Egyptian Pottery
© Colin A. Hope/Monash University
Pottery decorated in a distinctive pale blue color was in vogue in New Kingdom Egypt, particularly during the reign of Amenhotep III and Ramesses II.
While most Egyptian pottery was undecorated, during the New Kingdom - Egypt's Golden Age - a variety of pottery was elegantly decorated in a distinctive pale blue.

The pottery has been found at many sites in Egypt, the Middle East, and Sudan, with most of it being in Egypt. Given the restricted use of the blue paint, it probably was available only to artisans associated with major royal residences.

To obtain an uncontaminated sample of the blue paint for chemical analysis, Dr. Jennifer Smith, who is an associate professor at Washington University in St. Louis (WUSTL), Dr. Colin Hope, associate professor and director of the Center for Archaeology and Ancient History at Monash University in Australia, and Paul Kucera, a doctoral student at Monash University, belly-crawled through a long, narrow tunnel at a desert oasis. The tunnel was carved in the rock by Egyptians at the time of the pharaohs.

Although some Egyptian sculptures were colored by adding ground copper to ground quartz, the pale blue pottery was thought to be decorated by cobalt.

"Copper-based pigments must be applied in thick layers and were added after firing, so they tended to flake off when an object was handled. Instead of copper, the colorant used on most of the blue painted pottery is cobalt, which was fired onto the pots," said Hope in a press release.

Analysis of the paint obtained from the site showed that the cobalt was accompanied by trace amounts of zinc, nickel, and manganese, a mixture of elements distinctive enough to serve as a chemical fingerprint.

It was known that at the height of its power, the Egyptian administration of the Nile Valley sponsored mineral exploitation of the valley and surrounding desert regions. This oasis, where deep water is able to reach the surface along fractures and faults under its own pressure, has been continuously inhabited for perhaps as long as 400,000 years.

While searching for the paint, the scientists looked into the waste of eastern Sahara resting against the limestone escarpment that separates the desert from the Nile Valley in the Dakhleh Oasis, where the tunnel is situated.

In the lower foothills of the escarpment at the western end of Dakhleh, four mine shafts were meticulously hand-cut into the rock. Steps carved along the shafts allowed a safe descent. The shafts provided access to horizontal galleries, some as long as 15 meters (about 49 feet), that followed horizontal veins of the mineral alum.

A few centimeters thick, the alum veins are fibrous, pale gray to pink in color, and slightly astringent.

Wondering whether the alum was the secret ingredient used to make the blue paint, Smith obtained a sample for analysis.

"When we characterize a natural mineral, we want to know two things: its chemical composition and then how the elements that make it up are arranged, or its crystal structure," said Smith.

In the case of the Dakhleh alum, the crystal structure was of little use because it would have been destroyed in preparing the paint. Only the composition could connect the alum to the pottery.

Smith's results showed that the alum did contain cobalt, although it wasn't particularly rich in this element. The cobalt, however, was accompanied by trace amounts of manganese, nickel, and zinc - the same mixture of elements found in the blue paint.

Surprised by the low concentration of cobalt, Smith wondered if the ancient artisans found a way to concentrate it on site. From one sample she collected, she found that crust at the edge of a partially flooded mine shaft had a higher cobalt content than the others. Smith said that probably because the sulfate in the alum dissolves easily, the leftover cobalt content would therefore become higher.

"I look at all these different veins of sulfate, and I don't know which are useful for which purposes without doing analyses, but they must have had ways of telling from observable properties which ones to mine. That's impressive," she said.