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
“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
By using analytical techniques that have highlighted strong synergies between
archaeology and other scientific disciplines, the researchers were quickly able
to confirm their original theory, with the results appearing in the journal
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.
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
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.