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Aztec turquoise, Florentine Codex Book XI

The properties of conch shell in making turquoise mosaics

Scanning electron microscopy has long been an essential tool at the British Museum (BM) to enable scientists to extract as much information as they can from each object they study. For over ten years, Dr Caroline Cartwright, a Senior Scientist at the BM and member of our Panel of Experts, has been studying the nine Aztec turquoise mosaics from Mexico which are in the BM collections. She has generously written this introductory article for us on the latest fruits of this scientific research...

Pic 1: One of the BM turquoise masks; copyright Trustees of the British Museum
Pic 1: One of the BM turquoise masks; copyright Trustees of the British Museum (Click on image to enlarge)

By means of scanning electron microscopy (SEM), Dr. Cartwright has identified wood, plant fibres, animal skin and shells used in their construction. This has provided an understanding of the different materials traded from all over Mexico. For the shell, it has thrown new light on the reasons why particular types were selected because of their particular properties. The SEM study of the shell tesserae (little tiles) crafted for these mosaic objects examined to what extent both the microstructural and decorative properties of conch (Strombus) shells were well understood by Aztec and Mixtec peoples in central Mexico prior to the Spanish conquest in 1519. The same SEM procedure was used to examine red Spondylus (spiny oyster) shell and also of the iridescent Pinctada mazatlanica, mother of pearl shell. Both of these were also selected for use as tesserae on the turquoise mosaics.

Pic 2: detail of BM turquoise skull mask; copyright Trustees of the British Museum
Pic 2: detail of BM turquoise skull mask; copyright Trustees of the British Museum (Click on image to enlarge)

The whiteness of conch shell was used to full decorative impact on these mosaics, often being placed next to the turquoise tesserae (pic 2), the green of the malachite and the vivid red, pink and orange colours of Spondylus (see pic 2 [nose] and pic 3 ‘nose’ and gums). But there were a considerable number of useful bivalve and gastropod shells with white colours and smooth surfaces that the craftsmen could have selected. Why was none of these chosen?

Pic 3: detail of BM turquoise double-headed serpent mask; copyright Trustees of the British Museum
Pic 3: detail of BM turquoise double-headed serpent mask; copyright Trustees of the British Museum (Click on image to enlarge)

The second part of the answer lies in the requirements of key elements of the turquoise mosaics themselves, particularly the double-headed serpent. Raw material was required that could be carefully carved and most importantly, be hard-wearing and durable. Far more was needed than just the manufacture of small flat white shell tesserae; the three-dimensional carving of the serpent teeth and fangs (pic 3) necessitated a strong and tough raw material for which conch was the perfect choice.

Pic 4: Aztec craftsman handling a pearl, Florentine Codex Book XI: ‘they are really longed for, ever desirable, a sought-after thing...’
Pic 4: Aztec craftsman handling a pearl, Florentine Codex Book XI: ‘they are really longed for, ever desirable, a sought-after thing...’ (Click on image to enlarge)

Although the colours of shell species played a highly significant part in the decorative designs of the Aztec/Mixtec turquoise mosaics, coloured shell was not selected indiscriminately, irrespective of species, simply for its hue. Some species, such as Spondylus and Pinctada were preferentially selected on account of their symbolic, ritual and cultural importance, which was obviously more important than their working or material properties. Examining the comparative structure of these shells in the SEM showed that Strombus was much tougher and stronger than Spondylus which seems to have been selected primarily for its symbolic importance, then for its colours and lastly for its working properties.

Pic 5: detail of turquoise mosaic mask; copyright Trustees of the British Museum
Pic 5: detail of turquoise mosaic mask; copyright Trustees of the British Museum (Click on image to enlarge)

An even more pronounced example can be seen in the use of the silvery, iridescent shell of Pinctada on the turquoise mosaics which seems to have been chosen because of the importance of its decorative effect (pic 5) and symbolic properties. The structure of Pinctada is even less stable than that of Spondylus; fracturing and splitting are a constant threat. Whilst it clear that the selection of Spondylus and Pinctada by Aztec and Mixtec specialist craftsmen has been less influenced by their working properties as raw materials than by their symbolic value and by their colour or iridescence, the same cannot be said of conch shell. Only Strombus can claim to have been selected as much for its exceptional working properties as for its glossy white colour and high symbolic significance.

Pic 6: A Mexica craftstman working with shell, Florentine Codex Book XI
Pic 6: A Mexica craftstman working with shell, Florentine Codex Book XI (Click on image to enlarge)

Selected reading:-
• Cartwright, C. R. and Meeks, N. D. 2007 “Aztec conch shell working: high-tech design”. British Museum Technical Research Bulletin 1: 35–42.
• Cartwright, C. R. 2009 “Scanning electron microscopy at the British Museum and the Aztec turquoise mosaics from Mexico”. Hitachi Electron Microscopy News 3: 38–42.
• McEwan C., Middleton, A., Cartwright, C.R. and Stacey, R. 2006 Turquoise Mosaics from Mexico. The British Museum Press, London.
• McEwan C., Stacey, R.J. and Cartwright, C. R. (in press) “The ‘Tezcatlipoca’ skull mosaic in the British Museum collections: new insights and questions of identity” in Tezcatlipoca
• Stacey, R.J., Cartwright, C. R. and McEwan, C. 2006 “Chemical Characterisation of Ancient Mesoamerican ‘Copal’ Resins: Preliminary Results”. Archaeometry 48, 323-340.
• Stacey, R.J. and Cartwright, C.R. 2009 “Construction, modification and repair of Mexican mosaics: evidence from the adhesives” in Holding it all Together: ancient and modern approaches to joining, repair and consolidation, Ambers, J., D. Saunders and L. Harrison (eds.) Archetype, London.

Picture sources:-
• Pix 1, 2, 3 and 5: courtesy and copyright © Trustees of the British Museum
• Images from Book XI of the Florentine Codex scanned from our own copy of the Club Internacional del Libro facsimile edition, Madrid, 1994 (main picture at top of page shows pieces of turquoise, or xihuitl in Náhuatl).

This article was uploaded to the Mexicolore website on Jun 05th 2011

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