Scientific Analysis of the mural paintings of Aruch, Kobayr and Akhtalà

Transcription

1 Scientific Analysis of the mural paintings of Aruch, Kobayr and Akhtalà Nicola Ludwig 1, Maria Pia Riccardi 2, Letizia Bonizzoni 1, Michela Cantù 2, Marco Gargano 1, Fabio Giacometti 2. 1 Università Statale di Milano 2 Università degli Studi di Pavia 1 Fiber optic reflectance spectroscopy measurements with a portable spectrometer on the apse painting in the Akhtalà church. Introduction: The microanalysis for the study of ancient paintings. Non-destructive analyses directly on painted surface and micro sampling are useful in many cases to obtain information about the material characteristics of both pigments and fresco structure of ancient mural paintings. In our case artworks could not be studied under laboratory conditions, and the majority of data had to de recorded in a short time in the field, allowing only a small amount of micro samples, pivotal for understanding past artistic techniques, to be examined scientifically. Thus this kind of analysis was deemed important to complement the information about the mural paintings studied in the context of the Training Program implemented in Armenia. Another aim consisted in providing support to the Armenian Ministry of Culture in the execution of the conservation of murals. Analyses were conducted on the mural paintings of the left chapel of Kobayr Church (Fig. 2), and on some areas of the huge fresco in the basilica of Aruch in order to obtain a complete scientific investigation of pictorial materials used by the ancient painters. Numerous selected areas on the frescoes of the church of Akhtalà (Fig. 1), by virtue of its interesting nature and its closeness to Kobayr, were also studied. The identification of pigments was performed by visible-near infrared spectroscopy with a fiber optics probe (FORS) directly on the painting during the inspection (see Fig. 1). Collected micro samples of the powders were investigated in laboratory though energy dispersion X-Ray fluorescence spectrometry (ED- XRF), Microraman Spectroscopy (RS) and a SEM. Micro samples (0,3-3 mm) of ancient mural paintings were mounted in epoxy resin and examined with an optical microscope and SEM equipped in order to unravel their textural and chemical properties. The composition of pictorial layers, ground layers and plasters were studied with in-situ EDS (SEM) microanalyses that allowed to recognize the different components utilized by ancient painters. Where neither sampling nor contact analysis were possible, due to the high position of the paintings, an infrared false-color digital image was taken (Fig. 2). To verify the reliability of this last techniques, infrared false-false color digital images were also taken where contact analyses were possible and the comparison confirmed the good quality of these analyses. In cases where sampling would not modify the figurative composition, micro samples were collected in order to clarify doubts about the presence of pigments; only four of these samplings were made with a knife to preserve the stratigraphy of the mural painting. In some others areas, a very small amount (less than 0.1 g) of superficial pigment powder was sufficient to identify was sufficient to identify characteristic metals in mineral pigments (Fig. 7) by laboratory optical and/or elemental techniques

