Spectroscopic Characterization of Natural Dyes for Their Non-Invasive Identification on Pre-Columbian Codizes: The Maya Yellow

Beskrivelse

Mesoamerican books were colorfully painted manuscripts used by pre-Hispanic
indigenous societies, such as the Maya and the Aztecs to record by means
of images and logophonetic writing, religious, calendrical and astronomical
information [1]. Only a few of them survived the Spanish conquest as they were
brought to Europe, because of their unlikeness to any other object.
Because of their uniqueness and rarity it was applied a non-invasive analytical
methodology for all the codices studied so far [1,2,3]. The objective was to better understand the artistic ways of the Mayans and the Aztecs, by intensive study of their coloring materials and techniques, leading to better conservation methods.
Therefore, several colors were analyzed and described, such as the well-known
Maya Blue, made from a mixture of indigo and a clay called palygorskite, forming
an incredibly stable material where the dye is trapped inside the nanotubes of
the clay [4]. However, a bigger challenge lies with the study of the yellows used.
It has been proven that the Mesoamericans used the same clay-dye mixture to
produce their yellow colorants, and being organic dyes, they are harder to be
characterized, and understand the clay-dye interaction.
This study begins to unveil the mysteries surrounding the Maya Yellow,
presenting a better understanding of this hybrid material, as well as to provide a
database for yellow dyes not common in the European art history. For this, it is
necessary to understand also the differences between some classes of colorants,
such as the flavonoids and the carotenoids, present in plants that were most
likely used to produce the yellow color in the Mesoamerican culture. This study
tries to shed a light on their differences as colorants in the clay-dye hybrids, as
well as the type of interaction they might have with the clay to understand how
the Mesoamericans created such stable colorants. Also, since heating was used
as an important step for the production of these hybrids, a detailed study on
the effects of temperature on the molecules was performed. For this study a
flavonoid (morin) and a carotenoid (annatto) were used, as well as two types of
clay (palygorskite and kaolinite), in order to better understand the influence of
the clay structure in the hybrid. The mixture was prepared at a 20% of dye, and
several temperatures were employed from 140ºC to 220ºC. Analytical methods
were performed, such as Fourier Transformed Infrared spectroscopy, UV-Vis
absorption and emission, conventional Raman spectroscopy, as well as Surface enhanced Raman spectroscopy (SERS).
It was possible to realize the range of temperature that can be applied before the
degradation of the dye molecules in the hybrid. We could also confirm that noninvasive studies can help to characterize the difference between carotenoids and flavonoids, and their presence in the dye-clay hybrid. Finally, it was noticeable an interaction between the dyes and the clay, and also, that it varies between the different colorant species, becoming important the continuing of this study to form a database for the yellows used by the Mesoamericans, and therefore characterize which ones were employed in the codices.

Co-author with: P. Nabais 1, D. Buti 2, Ch. Grazia 3, A. Romani 2-3, A. Sgamellotti 2-3, C. Miliani 2-3 1 Department of Conservation and Restoration, Faculty of Science and Technology, New University of Lisbon, Quinta da Torre, 2829-516 Caparica, Portugal 2 CNR Instituto of Molecular Science and Technology, via Elce di sotto, 8, 06123 Perugia, Italy 3 Center of Excellence SMAArt, Chemistry Department, Perugia University, via Elce di sotto, 8,06123 Perugia, Italy

Periode	4 jul. 2014
Begivenhedstype	Konference
Placering	Vienna, Østrig