In the absence of anything like real science to guide them, most useful technologies in the ancient world were probably discovered by chance. But that doesn’t seem to bode well for understanding how, when and where these often transformative discoveries took place. Can we ever hope to know how, say, the Stone Age became the Bronze Age, or the Bronze Age became the Iron Age?
Thankfully, modern archaeologists are an optimistic and inventive lot. They figure that, even if the details are buried in the sands of time, we can make some good guesses by trying to reconstruct what the ancients were capable of, using the techniques and materials of the time. Researchers have, for example, built copies of ancient iron- and glass-making furnaces to figure out whether descriptions and recipes from those times really work.
One of the latest efforts now proposes that the production of glazed stones and ceramics – an innovation that profoundly affected trade across the globe – could have been made possible by a vital ingredient in a recipe assembled by serendipity. That possible ingredient? Cow dung.
The earliest glazes, dating from the late fifth millennium BC and found in the Near East, Egypt and the Indus Valley, were used for coating natural stones made from minerals such as quartz and soapstone (talc). As the technology advanced, the stones were often exquisitely carved before being coated with a blue copper-based glaze to make objects now known as faience. By the second millennium BC Egyptian faience was being traded throughout Europe.
Because these copper glazes appear during the so-called Chalcolithic period – the “Copper Age” that preceded the Bronze Age – it has been long thought that they were discovered as an offshoot of the smelting of copper ores such as malachite to make the metal. The glazes are forms of copper silicate, made as copper combines with the silicate minerals in the high temperature of a kiln. These compounds can range from green (like malachite itself, a kind of copper carbonate) through turquoise to rich blue, depending on how much salt (more specifically, how much chloride) is incorporated into the mix. The more of it, the greener the glaze.
Sometimes these copper glazes are crystalline, with regularly ordered arrays of atoms. But they can also be glassy, meaning that the atoms are rather disordered. In fact, it seems likely that copper smelting stimulated not only glazing but the production of glass itself, as well as the pigment known as Egyptian blue, which is a ground-up copper silicate glass. In other words, a whole cluster of valuable technologies might share a common root in the making of copper metal.
The basic idea, put forward by Egyptologists such as the Englishman William Flinders Petrie in the early twentieth century, was that other materials might have found their way by accident into copper-smelting kilns and been transformed in the heat. Glass, for instance, is little more than melted sand (mostly fine-grained quartz). To melt pure sand requires temperatures higher than ancient kilns could achieve, but the melting point is lowered if there is some alkaline substance present. This could have been provided by wood ash, although some later recipes in the Middle East used the mineral natron (sodium carbonate).
How exactly might a blue glaze have been made this way? Another early Egyptologist, Alfred Lucas, who worked with Howard Carter of Tutankhamun fame, proposed that perhaps a piece of quartz used to grind up malachite to make eye-paint found its way into a kiln, where the heat and alkali could have converted residues of the copper mineral into a blue film. But that would make the discovery independent of copper manufacture itself, and it’s not obvious how a grinding stone could slip into a kiln. Yet why else should a copper compound come to be on the surface of a lump of quartz?