Published: 11 April, 2002

From rock to metal (and we're not talking music)

How do you make a rock into a metal or a crystal?

After our ancestors found the answer to your question six thousand years ago, Cristi, they were able to replace their stone tools and weapons with much better ones made from metals. If that hadn't happened then, there would be no skyscrapers and jet aircraft today.

Folks in the Near East knew of metals because they found little pieces of copper at the bottom of streams. Artisans hammered and melted this copper to form beads and jewelry, but they didn't have enough for useful tools such as axes and plows. Early workers had to learn how to extract copper from copper-containing stones, called ores.

In most such ores, copper atoms are tightly bound to oxygen atoms. To free the copper, its ore must be heated with something that binds oxygen even more strongly than copper. This is called a reduction reaction, in which two different atoms compete for oxygen, with the more reactive one winning by binding to it. Carbon, such as the charcoal in our backyard grill, is more reactive than copper, so that it can reduce copper back to its metallic state. The oxygen binds to the carbon and goes off in the form of carbon dioxide gas, leaving behind copper metal. No one is sure how this first happened. It may have occurred when a potter fashioning a pot decorated it with malachite, a beautiful green copper-rich stone. When the clay pot was fired in a wood-fueled kiln, perhaps the malachite fell to the bottom into the partially burnt wood (charcoal), which bound to the oxygen, releasing metallic copper. Imagine the joy of our potter when, raking out the ashes from the kiln, he or she came upon a chunk of copper.

This revolutionary discovery led to the Bronze Age beginning 5,000 years ago and the Iron Age 3,000 years ago. Learning to extract metals from rocks using charcoal was a crucial step toward establishing our modern industrial world.