Elsevier

Analytica Chimica Acta

Volume 553, Issues 1–2, 30 November 2005, Pages 105-110
Analytica Chimica Acta

New insight on ancient cosmetic preparation by synchrotron-based infrared microscopy

https://doi.org/10.1016/j.aca.2005.07.067Get rights and content

Abstract

Synchrotron IR micro-spectroscopy is appropriately suited to the analysis of small particles of archaeological cosmetics. The sample originates from a 33 centuries old Egyptian cosmetic remain. Thanks to the high spectral quality and high lateral resolution achieved by this synchrotron-based microspectroscopic tool, new insights into the composition and spatial location of both organic and mineral components of the sample were obtained. Five different particles were analyzed and delivered similar findings: the core is fat rich, and surrounded with a mineral phase. In two particles, lead palmitate was clearly identified as a major component. Phosgenite (Pb2CO3Cl2), a lead salt synthesized in aqueous solution, was also located in region near the surface of the particles.

Introduction

Cosmetic containers have been frequently found in burials from Ancient Egypt. It emphasizes on the importance of the daily use of make-up during these periods. The role of these make-ups was fourth fold: aesthetic, hygienic, therapeutic and religious functions [1]. The chemical analysis of the remaining cosmetics in these containers (or reeds) is an essential step toward a deeper understanding of their making processes, and, in general, for the chemical and pharmaceutical knowledge and practices in Antiquity.

The analysis of such samples, from Cultural heritage, is more delicate since they are, very often, precious samples, which should be kept intact after their analysis. Moreover, they are heterogeneous at micron scale and may be composed of a mixture of organic and inorganic phases. In other words, their study requires non-destructive techniques, with high detectivity, high lateral resolution, and high chemical sensitivity (atomic, molecular and structural probes). Most of these constraints are fulfilled by synchrotron micro-analytical techniques, which are increasingly used in the Cultural Heritage research field. The techniques are mainly based on synchrotron X-ray beam (e.g. diffraction, scattering, edge-absorption, fluorescence). Yet, the synchrotron radiation also emits in the infrared domain, and offers a brighter source than classical globar sources (roughly three orders of magnitude) [2]. The lateral resolution is no longer limited by the photon flux (∼20 μm with thermal infrared source) but by the diffraction (half of the wavelength in the infrared domain, extending from 2 to 20 μm, in the mid infrared). By coupling an infrared microscope with a synchrotron source, highly contrasted 2D-chemical images can be obtained. The advantages of classical infrared spectroscopy (non destructive, simultaneous information on both organic and mineral phases and on both composition and structure) are enhanced by the use of synchrotron radiation brightness (high lateral resolution with high signal-to-noise ratio). Accordingly, spatially resolved data collection leads to mapping of the various functional groups, a feature which is essential to correlate the vibrational bands of the same compounds spatially, and ease their identification. In addition, structural and conformational information can be obtained and imaged in the different parts of the sample.

The cosmetic container studied in this article is a reed from Ancient Egypt (18th dynasty), kept in the Louvre Museum and referenced as “e11048b”. The reed is still full of cosmetics as evidenced by X-ray radiography (Fig. 1A) and bears a column of hieroglyphs specifying the quality of the product therein. This receptacle is in a very good conservation state. Inside the tube, a mixture of tiny particles of about 100 μm diameter in size, of different shape, colour, and composition was found. Fig 1B shows an optical image of such particles. Also reported on this figure, is the size of the aperture 6 × 6 μm2 used for infrared analysis.

The identification of the mineral phase by X-ray micro-diffraction has revealed the presence of different lead salts, some of them are natural, like galena (PbS) and cerusite (PbCO3), others are synthetic, like phosgenite (PbCO3Cl2) [3]. The discovery of this latter salt demonstrated that the Egyptians mastered some practices of synthesis in aqueous solution, already 33 centuries ago [4]. More recent Greco-Roman authors (Pliny the Elder and Dioscoride) described recipes for producing phosgenite, and stressed its pharmaceutical properties for skin and eye treatment.

Beside this complex mixture of mineral phases, the remaining cosmetic contains fatty components. Chromatographic analyses were previously performed on different fragments obtained by sampling of the container with a 1.5 mm diameter hollow needle, at various depths from the surface down to 20 mm. This analysis has revealed the presence of mixtures of fatty acids [4]. The presence of C15 and C17 iso and anteiso found in every sample is a signature of the animal origin of these greases. The determination of the total fatty acids quantity after saponification and methylation led to an average value of 6.1% (Table 1).

The objectives of this study were two-fold. First, we were particularly interested in understanding better the interactions between organic and mineral components as well as getting conformational information on the organic constituents. Second, we attended to image the distribution of the different constituents inside single particles, hoping to get more insights into the process followed, 33 centuries ago, for the preparation of this cosmetic. Such information was inaccessible with previously used techniques (X-ray micro-diffraction and chromatography).

Section snippets

Samples

Cosmetic particles are directly deposited onto a zinc sulphide-IR transparent window (Thermo-Optek, France). The brightest particles were localized among those deposited with the visible light of the microscope: it turns out that these particles are mainly composed of fat, as evidenced in the infrared spectra (Fig. 2). The acquisition mode was set in transmittance, meaning that some of the particles needed to be slightly flattened, in order to avoid the complete absorption of the IR signals.

Mapping of the molecular groups

Fig. 2 shows an infrared spectrum acquired at location indicated in Fig. 1B, together with that of a modern compound prepared on purpose, for identification. The most intense band is located at ∼3000 cm−1, and corresponds to Csingle bondH stretching modes. This reveals the presence of long fatty alkyl chains. The integrated intensity profile of the band extending from 2819 to 2977 cm−1 is reported in Fig. 3A. Concomitantly, the intensity profile of the carboxylate asymmetric stretching band (1566–1485 cm−1),

Conclusion

Synchrotron FTIR microscopy was used successfully for the analysis of both the mineral and organic constituents of a cosmetic from Ancient Egypt. The chemical images give additional information to better understand the FTIR spectra, as well as to locate the different constituents and imagine the whole process of the cosmetic. The very good spatial resolution provided by the synchrotron source enables to study very tiny quantities and, furthermore, to get detailed location of the different

Acknowledgments

The authors would like to thank Christopher Herm for fruitful discussions about metal soaps and Laurianne Robinet for providing us with the FTIR spectra of metal soaps. This work is supported by an ACI grant “Chimie, Beaute, Sante dans l’Antiquite” from the French Ministry of Research and collaboration between the CNRS and L’Oreal Research.

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