Blacksmith's forge
A controlled hot spot you can hold a bar in — the hearth every other trade's tools come out of.
Home: Metalworking, Level 2 · Forge, level 1 of 2 in its track. Serves 3 steps across 2 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 forge is not a fire. A fire heats everything near it; a forge produces a small, controllable, oxygen-starved hot zone that you can put one part of one workpiece into, at a chosen temperature, repeatedly.
Three things distinguish it from a bonfire, and all three are geometry:
- Air from below or from the side, not over the top. The blast has to pass through the fuel bed.
- A deep fuel bed above the air inlet. The work sits buried in glowing fuel, not on top of it. Above the inlet the atmosphere is reducing — starved of oxygen — which is what stops the steel burning up while you heat it.
- A hot spot small enough to heat one part while the rest of the bar stays cool enough to hold.
That last point is what makes a smith's work possible at all. If the whole bar heats, you cannot hold it and you cannot control where it bends.
Everything else — the size of the hearth, the height of the anvil, where the water bucket sits — is ergonomics, and ergonomics is what decides whether you can still do this at fifty.
Section through the firepot
Fig 1
Masonry / brickTimberLoose fill / grainWaterFire / hot zoneVoid / air
Working layout — the triangle
Fig 2
Masonry / brickTimberFire / hot zoneVoid / air
Keyed parts
| 1 | Firepot | The deep centre where the fire actually lives. Everything else is a shelf. |
| 2 | Fuel bed, 100–150 mm above the tuyère | The reducing zone. Work is buried in it, never laid on top. |
| 3 | Tuyère, 25–40 mm | Air inlet. Consumable — it burns back, so make it replaceable. |
| 4 | Clinker breaker / ash dump | Clinker forms every session and blocks the blast. This is how you clear it without dismantling the fire. |
| 5 | Hearth table | Working shelf around the firepot, for fuel, tools and resting long work. |
| 6 | Hood and flue | Carbon monoxide is produced continuously. This is a health control, not a comfort feature. |
| 7 | Water trough | Quenching, cooling tools, and damping the fire's edges to concentrate the heat. |
| 8 | Blast pipe from the bellows | With a gate or damper for fine control. Full blast is rarely what you want. |
Specifications
| Hearth height | 0.75–0.80 m | To the fuel bed. Set it to the smith, not to a standard |
| Firepot | 300 × 300 × 100 mm deep | Larger for heavy work, smaller concentrates the heat |
| Fuel above tuyère | 100–150 mm | The working variable. Adjusted constantly during a session |
| Tuyère bore | 25–40 mm | Match the bellows nozzle. Make it replaceable — it burns back |
| Hearth table | 0.9 × 0.6 m | Around the firepot, for fuel and resting work |
| Anvil face height | knuckle height | Typically 0.68–0.75 m. Measure it on the person, standing relaxed |
| Anvil weight | 50 kg or more | Mass returns hammer energy to the work. A light anvil wastes every blow |
| Fire temperature range | 700–1,300 °C | Controlled at the blast, not by piling on fuel |
| Fuel consumption | 3–6 kg charcoal/hour | Charcoal burns faster than coal; budget accordingly |
Bill of materials
| Material | Quantity | Notes and substitutes |
|---|---|---|
| Masonry or clay for the hearth | ≈ 0.5 m³ | Brick, stone or rammed clay on a timber frame. A simple earth-filled table works. |
| Firepot lining | 1 | Fireclay, or cast iron if available. Substitute: a shallow depression in rammed clay, relined as needed. |
| Tuyère | 2–3 | Fired clay or iron pipe. Make spares now: it is consumable and always fails mid-job. |
| Ash dump / clinker breaker | 1 | A simple hinged plate or a removable plug under the firepot. |
| Hood and flue | 1 | Sheet metal, clay, or a masonry chimney. Not optional — see the failure notes. |
| Anvil | 1 | 50 kg+. Substitutes: a large hardened steel block, a section of rail on end, or a heavy iron billet from your own bloomery. |
| Anvil stump | 1 | A solid section of tree trunk set into the ground. Mass and rigidity, not a stand. |
| Water trough | 1 | Also used to damp the fire's edges and concentrate the heat. |
| Charcoal | ongoing | See the charcoal clamp. Screened to 20–40 mm; fines choke the fire. |
Tools: Bellows (see separate build) · Tongs — several shapes, made first · Cross-pein and sledge hammers · Poker, rake and shovel for the fire · Wire brush · Hardy, punches and chisels — made as you go
Construction
Set the height to the person
Before building anything, have the smith stand relaxed with a hammer. The anvil face goes at knuckle height and the fuel bed at roughly the same level. Everything else follows. A forge built to a number instead of to a body produces a smith with a ruined back inside a decade.
✓ Check: Standing relaxed, the hammer face rests flat on the anvil with the arm hanging naturally.
