Jennifer Smith, PhD, associate professor of earth and planetary
sciences in Arts & Sciences at Washington University in St.
Louis, was belly crawling her way to the end of a long, narrow
tunnel carved in the rock at a desert oasis by Egyptians who lived
in the time of the pharaohs.
"I was crawling along when suddenly I felt stabbed in the
chest," she says. "I looked down and saw that I was pressing
against the broken end of a long bone. That freaked me out because
at first I thought I was crawling over bodies, but I looked up and
saw a sheep skull not too far away, so I calmed down. At least the
bones weren't human."
What was she doing in the tunnel?
The answer: seeking an uncontaminated sample of a mineral that
might have been the key ingredient in the blue used to decorate
"blue painted pottery" popular among the Egyptian elite during the
New Kingdom (1550 to 1079 BCE).
Colleague Colin A. Hope, an expert in blue painted pottery, had
asked if she wouldn't help him pin down the source of the blue
pigment by sampling and analyzing material fromt he mine.
Hope and Smith, together with Paul Kucera, a doctoral student at
Monash University who first identified the mines, describe the
pottery, the mines and the mineral in a chapter of Beyond the
Horizon, a festschrift for the Egyptologist Barry A. Kemp,
'Generic geologist'
In the wastes of the eastern Sahara, nestled against the
limestone escarpment that separates the desert from the Nile
Valley, lies the Dakhleh Oasis. This fortunate spot, where deep
water is able to reach the surface along fractures and faults under
its own pressure, has been continuously inhabited for a very long
time — perhaps as long as 400,000 years.
During that period there were roughly four glacial cycles and,
although Egypt itself was ice-free, the local climate oscillated
from hyperarid to semi-arid as the Earth's orbital position drove
changes in the location of the tropical rainfall belts.
Smith studies the impact of these climate fluctuations on
ancient oasis dwellers.
But Smith is also the "generic geologist" as she puts it, for
the Dakhleh Oasis Project, a long-term study of the oasis that
covers the entire stretch of Dakhleh history, from the Neolithic
through the Pharaonic, Roman, Islamic and modern settlements, and
employs — off and on — more than 50 specialists.
"The dig house is open from November until March," Smith
says.
As generic geologist, Smith was asked to help with a
material-sourcing puzzle that she says was "way outside her
period." During the 2007 season, Colin A. Hope, PhD, associate
professor and director of the Center for Archaeology and Ancient
History at Monash University in Australia, asked her whether a
mineral found at the oasis could have been used to color the blue
painted pottery.
It was a small question but an intriguing one.
Blue painted pottery
Most Egyptian pottery is undecorated, but during the New
Kingdom, the period when Egypt is at the zenith of its power, a
variety of pottery was elegantly decorated in a distinctive pale
blue.
The pottery has been found at many sites in Egypt, and also in
the Middle East and in Sudan.
The largest deposits, however, were found at New Kingdom sites
in Egypt, including Malqata (the palace complex of Amenhotep III),
Amarna (the remains of the city built by the Akenaten, the famous
pharaoh who moved the capital from Thebes and established his own
religion), the cemetary at Deir el-Medineh (the village where
artisans who worked on the tombs in the Valley of the Kings during
the New Kingdom lived), and the Great Temple of Amun (patron of
kingship during the New Kingdom).
"Walking over some sites, it is only a matter of minutes before
several shards of blue painted pottery or cobalt blue glass or
faience can be collected," Hope, who has written extensively about
the pottery, says.
Given the restricted use to which the pigment was put and the
archeological sites where remnants were found, Hope believes it was
probably available only to artisans associated with major royal
residences.
The pale blue is distinguishable at a glance from the brilliant
blues and blue-greens of the faience glazes common from the 3000 BC
onward. Faience, probably most familiar in the form of the small
statue of a hippo nicknamed William that is now in the collection
of the Metropolitan Museum of Art in New York, was made by adding
ground copper to ground quartz to create what ceramists today call
Egyptian paste.
But it is difficult to create durable patterns with copper
pigment on pottery, says Hope. "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.
Where did the cobalt-bearing mineral come from? Analysis of the
paint 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.
The mines of Dakhleh
At the height of its power, the Egyptian administration of the
Nile Valley sponsored mineral exploitation of the Valley and
surrounding desert regions. As early as 1980, it was suggested that
the cobalt might have come from the desert oases at Dakhleh and
Kharga.
In the lower foothills of the oasis 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, 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.
Alum is both the term for a specific compound and for a class of
compounds, all of which contain two negatively charged sulfate
groups and two chemical elements or groups bearing a positive
charge. The specific compound is hydrated aluminum potassium
sulfate but many other elements or groups can substitute for the
aluminum and potassium, and cobalt is one of these.
Alum was probably exploited for a variety of purposes in ancient
times, some having nothing to do with color. The Egyptians, for
example, used alum both to whiten skins during tanning and to
prepare cloth to absorb dye.
Alum is still used today in styptic pencils to stem bleeding and
in recipes for pickling cucumbers. More recently, it has been in
vogue as a "crystal deodorant" that is sold as more natural than
older deodorant products.
Was the Dakhleh Oasis alum used as a general-purpose astringent,
or did it have the same chemical fingerprint as the blue paint on
the pottery?
Analyzing the alum
To find out, Smith needed to sample the alum and analyze its
composition.
"I wanted to get relatively unaltered samples," she says, "which
is why I was crawling to the end of a gallery. The galleries were
small enough you couldn't really crawl on your hand and knees: you
had to belly crawl."
Smith brought the samples she collected back to Washington
University where she ran them through a variety of sophisticated
analytical instruments. "When we characterize a natural mineral,"
she says, "we want to know two things: its chemical composition and
then how the elements that make it up are arranged, or its crystal
structure."
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 they weren'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 hadn't found a way to concentrate it on site.
One sample she collected, a crust at the edge of a partially
flooded mine shaft, had a higher cobalt content than the others.
Because sulphate dissolves easily and the mines were much more
likely to have been flooded in the past, she wondered whether the
cobalt was mined not by chipping it out of the rock but instead by
ladling water out of the mines and collecting the sediment left
over when the water evaporated.
"But this is wild arm waving given the amount of data," Smith
says.
This small exercise in archeological problem solving left her
with a deep respect for the long-vanished miners.
"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 says.
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