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Sunday, January 23, 2022

The ancient origins of glass

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Enlarge / This glass fish was found in a fairly modest private house in Amarna, buried under a plaster floor along with a few other objects. It may once have contained ointment.

Trustees of the British Museum

Today, glass is ordinary, on-the-kitchen-shelf stuff. But early in its history, glass was bling for kings.

Thousands of years ago, the pharaohs of ancient Egypt surrounded themselves with the stuff, even in death, leaving stunning specimens for archaeologists to uncover. King Tutankhamen’s tomb housed a decorative writing palette and two blue-hued headrests made of solid glass that may once have supported the head of sleeping royals. His funerary mask sports blue glass inlays that alternate with gold to frame the king’s face.

In a world filled with the buff, brown, and sand hues of more utilitarian Late Bronze Age materials, glass—saturated with blue, purple, turquoise, yellow, red, and white—would have afforded the most striking colors other than gemstones, says Andrew Shortland, an archaeological scientist at Cranfield University in Shrivenham, England. In a hierarchy of materials, glass would have sat slightly beneath silver and gold and would have been valued as much as precious stones were.

But many questions remain about the prized material. Where was glass first fashioned? How was it worked and colored and passed around the ancient world? Though much is still mysterious, in the last few decades materials science techniques and a reanalysis of artifacts excavated in the past have begun to fill in details.

This analysis, in turn, opens a window onto the lives of Bronze Age artisans, traders, and kings as well as the international connections between them.

The Amarna Letters, clay tablets carrying the cuneiform correspondence of ancient kings and excavated at Tell el-Amarna in modern-day Egypt, include references to glass. A number from the Canaanite ruler Yidya of Ashkelon (like these shown) include one that comments on an order of glass for Pharaoh: "As to the king, my lord's, having ordered some glass, I herewith send to the king, my lord, 30 ('pieces') of glass. Moreover, who is the dog that would not obey the orders of the king, my lord, the Sun from the sky, the son of the Sun, whom the Sun loves?"
Enlarge / The Amarna Letters, clay tablets carrying the cuneiform correspondence of ancient kings and excavated at Tell el-Amarna in modern-day Egypt, include references to glass. A number from the Canaanite ruler Yidya of Ashkelon (like these shown) include one that comments on an order of glass for Pharaoh: “As to the king, my lord’s, having ordered some glass, I herewith send to the king, my lord, 30 (‘pieces’) of glass. Moreover, who is the dog that would not obey the orders of the king, my lord, the Sun from the sky, the son of the Sun, whom the Sun loves?”

Trustees of the British Museum

Glass from the past

Glass, both ancient and modern, is a material usually made of silicon dioxide, or silica, that is characterized by its disorderly atoms. In crystalline quartz, atoms are pinned to regularly spaced positions in a repeating pattern. But in glass, the same building blocks—a silicon atom buddied up with oxygens—are arranged topsy-turvy.

Archaeologists have found glass beads dating to as early as the third millennium BCE. Glazes based on the same materials and technology date earlier still. But it was in the Late Bronze Age—1600 to 1200 BCE—that the use of glass seems to have really taken off, in Egypt, Mycenaean Greece, and Mesopotamia, also called the Near East (located in what’s now Syria and Iraq).

Unlike today, glass of those times was often opaque and saturated with color, and the source of the silica was crushed quartz pebbles, not sand. Clever ancients figured out how to lower the melting temperature of the crushed quartz to what could be reached in Bronze Age furnaces: they used the ash of desert plants, which contain high levels of salts such as sodium carbonate or bicarbonates. The plants also contain lime—calcium oxide—that made the glass more stable. Ancient glassmakers also added materials that impart color to glass, such as cobalt for dark blue or lead antimonate for yellow. The ingredients melded in the melt, contributing chemical clues that researchers look for today.

“We can start to parse the raw materials that went into the production of the glass and then suggest where in the world it came from,” says materials scientist Marc Walton of Northwestern University in Evanston, Illinois, co-author of an article about materials science and archaeological artifacts and artwork in the 2021 Annual Review of Materials Research.

But those clues have taken researchers only so far. When Shortland and colleagues were investigating glass’s origins around 20 years ago, glass from Egypt, the Near East, and Greece appeared to be chemical lookalikes, difficult to distinguish based on the techniques available at the time.

The exception was blue glass, thanks to work by Polish-born chemist Alexander Kaczmarczyk who in the 1980s discovered that elements such as aluminum, manganese, nickel, and zinc tag along with the cobalt that gives glass an abyssal blue hue. By examining the relative amounts of these, Kaczmarczyk’s team even tracked the cobalt ore used for blue coloring to its mineral source in specific Egyptian oases.



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