Tin Deterioration on Polychrome Stone Sculptures of the San Jerónimo Church (Granada, Spain) / CAROLINA CARDELL (1*), ISABEL GUERRA (2), AFRICA YEBRA-RODRÍGUEZ (3)
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macla nº 16. junio ‘12 54 revista de la sociedad española de mineralogía Tin Deterioration on Polychrome Stone Sculptures of the San Jerónimo Church (Granada, Spain) / CAROLINA CARDELL (1*), ISABEL GUERRA (2), AFRICA YEBRA-RODRÍGUEZ (3) (1) Department of Mineralogy and Petrology, Faculty of Science, University of Granada, Campus Fuentenueva s/n, 18071 Granada, Spain (2) Scientific Instrumentation Centre, Campus Fuentenueva, University of Granada, 18071 Granada, Spain (3) Department of Geology-Associated Unit IACT (CSIC-UGR), Faculty of Experimental Science, University of Jaén, Campus Las Lagunillas s/n, 23071 Jaén,Spain INTRODUCTION products, and ii) the so-called tin pest (Bruker D8 Powder Advance which is the allotropic transformation of diffractometer). Chip samples were The church of San Jerónimo monastery the tin crystal structure from white analyzed after placing them onto a zero- is one of the most important examples metallic % form (tetragonal) into the grey background silicon plate with no of Spanish Renaissance thanks to the powdery non-metallic # form (cubic) that preparation. Diffraction data were architect and sculptor Diego de Siloé, takes place ca. 13.2 0C (MacLeod, recorded using Cu K# radiation, who worked there from 1528 to 1543. 2005). The allotropic transformation is operated at 40 kV, 40 mA, 0.02 deg In its interior Siloé executed the crossing influenced by the metal microstructure step size and 20 s integration time. and the architectonical stone sculptural (increasing with decreasing grain size) Diffraction patterns were scanned over a group placed in the upper apse. It seems and presence of impurities that either 20° < 2" < 90° range. Minerals were that these sculptures were polychromed delay/hinder the transformation (e.g. Sb, identified using the XPowder software in the 18th century using the temple Bi, Pb, Ag, Cd) or accelerate it (e.g.. Al, and the PDF2 database (Martín-Ramos, technique (Cardell-Fernández & Zn, Ge, Cu). On the other hand, tin 2004). Samples were also prepared as Rodríguez-Gordillo, 2003). The substrate oxidation increases with addition of polished thin sections and studied with of the polychromes is a biocalcarenite elements such as Fe, Pb, Mn, Sb and Bi, optical polarizing microscopy (OM) using (limestone) made of sand-size grains of and decreases with Zn or P (Chiavari et an Olympus BX60 and with a scanning fossils (Cardell 1998). This stone was al., 2006). electron microscope coupled with EDX covered with a gypsum-based microanalysis (Inca 350 version 17 preparation followed by one or two clay- Oxford Instrument, VP-SEM Leo rich primer layers (also containing 1430VP). SEM-EDX working conditions calcite, quartz and white lead –mainly were 500 pA filament current, 20 keV hydrocerussite), overlaid with diverse beam energy, and 10 eV/ch resolution painting color layers (lead white, raw for pinpoint analyses, and 1 nA filament and burnt Sienna pigments, red ochre, current and 20 eV/ch resolution for map yellow ochre and smalt) including gold acquisition. High-resolution maps (1024 and tin sheets. x 768 pixels) were obtained with 500 frames and 10 ms dwell time. Mineral The weathering of the paintings is fig 1. Paint stratigraphy: from inner to outer: gypsum maps were compiled from elemental alarming, mainly due to the presence of base, clay-rich prime, gypsum layer with dissolution distribution maps by applying the gypsum and clay minerals which are pores and re-crystallized gypsum (arrow-like Phasemap tool. This technique crystals), clay-rich layer and tin coat exhibiting bowl- very absorbent of humidity, leading to like shapes and detachments. Transmitted light and highlights the location, morphology and the formation of lead chlorides and crossed nicols. Bar = 0.5mm. composition of mineral phases in paint Tutton salts (double salts with formula stratigraphy. M2M'(SO4)2(H2O)6, e.g. (NH4)2Fe(SO4)2·6 In this work we present preliminary H2O) due to reactions between pigments results of the composition and Also tin samples were studied at and infiltration water (Cardell-Fernández microstructure of tin layers used as nanoscale by transmission electron & Rodríguez-Gordillo, 2003). As a decoration in polychromed stone- microscopy (TEM) using a Philips CM20 consequence, painting detachments and sculptures, and the corrosion products, equipped with EDAX solid-state ultrathin- micro-fissures are very abundant, as are to improve understanding of the decay window energy dispersive X-ray (EDX) the formation of bowl like shapes in processes affecting tin-rich paintings. detector. The acceleration voltage was those parts of the polychrome 200 kV, and the lens aperture 40 $m. containing tin layers (Fig. 1). EXPERIMENTAL Identification of mineral phases was done collecting selected area electron The literature reports two types of decay Fresh and decayed tin samples