Medieval Walls In Spain Contain Bits Of Bone

Scientists have found a 14th century oven used to bake animal bones for the purpose of strengthening the grand medieval walls in what is today, Granada, Spain. (Credit: American Chemical Society)

In a macabre discovery fit for Indiana Jones, archaeologists in Spain unearthed a 14th century brick oven with a unique role — to bake bones. Scientists report that the animal bones were burnt in the oven and mixed with other materials to produce a protective coating to strengthen the grand medieval walls of what is today Granada, Spain. Scientists now describe how they found these materials thanks to a powerful new testing method.

Carolina Cardell and colleagues point out that ancient decorative and protective layers, or patinas, covering the outside of very old buildings have been subject of many analyses in archaeology, conservation and chemistry. Patinas have been a popular finishing for building exteriors and walls for aesthetic and protective reasons since ancient times. “However, the results of this work are significant for archaeologists since this is the first report of burnt bones in a patina on a Muslim monument, as well as the archaeological artifacts — the oven and raw materials — used to produce them,” says Cardell.

Using a novel new method to identify the components of historical artifacts, the team found hydroxyapatite, the main component in bone pigments and animal bones, in the patina of Granada’s medieval walls. Their new test is inexpensive, identifies chemicals more accurately and — most importantly — does not harm the historical artifacts.

Excavations carried out at the 14th Century city wall of Granada have unearthed a brick kiln located next to stratified layers of bones and ashes. According to researchers from the University of Granada (UGR), this in situ evidence suggests that this kiln was used to manufacture a coating for the wall, which incorporated powder made from burnt bones.

“The bricks from the kiln that was discovered showed evidence of melting (blackened surfaces and small bubbles), which indicates that temperatures inside the kiln were in excess of those needed to fire bricks and Nazari ceramics. The wall was built under the rule of the Nazari sultan between 1333 and 1354”, Carolina Cardell, lead author of the study and a researcher at the UGR, tells SINC.

Until now, the presence of burnt bone powder had been demonstrated in monuments of Greco-Latin, Celtic and Christian monuments (in the Middle Ages), but never before in Medieval Muslim constructions.

By subjecting the bricks to micro X-ray diffraction, a non-destructive technique, it was possible to quickly identify their mineral content using moderately high spatial resolution (around half a millimetre). By analysing the mineral cartography from the map of elements acquired through Scanning Electron Microscopy and EDX (SEM-EDX) microanalysis, the scientists were able to identify the distribution and morphology of the mineral phases within the patina on the wall, as well as their abundance.

The use of analysis techniques suited to the diverse archaeological materials yielded data about their micro-textural and structural characteristics. These provide key information about the manufacturing techniques used to construct the Nazari wall, and also provide clues about how to better preserve the structure.

Non-destructive techniques

Cardell and her team also applied complementary analysis techniques to the artefacts – micro X-ray diffraction, SEM-EDX, Fourier transform infrared spectroscopy, gas chromatography coupled with mass spectrometry and carbon-14 dating.

These tests helped to detect hydroxyapatite, which comes from bones, in the patina on the wall, as well as mineralogical changes that had taken place within the bricks as a result of being exposed to extreme heat, which showed the researchers that temperatures in the kiln reached a little over 1000ºC.

One of the most striking findings of this research project is that the bones were not used only as pigment (the calcination of bones at around 800-900ºC results in white or black bone pigments), but that, according to Cardell “the bone powder was added to make the patina of the wall stronger and more durable”.

The results obtained could lead to these tools being used more widely in the study of cultural heritage artefacts. These “non-destructive” techniques have proved their effectiveness in the analysis of protected materials of great historical value, such as the wall of Granada, which is part of the Albayzín neighbourhood within the UNESCO world heritage site.

The researchers from Granada told SINC they were unaware of other similar discoveries – “either in terms of the use of burnt bone powder in the patina of a Muslim monument or of the archaeological artefacts (kiln and bone and ash remains) used to produce it”, concludes Cardell.