Kupferschmelzung aus Malachit — Von grünem Erz zu rotem Metall

Malachit zersetzt sich beim Erhitzen: Cu₂CO₃(OH)₂ → 2CuO + CO₂ + H₂O (bei ~300°C). Das entstehende Kupferoxid (CuO) wird dann durch Kohlenmonoxid aus brennender Holzkohle reduziert: CuO + CO → Cu + CO₂. Kupfer schmilzt bei 1085°C. Die Gesamtkupferausbeute aus reinem Malachit beträgt ungefähr 57,5%.

Zerlegen Sie Malachit mit einem Hammer und Amboss in Stücke kleiner als 1 cm. Je feiner die Zerkleinerung, desto schneller und vollständiger die Reduktion. Verwenden Sie bei Bedarf einen Mörser und Stößel für die endgültige Zerkleinerung. Streben Sie nach erbsengroßen oder kleineren Fragmenten an.

Verwenden Sie einen Tonschmelztiegel, der 1200°C+ standhalten kann. Ein Graphit-Ton-Schmelztiegel ist ideal. Der Schmelztiegel sollte mindestens 500 ml fassen. Wenn Sie einen selbst anfertigen, mischen Sie Schamotte mit Sand (2:1 Verhältnis) und brennen Sie ihn langsam in einem Ofen auf 1000°C vor Gebrauch. Vorgefeuerte Schmelztiegel reißen weniger.

Place a 2 cm layer of charcoal pieces at the bottom of the crucible. Add a layer of crushed malachite (2-3 cm). Top with another layer of charcoal. The charcoal both provides carbon for reduction and protects the copper from re-oxidation. Fill to about 2/3 full.

You need a forge or furnace capable of reaching 1100-1200°C. A simple charcoal forge with forced air (bellows or blower) works well. The crucible sits in a bed of burning charcoal with additional charcoal piled around and above it. The air supply must be controllable.

Copper smelting involves extreme heat, toxic fumes, and molten metal. Wear: leather apron, leather gauntlet gloves, face shield, safety boots (no synthetic materials). Work outdoors or in a well-ventilated space. Copper oxide fumes cause metal fume fever if inhaled. Keep a bucket of dry sand nearby for spill containment.

Light the forge charcoal and bring it to full heat over 30-45 minutes with gentle bellows action. Place the loaded crucible into the forge once the coals are glowing white-hot. Pack charcoal tightly around the crucible. Increase bellows speed gradually.

Keep the forge at 1100-1200°C for 2-3 hours. The malachite first decomposes (green changes to black CuO at ~300°C), then the CuO reduces to copper as temperature climbs. Add charcoal to the forge as needed to maintain heat. The crucible contents will shrink as CO₂ gas escapes.

After 2-3 hours, carefully lift the crucible lid with tongs and peek inside. You should see a bright orange-red pool of molten copper beneath a layer of slag (glassy dark material). If you see unmelted green or black chunks, continue heating for another 30 minutes.

While the copper melts, prepare a sand mold or a flat stone depression for pouring. Preheat the mold with a torch to prevent thermal shock cracking. A simple bar-shaped mold carved into a sandstone block works well. Ensure the mold is completely dry — any moisture causes explosive splattering of molten metal.

Using crucible tongs, lift the crucible from the forge. Tilt slowly to pour molten copper into the mold. The dark glassy slag floats on top and can be held back with a stick while the copper flows under it. Pour steadily — interrupted pours create cold shuts (weak seams).

Allow the copper ingot to cool in the mold for at least 30 minutes. Do not quench in water — rapid cooling makes copper brittle with internal stresses. Air cooling produces a softer, more workable metal. The copper surface oxidizes to a dark red-brown as it cools.

Once cool enough to handle (below 100°C), remove the ingot from the mold. Brush off adhering sand and slag with a wire brush. The ingot should have the characteristic salmon-pink color of fresh copper on freshly exposed surfaces. Any dark patches are trapped slag or oxide inclusions.

Weigh the clean copper ingot. From 2 kg of good malachite ore, expect 800-1000 g of copper (40-50% yield, accounting for slag losses and impurities). Pure malachite would yield 1150 g, but field-collected ore is rarely pure. Record the yield for future optimization.

Hammer the edge of the ingot on an anvil. Good copper deforms plastically without cracking — it is one of the most malleable metals. If it cracks, it contains too much slag or oxide. File a small area to expose fresh metal — pure copper is salmon-pink. A green tinge indicates incomplete reduction.

Copper oxidizes slowly in air (forming green patina over months). For short-term storage, keep dry. For long-term, coat with a thin layer of oil. Label with date, source ore, and weight. This copper is ready for casting, hammering into sheet, or alloying with tin to make bronze.