were diffraction (SAED) patterns. Prior to TEM affecting tin-base objects: i) atmospheric characterized at nano- and microscale analysis, particles were dispersed in corrosion in the form of oxidation of tin using a combination of complementary ethanol, sonic-vibrated for 30 s, and with SnO (romarchite) and SnO2 analytical techniques. Crystalline phases placed on Formvar© and C-coated Cu (cassiterite) as the main corrosion were identified with X-ray diffraction grids. palabras clave: Corrosión estaño, Policromías, SEM-EDX, XRD. key words: Tin corrosion, Polychrome, SEM-EDX, XRD. resumen SEM/SEA 2012 * corresponding author:[email protected] macla nº 16. junio ‘12 revista de la sociedad española de mineralogía 55 RESULTS AND DISCUSSION corroded parts the XRD study identified products. Identifying the nature of tin romanchite and ZnSnAs2. weathering is of key relevance for Decayed tin layers show tarnishing at restoration and conservation of these the surface at macroscopic scale and In the heavily altered tin layers the polychromes since while tin oxidized the formation of dark grey pustules, weathering process is more intense, as areas can be treated, those objects pinholes, cracks and bowl-like can be seen in Fig. 4 which is a false suffering from tin pest cannot be shapes/detachments. OM and SEM color map obtained with the INCA reconstituted. However, the tin pest investigation show that several tin layers Phasemaptool of the VP-SEM based on process was not possible to be were applied in the polychrome on top SEM–EDX elemental mappings: the established with these analytical of a layer rich in gypsum and calcite and green color corresponds to unaltered Sn, techniques. Therefore other more a clay-based layer (Fig. 1). More in detail, the blue color to tin oxide areas and the sophisticated techniques will be applied, the SEM study reveals that these tin red color marks zones rich in tin and such as the structural and chemical layers display different degrees of sulphur. In this layer tin-rich chlorides analyzer SCA, which is a tandem Raman weathering. Thus, there is a less-altered also were found with SEM-EDX and spectroscopy and VPSEM-EDX. tin layer showing a more porous identified as abhurite, Sn3O(OH)2Cl2. compact structure with polygonal fissures and scarce dark grey rounded stains, and a heavily altered dark grey tin layer, as shown in Fig. 2. fig.6. View of fresh tin % with TEM. fig.4. False color map of the most altered tin layer. ACKNOWLEDGEMENTS A false color map of a paint stratigraphy is shown in Fig. 5. Here it can be seen Financial support was provided by fig. 2. Contact between the light grey less corroded Andalusian Research Group RNM-179 and the intensely altered dark grey tin layers. that tin layers at the surface are made of Sn (green color) while the inner tin and MEC Project MAT2009-11332. We In the brightest areas (fresher areas) of layer (blue) is composed of tin oxide. The thank the Centro de Instrumentación the light grey layer only Sn was found red spots are made of tin-rich chlorides, Científica of the University of Granada with SEM-EDX and XRD. Also, SEM while the pistachio color corresponds to for the SEM-EDX, TEM and XRD analysis shows that solely in these fresh the calcite/gypsum preparation layer analyses. areas tin whiskers formation take place and the violet color to quartz grains. (Fig. 3). Tin whiskers are needle-like REFERENCES form conductive crystals that grow Cardell, C. (1998): Cristalización de sales en naturally from pure tin or high tin calcarenitas: aplicación al monasterio de finished surfaces and are related field San Jerónimo, Granada. Tesis Doctoral, failures (Fang et al., 2006). Universidad de Granada. 219p. Cardell-Fernández, C. & Rodríguez-Gordillo, J. (2003): Policromías de la Iglesia del Monasterio de San Jerónimo de Granada: composición, alteración y técnicas de ejecución. Bol. Soc. Esp. 26, 113–121. MacLeod, I. (2005): The decay and conservation of museum objects of tin. fig. 5. False color map of a paint stratigraphy Studies in Conservation, 50, 151-152. showing the fresh (light grey/green) and oxidized Chiavari, C. Martini, C. Poli, G. & (dark grey/blue) tin layers. Prandstraller, D. (2006): Deterioration of tin-rich organ pipes. J. Mater. Sci. 41, TEM analyses corroborated the presence 1819-1826. of tin % (Fig. 6) and tin oxide in the fresh Martín-Ramos, J.D. (2004): Using XPowder: A and oxidized areas respectively. software package for Powder X-ray fig.3. Tin whiskers extruding from polygonal cracks. diffraction analysis. www.xpowder.com, (GR 1001/04. ISBN 84-609- 1497-6). In summary, the combined analytical According to XRD, grey areas are made Fang, T. Osterman, M. Mathew, S. & Pecht, M. techniques used in this work (OM, SEM- of SnO (romanchite) (see small spots in (2006): Tin whisker risk assessment, EDX, TEM) have allowed a good Fig. 3), while in the darker grey rounded Circuit World 32, 25-29. characterization of the tin layers used in stains (Fig. 3), SEM-EDX identified the polychromes of San Jerónimo church diverse elements other than Sn, e.g.