2 Kobayr Two samples of blue paintings were collected and they revealed different techniques in the preparation of the pictorial mixtures. Sample CMK1 (K03) The superimposition of different pictorial layers was observed in this sample. The blue pictorial layer was applied on a white ground layer (Fig. 3). No fragments of the underlying plaster were collected. The ground layer consists of lime and, with minor apatite, which were probably utilized to have a white base on which the blue pictorial layer could be applied. Si and Al peaks in EDS spectra are ascribable to the contamination from the overlying pictorial layer. No evidence of weathering of occurs. The blue layer consists of crystal fragments mixed with, lime and an organic binder. Crystal fragments correspond to lazurite, therefore indicating the usage of lapis lazuli as a pigment. Sample Cmk4 (K08B) This sample has a complex stratigraphy, revealing the superimposition of two coloured layers on a ground layer (Fig. 4). The ground layer consists of a homogeneous fine-grained lime aggregate with small secondary gypsum fibers as confirmed by the S-peaks in the EDS spectra. The first pictorial layer is green and was obtained by the mixture of a yellow and a blue pigment. The yellow pigment 2 Digital Infrared False-Color image of Christ in the apse of Kobayr church. Yellow and red areas appear painted in Vermilion and Ultramarine blue respectively. consists of orpiment (As2S3) as revealed by EDS spectra. The blue pigment has a complex composition probably corresponding to that of an unidentified clay mineral. EDS investigation evidence indicates that the silicate utilized does not correspond to glauconite and/or celadonite which are the most abundant components of greenish soils. Further investigations are needed to unravel the nature of this pigment. Fragments of a spinel group mineral (with intermediate composition between chromite and spinel s.s.) are widespread and could be responsible for the blue component in the pigment. The blue layer consists of a mixture of, lime and the same blue pigment as that found in the underlying green layer. The uppermost portion of this sample consists of a sequence of (< 10 μm) organic layer on which a μm gypsum layer and a μm red soil layer were applied on the blue painting. Akhtalà The most important frescoes of Armenia (Fig. 1) are found in the church of Akhtalà. The absence of the cupola in the central part of the church and the lack of windows frames and the damaged roof have been causing water penetration. Point /sample name RED K01 Vermilion -- K07 Vermilion over litharge Hg, Pb, Ca (Sr, Fe) K08R / CMK4 Vermilion Pb, Hg (Fe) K09 Vermilion Hg, Pb (Ca, Fe, Sr) BLUE K03/ CMK1 Ultramarine (lapislazuli) -- K08B / CMK4 Ultramarine (lapislazuli) and Pb, As, Ca (Fe, Co) orpiment GREEN K13 Cobalt green (green earth) -- WHITE K04 Gypsum - Calcium carbonate Ca (Fe, Hg, Pb) (bianco di S. Giovanni) 4 K10 Gypsum - Calcium carbonate Ca (Fe, Pb) K15-halo Gypsum - Calcium carbonate -- *1 Calcium carbonate white Ca (Fe, Pb, Sr) lead *2 Calcium carbonate white Ca (Fe, Pb, Sr) lead YELLOW K02 Yellow ochre Pb (Ca, Fe, Cu) K12 Yellow ochre -- BLACK K14 Organic carbon -- 3 Sample of a blue pictorial layer on a white ground layer. The optical microscope image (left) and BSE image (right) reveal the presence of crystal fragments utilized as a pigment. EDS spectra indicate the composition of the two layers. 5 Frescoes of Akhtalà church. -- = no sampling was done Calcium and iron are always present and are mainly due to the plaster base. Table 1 shows the data split in six main chromatic classes Red, Blue, Green, White, Yellow and Black

3 However the paintings show a good state of conservation thanks both to the presence of a continuous surveillance by the priest working there and to an important restoration intervention realized in the 70s. A strong reminder of this intervention can be found in the wide use of cobalt blue, singled out in many point of the paintings in both the side of the church and in the apse. Cobalt blue has been identified thanks to several characteristic peaks of reflectance in spectral region between 450 and 630 nm as shown in Fig. 7. Calcium and iron are always present, which is mainly due to the plaster base. One sample of a blue pigment from this site was analyzed showing similarities with one of the samples from Koybar s minor church. Sample of a blue pictorial layer superimposed on a white ground layer. The optical microscope image (left) and BSE image (right) reveal the occurrence of only one pictorial layer. EDS spectra give insights on the mixture utilized for the two layers Sample with a complex sequence of superimposed pictorial layers (Cmk4). The optical microscope image (left) and BSE image (right) reveal this complex stratigraphy (lower right) and EDS spectra give insights on the mixture utilized. Blue pigments of the Akhtalà church frescoes.. BLUE sample name A20 Ultramarine (lapislazuli) and Pb, Ca, (Fe) A21 Cobalt blue and Pb, Ca (Fe, Zn, Co) A24 Cobalt blue and -- A26 Cobalt blue and -- A28 A31 A32 A41 Cobalt blue Ca (Fe, Pb, Sr, Co) A43 Sample Blu AK In this sample the blue pictorial layer is applied on a white ground layer (Fig. 8). The ground layer consists of Mg-rich lime mixed with, which was

