Smelt and cast copper

Roast green-stained ore with charcoal until beads of shining metal form, then pour melted copper into clay molds.

hard⏳ a weektradition — not yet hand-tested

Copper was the first metal people ever pulled out of stone, about seven thousand years ago, and you are about to repeat that moment. Copper melts at 1084°C — your furnace can reach that. The trick is not just heat: charcoal burning short of air makes a hungry gas that strips the ore down to bare metal. One smelt takes a long day; this week covers preparing, smelting, and casting your first object.

You need

• Green copper ore (malachite), crushed to pea size — a basketful, several kilograms. Richer ore means more metal.
• Charcoal — lots. Plan on four to five times the weight of your ore. Smelting eats fuel.
• Your furnace, fully dried and test-fired, with bellows and a spare tuyere.
• A crucible — a thick clay cup, fist-sized, fired hard in your kiln — and one or two clay molds carved or pressed with the shape you want (an axe blank, a bar). Made days ahead and bone dry.
• Long green-wood tongs — two fresh springy sticks lashed at one end — for gripping the crucible. Green wood chars slowly instead of catching.
• A helper to trade off on the bellows.
• Water bucket and a pile of sand nearby — for you and stray fires, never for the metal.
• A dry day and outdoor air. Never smelt in an enclosed space.

Steps

1. Prepare the ore: crush it on a flat stone to pea-sized bits, picking out plain rock. Then roast it — spread it on an open wood fire for an hour or two. Roasting drives off water and gas; malachite turns from green to black, which is copper oxide, ready to smelt.
2. Preheat the furnace: fill the shaft with charcoal, light it through the tuyere hole or from the top, plug the front arch, and pump gently for about an hour until the whole column glows.
3. Charge in layers from the top: one cup of roasted ore, then one or two cups of charcoal, again and again. Keep the furnace topped up — never let the level fall below half.
4. Pump steadily for 2–4 hours. A calm, even rhythm beats furious bursts. Trade with your helper before your arms fail; the fire must never wait.
5. Read the signs: quick side-glances at the tuyere hole should show white-yellow glow. Wisps of green in the flame at the top are copper's own color — a good omen.
6. Let the last charge burn down, then open the front arch and rake everything out onto dry ground: ash, glowing charcoal, and slag — slag is the melted rock waste, dark, glassy, and shiny. The copper sits at the bottom as beads and lumps, reddish where scratched.
7. Let it all cool in the air, then crush the slag and pick out every bead. A fist-sized haul from a basket of ore is a real success for a first smelt.
8. Cast: pile the beads into the crucible and set it in the glowing furnace or a bellows-blown charcoal hearth. When the metal is fully liquid — a bright mirror surface that shivers like water — grip the crucible with the tongs and pour in one smooth, steady motion into the dry mold. Do not stop mid-pour.
9. Wait long. The casting looks dull within minutes but burns skin for an hour or more. Let it cool completely in air, then break it free and inspect your first metal thing.

Watch out

• Carbon monoxide. The furnace breathes out an invisible, odourless poison gas — the very gas that frees your metal, described below. Outdoors only, always; stand upwind, and never lean over the top breathing the fumes while you charge. Charging is the moment of danger, because it puts your face where the breath comes out: drop the ore in and get your head back. A headache, dizziness, or a strange dullness at the furnace means bad air — walk away into clean air at once, and do not go back until your head is fully clear. This one gives no warning smell, so unlike arsenic below, your nose will never tell you it is there.
• Water plus molten metal explodes. A damp mold or crucible flashes its moisture to steam and sprays liquid copper. This is the injury that ends smelting careers. Dry molds for days, warm them beside the fire before pouring, and never pour over wet ground.
• A garlic smell from the fire means arsenic — some copper ores carry it, and its fumes are poison. Stay upwind at all times; if the garlic smell is strong, abandon that batch of ore and find a cleaner source.
• Only smelt ore you found under green stains. Heavy shiny gray ore may be galena — lead — whose fumes poison slowly and invisibly. Copper first, always outdoors, always upwind of the chimney.
• If the bellows stop, the smelt dies. The furnace cools and everything freezes into a half-melted mess. Keep air moving from first charge to last.
• Everything stays hot after it stops glowing. Slag, tools, castings — hover the back of your hand near a piece to feel for heat before you ever touch it. When in doubt, let time do the cooling.
• Do not stare into the tuyere glow. It is bright enough to ache your eyes. Quick glances from the side.

How it works

Deep in the charcoal bed, the fire runs short of air and makes carbon monoxide — a gas so hungry for oxygen that it steals it straight off the roasted ore. Copper oxide loses its oxygen and becomes copper; the leftover rock melts into slag and drains aside. Metal is heavier than slag, so gravity sorts your prize to the bottom, where you rake it out.

That same hunger is why the furnace's breath is poison, and it is worth seeing that these are one fact and not two. Your blood carries oxygen for a living; carbon monoxide takes those seats and will not give them up, so a lungful of the gas that so eagerly strips the ore will just as eagerly strip you — quietly, with no smell and no taste, while you feel only a little tired. The wonder and the warning are the same gas. Stand upwind.