The Seville Cathedral altarpiece: a microbiological and chemical survey of the dust

The altarpiece of the Seville Cathedral was built between 1481 and 1565 and has a total perimeter of 20.10 m and 23.41 m height, which makes this altarpiece the largest in the world. The last restoration was performed in 1977, centered on ensuring its structural stability and consolidation of wood structures and polychromy. A new intervention to appraise the state of conservation is currently in progress. Our study is centered on the assessment of the dust accumulated on the surface including a survey of significant alterations from biological origin. The approach carried out combined molecular biology methods and analytical procedures. The composition of the dust samples is very heterogeneous, comprising mineral particles resulting from the deterioration of the building materials, fragments of decorative elements particularly gold leaf from the gilded wood, products from air pollution (soot), as well as biological fragments of insects, arachnids and bird droppings, in addition to bacteria and fungal spores. 2 MATERIAL AND METHODS Seven dust samples were collected in March 2012 from the surface of the altarpiece (Fig. 1). The scientific approach carried out combined microbiological analyses: isolation of bacteria and fungi on culture media and molecular identification by sequencing of rDNA, and physicochemical analyses: observations by stereomicroscopy and field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), X-ray fluorescence (XRF) and Fourier-transform infrared spectroscopy (FTIR). Before analysis, the samples were sieved and divided into five fractions depending on their particle size. Sieves were used with the following pore diameters: 475, 250, 150 and 75 μm. Stereomicroscopy observations were performed to characterize the physical and morphological properties of the different particles found within the samples. The finest fraction (<75μm) was then analyzed by XRD and XRF. Additionally, other fractions were analyzed by XRD and FTIR. Aerobiology methodology for identification of airborne bacteria and fungi was described elsewhere (Fernandez-Cortes et al. 2011). Figure 1. a. The altarpiece of the Seville Cathedral showing the areas where the dust samples (1-7) were collected. b. Cristo del Millón. c. Detail of the María Magdalena unge los pies de Jesucristo panel. d. Detail of the Resurrección de Jesucristo panel. e. Pecado original y expulsión del Paraiso. 3 RESULTS AND DISCUSION 3.1 Microbiological characterization All analyzed dust samples contain high amount of microorganisms. High concentrations of bacteria and fungi, between 10 and 10 colony forming units per gram (cfu/g), were detected (Fig. 2, Table 1). According to the molecular identification, the bacterial were dominated by species of Bacillus, followed by species of Massilia, and a small portion of Staphylococcus. The most abundant fungal species belonged to the genera Cladosporium, Penicillium, Chaetomium, Aspergillus and Trichoderma. These microorganisms have high sporulation rates which facilitate their dispersion. All detected microorganisms have a cosmopolitan distribution and are frequently isolated from any environmental sample. Table 1. Counting of microbial colonies grown on culture media as colony forming units per gram Dust sample Bacteria (cfu/g)* Fungi (cfu/g)* 1 1.98 x 10 9.28 x 10 2 2.78 x 10 5.04 x 10 5 1.25 x 10 1.21 x 10 Figure 2. Isolations on culture media. a. Bacterial colonies on Trypticase Soja Agar + Cycloheximide. b. Fungal colonies on Dichloran Rose Bengal Chlortetracycline Agar. 3.2 Physicochemical characterization Stereomicroscopy observations revealed that the composition of the dust samples was very heterogeneous, comprising different components of the building materials (Fig. 3a), fragments of decorative elements particularly gold leafs from the gilded wood and pigments from the polychromy (Fig. 3b-d). Also, abundant fragments of insects, arachnids (Fig. 3e) and bird droppings enriched in uricite were detected (Fig.3f). XRD analysis revealed the presence of calcite, quartz, dolomite, gypsum, halite, illite and anhidrite, which are mostly the mineral constituents of the building materials. A lead chromate, likely PbCrO4, was detected by XRD, suggesting that a yellow pigment was used in the form of chrome yellow on the sculptures of the altarpiece. It must be mentioned that the chrome pigments were in use by 1816 (Douma 2008), which indicates that this pigment was used in conservation intervention performed in 1977 or before, but not at the time of the altarpiece construction. Barium sulfate, BaSO4, was also identified by XRD, which sometimes is used in restoration works as a partial replacement for titanium dioxide in paintings, reducing pigment costs. Moreover, copper resinate was identified by FTIR. In the 15th and 17th centuries artists used copper resinate to add glaze on paintings laying a layer of copper resinate over verdigris to form a deep saturation of green color (Douma 2008). Figure 3. Stereomicroscopy images from different fractions of the dust samples. a. Grains of building materials. b. Fibers and gold leafs. c. Pigments. d. Fragments of gilded wood. e. Fragments of insects and arachnids. f. Bird droppings. According to FESEM analysis, abundant pollen grains (Fig. 4a) with different morphologies were observed within the dust samples, together with soot particles probably resulting from air pollution and frequent liturgical activities inside the cathedral. Note that the cathedral is in the centre of the city, an area that until 2006 suffered from intense automotive traffic, which produced an atmosphere of carbon monoxide, sulfur dioxide, soot, hydrocarbons and other substances from vehicle exhausts (Saiz-Jimenez et al. 2004, Reyes et al. 2006). In addition, some diatoms were observed (Fig. 4c) probably derived from the building stones and/or water infiltrations in the roof of the cathedral. Figure 4. FESEM images of the dust samples depicting: a. Pollen grain, b. Soot particle, and c. Diatom.