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If the walls of Granada could talk, they would tell you about their unusual patina. In a recent AC paper (DOI 10.1021/ ac8022444), Carolina Cardell and colleagues at the University of Granada (Spain) describe a 12 ft rampart built in the 14th century by the Moors; the rampart is covered in a patina made of calcium carbonate and powdered bones. The investigators suspect that the medieval masons added the bone powder to the patina to make the coating more durable. Burnt, powdered bones in patinas on monuments are unusual, but cases have been cited in the literature about Greek, Roman, and Celtic structures, as well as medieval churches. The bones in these instances either played an aesthetic role or increased the mechanical strength of the patina. The work by Cardell and colleagues is the first report of bones in patinas of medieval Moorish buildings. Clues that Granada’s earth rampart may be unusual surfaced about 2 years ago, says Cardell. The structure stands in the city’s Albayzin quarter, a UNESCO World Cultural Heritage site. As archaeologists were excavating around the wall for a major restoration project, they stumbled upon a brick oven with a pile of black ash and bones next to it. Suspecting that the oven, ash, bones, and wall were connected, the archaeologists asked Cardell to tease out the relationship. The patina on the wall had been analyzed by conventional X-ray diffraction, but the technique had a high limit of detection and didn’t provide much insight. Cardell instead armed her team with micro-X-ray diffraction, SEMenergy-dispersive X-ray spectrometry (SEM-EDX), and IR spectroscopy, which all have lower limits of detection. Cardell says it was obvious by just looking at the oven that its interior used to get extremely hot. The oven bricks were black on the surface and pistachio-green just below the surface, but underneath that, the bricks were reddish-cream. The gradation of color was presumably a consequence of being exposed to high temperatures. 858
ANALYTICAL CHEMISTRY /
COURTESY OF CAROLINA CARDELL
Baring the bones of a medieval Moorish wall
(Top) A 12 ft medieval clay rampart in Granada was strengthened by a coating made of burnt, powdered bones and calcium carbonate. (Bottom) A pile of ash and bone remains were discovered next to a brick oven (the pen is for scale).
But the investigators wanted to know exactly how hot the oven got, so Cardell and her colleagues studied the minerals in the bricks. At particular temperatures, minerals irreversibly break down or form new compounds; with that knowledge, scientists can estimate the temperature of an object. Cardell illustrates the concept with an example: “Calcium carbonate decomposes at over 800 °C,” she says. “If you don’t find calcium carbonate, that means that the temperature of the oven surpassed this temperature.” By analyzing the nature and distribution of the minerals in the bricks, the investigators established that the oven reached a temperature of ⬃1000 °C.
FEBRUARY 1, 2009
Next, the investigators turned their attention to the pile of black ash and bones. The black ash lay below the bones, as if someone had cleaned out the oven and dumped the ash and bones next to it. With their analyses, Cardell and colleagues discovered that the ash and bones consisted mostly of hydroxyapatite, a calcium phosphate-based mineral that is the main component of bone. The wall, which had a thin orangebrown coating, was also scrutinized. The researchers discerned that the contact between the wall and coating was very smooth, suggesting that the patina had been deliberately applied by hand (and that it wasn’t a coating that had accumulated over the passage of time). From the detailed mineral maps obtained by SEM-EDX, the investigators found that the coating was rich in an apatite-based mineral. Further analysis confirmed that more specifically, the mineral was hydroxyapatiteOthe same compound as was found in the ash and bone pile. Piecing together the different analyses, the investigators concluded that the brick oven was used to burn bones down to a powder. The powder was then mixed with calcium carbonate to make the patina. The investigators recently confirmed that the bones came from animals. The calcium carbonate was extracted from limestone that is typical of the kind found in mountains near Granada. The mineral was so well preserved that Cardell says, “I could see even remains of fossils in the patina!” The investigators attempted to pinpoint the age of the bones by radiocarbon dating of collagen, but any of the protein associated with the bones had long since vanished. So the investigators have no idea when the patina was manufactured. The patina isn’t peculiar to this particular rampart. Cardell says, “We have discovered another Moorish building from the same time period that apparently has this patina.” In addition, Cardell is now studying a Moorish water park where it seems the interior walls of a swimming pool are also covered with the patina. —Rajendrani Mukhopadhyay
10.1021/AC802574N 2009 AMERICAN CHEMICAL SOCIETY
Published on Web 12/19/2008