4 probably used to obtain a white base. Minor aluminum and silicon peaks in the EDS spectra reveal the presence of small amounts of silicates in the mixture. The blue pictorial layer consists of azurite fragments mixed with an organic binder. Additionally, in this case lapis lazuli was (natural ultramarine) utilized as pigment. Aruch The church of Aruch is the most important basilica style building of ancient Armenia. A wide portion of a fresco in the apse shows some vestiges of a former very large fresco of the seventh century. The pictorial palette shows typical compositions of frescoes with white parts obtained by the use of and studied samples. Sample MAR7 A blue-greenish pictorial layer was painted on a ground layer and not directly on the plaster (Fig. 10). SEM-EDS investigations evidenced that the plaster is essentially made of gypsum whereas a mixture of lime and clay minerals was utilized as a ground layer. S-peaks in the EDS spectra of the ground layer could be due to contamination from the underlying plaster during the application of this lime and clay mixture. EDS spectra also suggest that a copper sulfide (probably chalcocite, XRF spectra of the same area red/blue area of fig. 4 and 5. Hg mercury - is attributed to Vermilion pigments and As arsenic - to the orpiment pigment respectively. Lead Pb - is due to the presence. 9 red ones obtained using with ochre. In contrast, early painters used a mixture of blue and yellow pigments in order to obtain different green hues. Green areas of grass or gems depicted on the books held by the saints show the use of a different mixture of azurite and litharge (Fig. 9). Calcium and iron are always present, which Point /sample name GREEN D00 / MAR7 Azurite and litharge /orpiment Cu (As, Ca, Fe) D01g Azurite + litharge -- D15g Azurite + litharge -- D20g Azurite + litharge -- D14g Azurite + litharge -- WHITE Dx - scroll White lead Pb (Fe, Ca) D14 White lead -- D22 White lead -- D25 White lead -- RED D11 Red ochre -- D12 Red ochre -- D23 Red ochre -- is mainly due to the plaster base. Samples of blue paintings were collected and revealed complex stratigraphies. Although it is not possible to define an absolute chronology of the coating events, the local superimposition of different pictorial layers is apparent in some of the Sample of blue-greenish mural painting with the underlying ground layer and plaster. Top left: optical microscope image. Lower left: correspondent BSE image in which the three different layers (plaster, ground layer and pictorial layer) are shown. Right: EDS spectra of the average composition of the three layers. 10 whose weathered product - malachite - is typically green) was utilized as a pigment in the pictorial layer. Moreover, the locally high C and/or Ca concentrations suggest that this layer was prepared by mixing the pigment with lime and an organic binder. The observed P-peaks are unlikely to be due only to environmental contamination and are probably ascribable to the presence of apatite in the raw materials utilized. Sample A21B Three different layers are superimposed and applied directly on the plaster, essentially consisting of lime with calcite fragments (Fig. 11). EDS spectra reveal that the first pictorial layer was obtained mixing lime with an Arsenic rich component. Due to chemical weathering this layer is currently black and does not reveal its original color. The second pictorial layer is blue and has a complex chemical composition, revealing the usage of multiple components. In addition to lime, small vegetal fragments are pervasive, indicating that the pigment is a blue indigo. Very finegrained gypsum crystals are observed and their fibrous appearance indicate that they were formed at secondary phases due to environmental contamination. The occurrence of small apatite and silicate crystals could explain the observed phosphorus, silicon and aluminum peaks in the EDS spectra. Arsenic peaks could be due to contamination from the underlying black layer during the application of the blue layer. The uppermost pictorial layer consists of a weathered

5 11 pigment and lime. Fibrous secondary gypsum crystal are prevalent in this layer due to weathering and environmental contamination. Sample A41 This sample consists of a partially weathered blue pictorial layer that was applied on a lime plaster (Fig. 12). The pictorial layer is made of lime mixed with vegetal fibers, which bear testimony to the usage of blue indigo as a pigment. BSE images show that the uppermost part of this fragment consists of a μm thick black crust containing small gypsum and silicates (possibly soil-derived dust) crystals Sample of multi-layered mural painting. Optical microscope and BSE images show that the three different pictorial layers were applied directly on a lime plaster. EDS spectra help us identify the composition of the three layers. that are ascribable to environmental weathering. Technical note: the instrumentation for in situ and laboratory measurements Non-destructive EDXRF analyses have been performed on untreated whole ceramics with a portable spectrometer (Assing Lithos 3000) equipped with a low power X-ray tube with Mo anode and a Peltier cooled Si-PIN detector. A Zr transmission filter between the X-ray tube and the sample guarantees monochromatic radiation at Mo Ka energy (E =17.4 kev), with a 4mm diameter collimator. The area irradiated on the sample is about 25 mm2. The distance between the sample and the X-ray tube is 1.4 cm; the same applies for the distance between the sample and the detector. The working conditions are 25 kv and 0.3mA with an about 60 s. acquisition time. FORS measurements in the visible and near infrared range were carried out using an Ocean Optics HR4000 spectrophotometer with linear CCD-array ( nm, 2.5 nm resolution) in reflection mode; spot on samples is about 5 mm2. The system is equipped with a plastic holding probe able to apply a 45 x:45 detection geometry. Micro-textural investigation and mineral analyses were carried out on samples mounted in a epoxy resin by scanning electron microscopy with a Tescan Serie Mira XMU scanning electron microscope (SEM) equipped with an energy dispersive X-Ray spectrometer (EDS). Accelerating voltage was 20 kv, the beam current 20 µa, and the spot diameter about 5 micron Sample of a partially weathered blue pictorial layer on a lime plaster. BSE images show the presence of a black crust on the uppermost portion of the blue layer. Up to 0.2 mm vegetal fragments (up left) were utilized as a pigment. EDS spectra indicate the composition of the components of the three layers. Frescoes in Kobayr church