. Keep the bore even — a waisted or bulging shaft causes the charge to hang and then collapse. Form the tuyère port and the tap hole as you pass them.
✓ Check: Bore within ±20 mm of 280 mm at every height; walls plumb.
Dry it slowly. Then dry it more.
Air-dry for at least two weeks, longer in damp weather. Then run small fires inside, increasing over two or three days. Water trapped in the wall turns to steam and blows sections out of the shaft during the smelt, which ends the smelt and can injure whoever is nearest.
✓ Check: No cold spots on the outside wall after an hour of moderate fire; no steam anywhere.
Roast and crush the ore
Heap the ore on a wood fire and burn it down. Roasting drives off water and sulphur, converts the iron minerals to a more reducible form, and makes the ore friable so it crushes easily. Then crush to under 10 mm and screen out the dust.
✓ Check: Roasted ore is red-brown, crumbles under a hammer, and rings dead rather than sharp.
Preheat and establish the column
Light a wood fire in the base and let the shaft come up to temperature over 2–3 hours. Then fill with charcoal alone and get the blast going until the whole column is burning and the top glows. Only then start adding ore. Charging ore into a cold furnace produces a shaft full of roasted rock.
✓ Check: Flame at the mouth, and charcoal glowing right up the shaft before the first ore goes in.
Charge, and keep charging
Alternate charcoal and ore in measured amounts — a fixed basket of each — and keep the shaft full at all times. Record every charge and the time. As the charge descends the level drops; top up. The rhythm matters more than the exact ratio, because a consistent process is one you can diagnose afterwards.
✓ Check: Level never falls more than ~150 mm below the mouth.
Keep the air on, without exception
This is where smelts are lost. The blast must be continuous for the whole run — four to eight hours. Work the bellows in shifts with a proper handover, and never let it stop for a shift change. If the blast stops the hot zone collapses in minutes, the slag freezes around the tuyère, and the smelt is over.
✓ Check: Tuyère stays clear and bright; no dark crust creeping across it.
Tap the slag
Periodically open the tap hole with a hot bar and let the molten slag run out into the pit. If slag builds up past the tuyère it drowns the blast and the smelt dies. Tapped slag should run like thick honey and be glassy and dark when cool — that is what a working furnace looks like from outside.
✓ Check: Slag runs freely and cools glassy. Pasty, crumbly slag means the furnace is too cool.
Pull the bloom and consolidate it hot
Break out the front of the furnace or lift from the top, and get the bloom out white hot. Then hammer it immediately and repeatedly to squeeze out the trapped slag, folding as you go. This is heavy, urgent work and it must not wait — a cooled bloom has to be reheated, and reheating a slag-soaked bloom is far harder than working the one you just pulled. Expect to lose half the mass as expelled slag and scale.
✓ Check: The consolidated bloom rings when struck and shows a fibrous grey fracture, not a crumbly one.
Tolerances that matter
| Dimension | Target | Why it matters |
|---|---|---|
| Tuyère height above hearth | 200 mm ±20 | Too low, slag drowns the blast. Too high, the bloom forms above the hearth and cannot be extracted whole. |
| Bore consistency | ±20 mm over full height | A waist makes the charge hang, then drop suddenly, punching the hot zone out. |
| Wall drying | ≥ 2 weeks air, then staged fires | Trapped water flashes to steam and blows the wall out during a smelt. |
| Ore size | < 10 mm | Larger lumps pass through the hot zone with an unreduced core and end up in the slag. |
| Blast continuity | no stop > 60 s | The hot zone is small and has almost no thermal inertia. A pause freezes the tuyère. |
| Charcoal size | 20–40 mm, screened | Fines pack the column, block the blast and stall the furnace. |
How this fails
Insufficient or intermittent air. The overwhelming first-attempt failure. The furnace never reaches temperature, you get a shaft of roasted rock, and it is easy to blame the ore. Build the bellows before the furnace and test it independently.
Firing a wall that was not properly dried. Steam blows out a section mid-smelt, with molten slag and burning charcoal at head height. This is the most dangerous failure mode here, and it is entirely prevented by patience.
Expecting molten iron. People pull a bloom, see spongy grey mass rather than a cast, conclude it failed, and throw it away. That mass is the product.
Slag flooding the tuyère. Tap regularly. Once slag freezes across the air inlet the smelt cannot be recovered.
Running out of charcoal. A smelt eats far more than seems reasonable. Stopping to burn more means stopping the blast, which ends the run.
Standing over the charging mouth. It vents flame, sparks and carbon monoxide. Charge from the side, and never work a furnace inside an enclosed space.
Letting the bloom cool before consolidating. Working it hot from the furnace takes hours of hammering; working it cold and reheated takes days and yields less.
How you know it worked
VerificationYou have hammered a bloom into a consolidated bar, forged that bar into a tool, and the tool holds an edge. Anything short of that — including a bloom that looks convincing — has not proved the furnace.
Scaling and variants
Smaller: a 200 mm bore shaft about 0.9 m tall can be blown by a single person on bag bellows and will produce 1–2 kg blooms. This is the right size to learn on, and the losses from a failed run are hours rather than days.
Larger: above roughly 400 mm bore, hand bellows cannot supply the air and you need water-powered blowing. That is the historical route from bloomery to blast furnace, and it changes the product: with more air, more fuel and a taller shaft the iron absorbs enough carbon to melt, and you get liquid cast iron instead of a solid bloom. That is a different machine with a different purpose, not a bigger version of this one.
Copper instead: the same shaft smelts copper ore at a lower temperature and gives a genuine pour rather than a bloom, which makes it a much more encouraging first project. If you have any copper ore at all, run copper before you run iron.