Build the hearth table
A masonry or timber-framed table filled with rammed clay or sand, with the firepot depression formed centrally. Slope the surface very slightly toward the firepot so raked fuel returns to the fire.
✓ Check: Table solid enough to take a leaning workpiece and a dropped hammer.
Fit the tuyère and ash dump
Bring the tuyère up into the base of the firepot from below or in through the side. Make it replaceable without dismantling the hearth — it will burn back and need changing, always at an inconvenient moment. Fit the ash dump directly below so clinker can be dropped clear.
✓ Check: Tuyère can be withdrawn and replaced in a few minutes with the hearth cold.
Line the firepot
Fireclay or a well-tempered clay, formed to a shallow dish. Dry slowly, then fire gently before use. Expect to reline it periodically — it is a wear part.
✓ Check: Lining dry, hard and free of through-cracks after a slow first firing.
Build the hood and flue
A forge burns fuel continuously in a confined space and produces carbon monoxide the entire time. Build a hood over the fire and a flue that carries out of the building. Then work with a door or window open anyway.
✓ Check: Smoke from a smouldering fire draws up the flue cleanly, with none spilling into the room.
Connect the blast with a damper
Pipe from the bellows to the tuyère, with a gate or sliding damper in the line. Fine blast control is how you control temperature — a smith spends most of the day at part blast, not full.
✓ Check: Damper gives a smooth range from barely-alight to full blast.
Set the anvil and the triangle
Anvil on a solid stump, set into the ground if possible, at one pace from the fire. Vice and water within a turn. Nothing within a hammer swing. Long stock should be able to pass through the fire without hitting a wall.
✓ Check: You can move fire → anvil → vice → water without taking a step.
Light, bank and learn to read colour
Light with kindling, get a bed established, and bank the fuel up around the hot centre. Then learn the colours, in dim light — this is the actual instrument: dull red ~700 °C, cherry ~800, orange ~900, yellow ~1,000, white and throwing sparks ~1,300 and burning. Every operation has a correct colour and working outside it either does nothing or destroys the work.
✓ Check: You can hit a specified colour on demand and hold it.
Clear clinker every session
Clinker — fused ash and impurities — forms in the bottom of the firepot and blocks the blast. It looks like grey-green glassy slag. Break it out and drop it through the ash dump at the end of each session, and during the session if the fire goes dull for no reason.
✓ Check: Firepot bottom clear; blast reaches the fuel evenly across the pot.
Tolerances that matter
| Dimension | Target | Why it matters |
|---|---|---|
| Anvil face height | knuckle height ±20 mm | Directly determines hammer efficiency and long-term back and shoulder injury. |
| Fuel depth above tuyère | 100–150 mm | Below this the blast oxidises the work and burns the steel; above it the heat never reaches. |
| Anvil mass | ≥ 50 kg | Rebound returns energy to the work. On a light anvil most of every blow is wasted moving the anvil. |
| Anvil mounting | dead solid, no ring or bounce | A rocking anvil absorbs the blow and is exhausting to work on. |
| Tuyère replaceability | removable cold in minutes | It is a consumable; a built-in tuyère eventually means rebuilding the hearth. |
How this fails
No flue, or working in an enclosed space. Carbon monoxide is odourless and a forge makes it continuously. This is the failure mode most likely to kill somebody on this page, and it kills quietly, often in someone who simply sat down for a rest.
Anvil at the wrong height. Not dramatic, entirely cumulative. It ends careers.
Work laid on top of the fire. It oxidises heavily, scales badly, and at high heat the steel burns — throwing sparks, and once burnt it is scrap. Bury the work in the fuel.
Clinker left to build up. The fire goes dull for no apparent reason and no amount of extra blast helps. Clear it.
Charcoal fines not screened out. They pack around the tuyère and choke the blast.
Quenching in the water trough by reflex. Quenching hardened steel cracks it if it was not meant to be quenched, and quenching hot tongs weakens them. Know which you are doing.
No eye protection. Scale flies off hot work at every blow, hot and sharp, at eye height.
How you know it worked
VerificationYou can bring a 20 mm bar to an even orange heat through its section in under three minutes, hold that temperature while working, and finish a session with the firepot clear of clinker.
Scaling and variants
Smaller: a hole in the ground with a clay-lined depression, a tuyère and a pair of bag bellows is a complete working forge and has been for several thousand years. It will do everything a built hearth will, less comfortably. Build this first and learn on it.
Side-blast instead of bottom-blast: the tuyère enters horizontally through the hearth wall rather than from below. It handles charcoal particularly well, clinkers less, and is easier to build without metal — but the tuyère needs protecting from the heat, traditionally with a water-cooled jacket. Where charcoal is your only fuel, this is arguably the better choice.
Larger: a second person on the sledge roughly triples what can be moved per heat, and is the difference between making tools and making structural ironwork. Beyond that, the next step is a water-powered trip hammer, which is the same jump the water wheel makes possible everywhere else.