Blast furnace and cheap iron
Make iron by the tonne instead of by the kilogram.
Everything for this step
- Field guide
- Minerals and ores → 3 sections
- Builds
- None yet.
- Reference sheets
- None.
- Track
- Metals, level 3 of 4 · next: Electrochemistry and light metals
Why this matters
A bloomery gives you kilograms after a day of hard work. A blast furnace runs continuously for months and pours tonnes. That change in quantity is a change in kind: it makes rails, boilers, machine frames, pipes and structures possible for the first time.
The shape of it
- Build tall and line it well. A blast furnace is a continuously charged shaft — ore, coke and limestone in at the top, air blown in at the bottom, molten iron and slag tapped from the base. The refractory lining is the limiting component.
- The blast must be powerful and continuous. This is why the furnace waits on steam or a large water wheel; hand bellows cannot do it. Preheat the blast if you can — hot blast cut fuel consumption dramatically and was one of the highest-return improvements ever made to the process.
- Flux with limestone to carry away silica as a fluid slag. Get this wrong and the furnace chokes solid, which can end the campaign and sometimes the furnace.
- Understand the product. Blast furnace iron is pig iron — around 4% carbon, hard, brittle and unforgeable. It casts beautifully and is excellent in compression, so use it for machine frames, pipes, columns and cookware.
- Then decarburise for wrought iron: puddling — stirring molten pig iron in a reverberatory furnace so oxygen burns the carbon out. It is brutal, skilled, and it was the backbone of nineteenth-century industry.
- Run it continuously. Blowing a furnace out and relighting is expensive and damages the lining. A campaign runs for months, which means your ore, coke and flux supply must be continuous too — this is a logistics problem as much as a metallurgical one.
- Never stand near a tap or a slag notch without knowing where the metal will go. Molten iron finds water and turns it instantly to steam, throwing metal. Keep the casting floor bone dry.
Doing it, step by step
The summary above says what has to happen. This says how, with the numbers that decide whether it works.
1 · Do the logistics arithmetic before building
A blast furnace is a logistics problem as much as a metallurgical one. It runs for months, so ore, coke and flux must arrive continuously.
Budget per tonne of pig iron, as rough planning figures that depend heavily on ore grade and furnace design: about 1.5–2 tonnes of ore, around a tonne of coke or more for an early coke furnace, several hundred kilograms of limestone, and several tonnes of air. Multiply by the daily output and the campaign length. A furnace that runs out of coke on day 40 has to be blown out, which damages the lining and costs weeks.
2 · Build tall, line it with the best refractory, and dry it slowly
A continuously charged shaft: ore, coke and limestone in at the top; air blown in at the bottom through tuyères; molten iron and slag tapped from the hearth. The stack widens to the bosh and narrows to the hearth.
The refractory lining is the limiting component — build it of the best firebrick you have. Dry the new furnace and heat it up slowly over days before the first charge: water in the brickwork cracks the lining.
3 · Blow hard and continuously — and preheat the blast
Hand bellows cannot do this. The blast needs blowing engines driven by steam or a large water wheel, with a spare. An intermittent blast chills the furnace, and a chilled hearth can freeze solid.
Preheat the blast: passing it through heated pipes or, later, brick regenerative stoves heated by the furnace's own waste gas. Hot blast cut fuel consumption dramatically and was one of the highest-return improvements ever made to the process.
4 · Adjust the flux by watching the slag
Limestone carries silica and other impurities away as a fluid slag. Get it wrong and the furnace chokes.
Watch the slag at every tap: it should run freely and cool to a glassy or stony mass. Stiff, sticky slag usually means too little lime; very runny slag that sets as a weak powder can mean too much. Change the limestone in the charge in small steps — a few percent at a time — and log each change against the slag behaviour.
5 · Tap onto bone-dry sand
Tap iron every few hours into channels and pig beds of dry sand, and run slag off through a separate notch.
Everyone on the casting floor wears heavy boots, leather or wool clothing, and face and eye protection, and nobody stands in the path of the runners. Know where the metal will go before the tap is opened.
6 · Use pig iron for what it is good at, and puddle the rest
Pig iron is about 4% carbon — hard, brittle and unforgeable. It is not a failure. It casts beautifully and is excellent in compression: machine frames, pipes, columns, cylinders and cookware.
For wrought iron, puddle it: melt pig iron in a reverberatory furnace, where flame passes over the metal without touching the fuel, and stir it so air burns the carbon out. As carbon falls the melting point rises, the iron thickens into a pasty ball, and it is drawn out and hammered to squeeze out slag. Brutal, skilled work — and the backbone of nineteenth-century industry. For steel, see steel.
7 · Capture the furnace gas, and log the campaign
The gas leaving the top of the furnace is rich in carbon monoxide — lethal, and fuel. Close the top with a charging bell, pipe the gas to fire the blast stoves and boilers, and never let anyone work at the furnace top in still air without a second person.
Log every charge, blast pressure and temperature, tap weight and slag behaviour. A campaign runs for months, and the log is how the next campaign runs better.
How this fails
How you know it worked
What this unlocks
What this makes practical
These were already buildable without this step. What this one changes is the cost, the scale, or the safety of them.