Kid Asked

How is metal made?

Tap the yellow button to begin.

For the grown-up

The short answer. Out of rock, with fire. Iron, the metal we use most, is locked inside red rock called ore, where it is joined to oxygen. In a very hot furnace, burning charcoal takes the oxygen away and leaves the iron, which melts and runs to the bottom. Poured into a shape, it cools hard and keeps that shape. Left out in the rain, it slowly takes hold of oxygen again, and that is rust.

These pages make iron, because iron is the metal we use most: about 95 of every 100 tonnes of metal made in a year are iron and steel. Here is what the tall oven is really doing, where the pictures stop being true, what to ask, and what to look for outside.

How iron comes out of rock

  1. Ore. Iron does not lie in the ground as metal. It is locked up in minerals such as hematite, which is iron joined to oxygen: two parts iron to three parts oxygen. Hematite is named after the Greek word for blood. Lumps can be red, brown or gray, but the powder is always rust-red.
  2. Charcoal. Almost pure carbon. It is the fuel, and it is also the thing that takes the oxygen away.
  3. Air. A blast of air drives the fire to about 3,000 °F (1,650 °C). A kitchen oven stops below 300 °C.
  4. The grab. Burning charcoal gives off a gas, carbon monoxide, that is hungry for oxygen. It takes the oxygen out of the ore and leaves as carbon dioxide. What stays behind is iron.
  5. Liquid iron. Pure iron melts at 1,538 °C. On its way down the furnace the iron soaks up carbon, which lowers its melting point, so it reaches the bottom as a liquid. The rest of the rock floats on top of it as slag and is run off first.
  6. Into the sand. A worker knocked a clay plug out of the furnace, and the iron ran along channels in a floor of sand and clay. A furnace like the one in the pictures gave one to one and a half tons at a time, twice a day, and burned for months without stopping.

The bits in the pictures

Gray bits are iron. Coral bits are oxygen, which is about a fifth of the air; they are the same coral bits as in “How does light travel faster than sound?”. Black bits are carbon, which is what charcoal is.

That gives one idea for three pages. In the ore, iron is holding on to oxygen. In the fire, carbon takes the oxygen and the iron is left by itself. In the rain, iron takes hold of oxygen again, and that is rust.

The words “holds on”, “grabs” and “lets go” stand for chemical bonds. No hammer, sieve or magnet separates iron from its oxygen. Only chemistry does.

Where the pictures stop being true

“It hides inside rocks”
True of iron. A few metals do lie in the ground as metal: gold nearly always, copper and silver sometimes. Those only need digging and washing.
“Charcoal and ore”
A third thing went in: limestone, which carries off the part of the rock that is not iron. And nobody pumped by hand. A water wheel worked the bellows, day and night.
Blowing air in to take air out
It sounds backwards. The air goes in at the bottom, where it burns charcoal and makes the heat. That uses its oxygen up. The gas that rises through the ore has none to spare, so it takes the ore’s.
“It is free”
Not quite pure. Iron straight from the furnace holds about 4% carbon and is brittle. Nearly all of it is then made into steel, which is iron with only a little carbon left in.
The pan
The furnace’s own iron ran into rows of bars called pigs, because they lay along a feeder channel like piglets beside a sow. A pan is cast later, from iron melted a second time, inside a block of packed sand. One maker of iron pans still works this way, at over 2,000 °F, and uses the sand again. Steel is usually shaped another way: it is cast into slabs and bars, and then rolled.
“Yellow. Orange. Red. Gray.”
The order is real and the speed is not. Hot metal glows white above about 1,300 °C, yellow near 1,100 to 1,250, orange near 1,000, cherry red near 850 and dark red from about 700. A casting takes far longer than a few seconds to get there.
“Just like in the red rock”
A close cousin, not a twin. Rust is iron joined to oxygen with water built in. Its main ingredient is a mineral called goethite, which is itself dug up as “brown iron ore”.
Today
The idea has not changed in centuries, only the size. Coke made from coal took the place of charcoal after 1709, and the air is now heated to about 1,000 °C before it goes in. A furnace like the one in the pictures made two or three tons of iron a day. The largest today makes about 5.6 million tonnes a year, which by our own sum is some 15,000 tonnes a day.
Other metals
Each has its own way out of the rock. Aluminium is won with electricity, not charcoal, by a method found in 1886. The golden bell on the first page is brass, which is copper mixed with zinc.

Questions to ask

  • Before you start: “Where do you think metal comes from?” Let the guess stand, then find out together.
  • “What in this room is made of metal? How can you tell?” Cold to touch, hard, often shiny, and it rings when you tap it.
  • “Why couldn’t the hammer get the iron out?”
  • “Why do we paint gates and bikes?” Paint keeps the water and the air away from the iron.
  • “What else goes runny when it is hot and hard when it is cold?” Chocolate, butter, candle wax.

Try it

  • Rust hunt. On a walk, look at railings, bolts, an old bike chain. Red-brown and flaky means there is iron underneath.
  • Make rust. Lay an iron nail in a saucer with a little water, and a dry one beside it. Look every day. Rust needs both water and air.
  • Pour and wait. Pour melted chocolate into an ice-cube tray, or juice into an ice-pop mold. Runny going in, hard coming out, and it keeps the shape it was poured into.
  • Find something poured. A drain cover in the street is usually cast iron, poured into sand. The letters and the pattern on it were pressed into the sand first. Look from the sidewalk.
  • Magnet hunt. A fridge magnet finds iron. It sticks to most steel, which is nearly all iron. It does not stick to kitchen foil, which is aluminium. Some stainless-steel spoons and sinks ignore it too.

Hot metal stays in the pictures

Nothing here needs heat. Iron does not melt until far past 1,000 °C, and melting any metal is not a home experiment. The fire on these pages is for tapping.

Where this comes from

The voice is a computer voice. With the sound off, the words stay on the page for you to read aloud.

Every word, page by page
  1. Look. A spoon. A key. A pan. A bell. They are all made of metal. But how is metal made? Let's find out!
  2. Metal comes out of the ground! It hides inside rocks. This red rock has iron in it. Tap the red rock to dig some out. A rock with metal inside is called ore. This is iron ore. And iron is the metal we use the most.
  3. But where is the iron? Let's break the rock open. Tap it with the hammer. Crack! No shiny iron. Only red crumbs. Tap the crumbs to look much, much closer. There it is! Tiny bits of iron. Each one holds on tight to bits of air. Iron and air together make red rock. A hammer cannot pull them apart.
  4. We need a helper. Black charcoal! Fill the tall oven with charcoal and ore. Tap, tap, tap. Now make the fire hot. Blow air into it. Push the pump, again and again! It glows! So, so hot. Now the charcoal grabs the air bits away. The iron lets go. It is free! Fill. Blow. Glow. Flow! The free iron melts. It runs down to the bottom.
  5. The iron is so hot that it glows. And it is runny, like water. Pull out the plug! Whoosh! The iron runs into a hole in the sand. The hole is the shape of a pan. Now we wait. It cools down. Yellow. Orange. Red. Gray. Now lift it out. A pan! Hot iron is runny. Cold iron is hard. And it keeps its shape.
  6. This gate is iron. But air bits are all around it. Iron wants to hold them again! Water helps. Tap the cloud, again and again. Red-brown spots! That is rust. Rust is iron holding on to air again. Just like in the red rock. Next time you see rust, remember. That metal was once a rock in the ground.
  7. Now you make the metal. Draw a path in the sand with your finger. Lead the hot iron to a shape! You did it! You made metal. From a rock, with fire. Make some more!

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