Analytical characterization of the roman wall paintings of
the archaeological site of Cástulo (Spain)
1. Authors of the research
Antonio Doménech-Carbóa, María Teresa Doménech-Carbób, Francisco López-Lópeza, Francisco Manuel Valle-Algarraa, Laura Osete-Cortinab, Estrella Arcos Von Haartmanc
aDepartament de Química Analítica. Universitat de València. Dr. Moliner, 50, 46100 Burjassot (Valencia), Spain
b Institut de Restauració del Patrimoni. Universitat Politècnica de València. Camino de Vera 14, 46022 Valencia, Spain
c Quibla Restaura S. L. Universidad de Málaga, Travesía Monte Sancha 8, 29016 Málaga, Spain
2. Archaeological site of Cástulo
The Roman city of Cástulo is an archeological site placed in the Andalucia Region (Spain). This city dates back to the 3rd millenium BC when an Iberian settlement was occuping this place. Further, in the 3rd century BC this place becomes Roman city and was progresively enlarging with a number of outstanding monuments such as an acueduct, termas, hipocaustum with opus spicatum and a number of nobiliary houses.
The present study is focused on the analytical characterization of the splendid wall paintings found in one of these houses, which exhibit a rich color palette including characteristic pigments corresponding to the Roman period from local sources.
3. Materials and methods
3.1 Conventional techniques
Optical microscopy. Microsamples were prepared as cross sections by embedding them in polyester resin (Seryfix, Struers) and then mechanically polished to a 4000 silicon carbide grit finish in a polishing machine ( Knuth-rotor 2, Struers, Erkrath, Germany) for examination under a Leica DMR microscope using polarized reflected light at X25-X400 magnification.
Scanning electron microscopy-energy dispersive x-ray microanalysis. Samples prepared as cross-sections were studied by SEM-EDS using a Jeol JSM 6300 scanning electron microscope operating with a Link-Oxford-Isis X-ray microanalysis system. The analytical conditions were: 20 kV accelerating voltage, 2x10-9A beam current and 15 mm as working distance. Samples were carbon coated to eliminate charging effects. Elemental analysis was performed in parallel to the morphological examination of the microsamples. Qualitative analysis was performed by point and area acquisitions. Quantitative microanalysis was carried out using the ZAF method that is based on the correction of the matrix of multi-elemental effect that takes place in the simultaneous determination of the concentration of each element present in a multielemental material. This theoretical method provides a correction of the x-ray intensity of each element present in the material by means of the application of a series of correction factors for the atomic number effect (which describes the depth of electron penetration and the fraction of electrons that backscatter from the sample), the absorption correction (which describes the absorption of x-rays in the matrix as function of the composition and depth of electron penetration) and the fluorescence correction (which describes the secondary fluorescence of one element by the others present). In the present study, the counting time was 100 s for major and minor elements. Concentrations were calculated by stoichiometry from element percentages generated by ZAF software on the Oxford-Link-Isis EDX instrument.
Fourier Transfrom Infrared Spectroscopy: IR spectra of the samples were obtained using a Vertex 70 Fourier-transform infrared spectrometer with a FR-DTGS (fast recovery deuterated triglycine sulphate) temperature-stabilized coated detector. Number of co-added scans: 32, obtained with a spectral resolution of 4 cm-1.
3.2 Nanoelectrochemical techniques
Voltammetry of microparticles: Voltammetric measurements were performed at sample-modified paraffin-impregnated graphite electrodes immersed into deareated 0.50 M sodium acetate buffer at pH 4.85, using a CH I420 equipment. A conventional three-electrode arrangement was used, with a Pt-wire auxiliary electrode and an AgCl (3M NaCl)/Ag reference one. Samples (ca. 1 µg) were pressed on a freshly polished graphite electrode and subjected to voltammetry analysis.
Square wave voltammograms (SWVs), and cyclic voltammograms (CVs): were obtained using both one touch and conventional abrasive VMP protocols involving graphite pencil. The electrolyte solution was renewed after each electrochemical run to avoid contamination due to copper ions eventually released to the solution phase during electrochemical turnovers.
Electrochemical experiments were performed at 298 K in a three-electrode cell under argon atmosphere using a CH I660C device (Cambria Scientific, Llwynhendy, Llanelli UK). A platinum wire counter electrode and an AgCl (3 M NaCl)/Ag reference electrode completed the three-electrode arrangement.
Scanning electrochemical microscopy (SECM): experiments were performed on deposits of the studied materials on a graphite plate acting as a substrate electrode in contact with 5.0 mM K4Fe(CN)6 solution in 0.25 M HAc/NaAc (pH 4.75). Experiments were performed with CH 920c equipment using a microdisk platinum electrode tip (CH 49, diameter 20 mm) and a Pt substrate electrode. The bipotentiostat mode was used to apply potentials to the tip (ET) and the electrode substrate (ES).
Atomic force microscopy-voltammetry of nanoparticles (AFM_VNP): In situ AFM-monitored electrochemical experiments were performed with a multimode AFM (Digital Instruments VEECO Methodology Group, USA) with a NanoScope IIIa controller and equipped with a J-type scanner (maximum scan size of 150 µm × 150 µm × 6 µm) was used. The topography of the samples was studied in contact mode. An oxide-sharpened silicon nitride probe Olympus, VEECO Methodology Group, model NP-S has been used with a V-shaped cantilever configuration. Transference of sample particles to a carbon plate were performed by means of a diluted solution of acrylic polymer.
4. Acknowledgements
Financial support is thanked to the Spanish (MICINN) R+D Project CTQ2011-28079-C03-01 and 02 also supported with ERDF funds. Research was conducted within the “Grupo de análisis científico de bienes culturales y patrimoniales y estudios de ciencia de la conservación” microcluster of the University of Valencia Excellence Campus (Ref. 1362). The authors wish also to thank Mr. Manuel Planes i Insausti and Dr. José Luis Moya López, the technical supervisors responsible for the Electron Microscopy Service of the Universitat Politècnica de València, technical supervisor of Scientific.
5. Conclusions
Ground: typical fresco technique has been used for this painting. In Figure 1 is shown this ground layer made with lime and sand.
Color palette: the pigments identified by means of the multi-technique approach applied
were:
Blue: Egyptian blue
Green: Green earth
Red: Iron oxide red from Spanish ores (Hematita española)
Orange: Red earth, Vermillion
Brown: Sienna raw
Black: Carbon black
6. Publications and communications
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Antonio Domnech-Carbó, María Teresa Doménech-Carbó, Francisco López-López, Francisco Manuel Valle-Algarra,a Laura Osete-Cortina Estrella Arcos-Von Haartman
Electrochemical Characterization of Egyptian Blue Pigment in Wall Paintings Using the Voltammetry of Microparticles Methodology
Electroanalysis 2013, 25, No. 12, 2621 – 2630.
Analytical characterization of the roman wall paintings of the archaeological site of Cástulo (Spain)
WAC-7 Seventh World Archaeological Congress, 13-18 enero 2013, Dead Sea (Jordania).