Today, we ask why the Aztecs didn't make full use of metal. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

One question that torments historians of technology is the "Why didn't?" question. Why didn't the Chinese, with all their inventions, produce the industrial revolution? Why didn't the Romans ever make full use of water wheels? Why was Europe 400 years behind China in printing with movable type? All those questions come back upon the present, of course. Why aren't we doing the right thing today -- whatever that might be?

So: why didn't the Aztecs ever emerge from the stone age? Why did such a remarkably advanced people make such limited use of metal? Anthropologist Terry Stocker offers a troubling answer. When you already have a fine technology, you don't see beyond it. And the Aztecs had obsidian for their axes and knives.

Obsidian is a naturally-occurring glass, usually black and opaque. It's harder than steel, and it fractures smoothly. By splitting it, you can create murderously sharp blades. For the early Greeks and Egyptians, obsidian was a profitable medium of trade, not so easily available. Once artisans had shaped cutting tools from bronze, they had reason to give up obsidian.

That never happened in the Aztec world. Southern Mexico was richly endowed with obsidian. Anthropologists now think the huge and mysterious pre-Aztec city of Teotihuacan, near Mexico City, was the center of an obsidian industry.

Aztec swords were made with rows of small obsidian teeth. They were murderous weapons for cutting an enemy. For a long time, historians have marveled at the amount of ceremonial self-mutilation the Aztecs underwent. Now we find that being cut with obsidian is less painful than you'd think, because it makes such a sharp edge.

So obsidian became woven into Aztec worship as well as Aztec function. What need could there be for a replacement material? The Aztecs didn't develop their use of metal because they couldn't see beyond obsidian.

Then the Spanish came with their steel guns, swords, and cannon. They conquered the Aztecs and tried to erase their history. The sublime irony of that is, we now use Aztec obsidian to reconstruct that history. For obsidian carries the imprint of its own past. Once fractured, obsidian slowly reacts to water in a chemical process called hydration. It's possible to read the age of artifacts by seeing how far that process has gone.

I have a little Aztec image on my desk. It's smooth and inky black, carved from Mexican obsidian. It catches the light and rewards the touch. It tells me, as surely as history does, why the Aztecs continued to write their story in this hard, hypnotic, even magical stuff, when simple reason would've preferred gray steel.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

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Stocker, T., A Technological Mystery Resolved. American Inventions: A Chronicle of Achievements that Changed the World. New York: Barnes and Noble Books, 1995, p. 33.

Advances in Obsidian Glass Studies: Archaeological and Geochemical Perspectives (R.E. Taylor, ed.). Park Ridge, NJ: Noyes Press, 1976.

Torrence, R., Production and Exchange of Stone Tools. Cambridge: Cambridge University Press, 1986.