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ANALYZING A SICILIAN RENAISSANCE PORTAL
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he church of Santa Maria la Vetere in the historic village of Militello Val di Catania (Sicily) was heavily damaged during the earthquake of 1693, and only the right aisle and a part of the front remain today. The portal, known to have been built in 1506, has a series of Renaissance low reliefs representing the busts of prophets and kings and, in the center, the Virgin Mary and the infant Jesus adored by two angels. All of the reliefs were enriched by bright
Enrico Ciliberto, Ignazio Fragalá, Giuseppe Spoto Universitá di Catania Carmelo Di Stefano Soprintendenza ai Beni Culturali ed Ambientali di Catania Geoffrey C. Allen University of Bristol 0003-2700/95/0367-249A/$09.00/0 © 1995 American Chemical Society
areas and crumbling of the stonework. Microanalytical The original coloration is, however, present in the lunette, although a 500techniques yield ΙΟΟΟ-μπι-thick yellow-brown patina covers all of the brightly painted areas. valuable information A massive restoration of the portal is in and several investigations are about the materials progress, being carried out to determine the origin and nature of the superficial coating and and techniques used the chemical composition of the materi als used in the coloration process. The fi during construction of nal goal is to restore the remains as closely as possible to their original condition a Renaissance church while controlling the degradation process. colorations but, unfortunately, only a few such regions are still visible. The portal has deteriorated badly, showing gaps in the stonework, deep erosion of surfaces, and the formation of cavities, leading to an almost complete loss of the colored
We have used scanning electron micros copy (SEM), energy-dispersive X-ray mi croanalysis (EDX), secondary ion MS (SIMS), electron spectroscopy for chem ical analysis (ESCA), and X-ray diffraction (XRD) techniques to study the most sig nificant elements of the portal.
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These microsampling techniques allow us to carry out detailed analyses on exceedingly small samples, thereby preventing further damage to the building. Tiny polycrystalline phases can be easily studied by SEM, identified by XRD, and analyzed by EDX; surface phases can be studied by ESCA and SIMS to provide information on the nature of the materials used. Sampling and analysis
Stone characterization. The surfaces of limestone material, patina, and col-
ored areas were scraped under a stereomicroscopic light using a microlancet. Samples of stone surfaces were removed from the bottom of the portal, where no traces of colored material could be identified, and patina fragments were removedfromthe wing region of one of the angels while samples of coloring material were takenfromother areas of interest. Azure pigment was removedfromthe angel's wing, blue pigmentfromthe angel's clothing, and red pigmentfromthe lunette background and the internal side of the carving of the Virgin's mantle
Figure 1 . XRD traces. (a) Portal stone, (b) patina, (c) red ocher pigment, (d) cinnabar red pigment, and (e) azure pigment. Key: C = calcite; g = gypsum; w = weddellite; Q = quartz sand; Pb = white lead; Ce = PbC03 a = azurite; Hg - HgS; Et = Ti-Fe203 250 A
Analytical Chemistry, Vol. 67, No. 7, April 1, 1995
sleeve. Samples were also taken with great care from the golden coloration on the angel's hair. The portal stone is characterized as calcilutite with inclusions of foraminifera, probably mined in the Iblei mountains near Syracuse. The powder XRD spectrum shown in Figure la clearly reveals a pattern characteristic of calcite, with very weak peaksfromgypsum and weddellite, the presence of which is indicative of a degradation process caused by weathering or biological action (1). Similar conclusions resultfromX-ray photoemission measurements, which show an intense band centered at a binding energy of 169 eV attributable to the 2p core-level ionization of sulfur atoms in oxidation state (VI), indicating the presence of a sulfate coating on the limestone surfaces. The region between 280 and 300 eV shows two peaks that can be readily assigned to ionization of the carbon Is level; the band at 289.7 eV represents carbonate ions, whereas the feature at 284.3 eV is associated with organic and/or graphitic carbon. Finally, the weak band centered at 390 eV, probably attributable to ionization from the nitrogen Is level, would appear to confirm the presence of microorganisms at the stone surface. Patina. The portal lunette was entirely covered with a 500-1000^m-thick yellowish patina, probably spread during the recent repairs. The scanning electron micrographs of patina samples (Figures 2a and b) show the different textures of the inner and outer areas; the X-ray powder diffraction pattern (Figure lb) reveals several crystalline phases, among which the most dominant is gypsum. Particularly noteworthy is the presence of weddellite, probably as an organic metabolite. The EDX spectrum of the inner side of the patina (Figure 3a) shows X-ray lines from sulfur (K„p), calcium (K^p), copper (K^p), lead (L ap ), and chlorine (K^p). An identical measurement recorded from the outer region (Figure 3b) is completely different, showing only low-intensity X-ray peaksfromsulfur and titanium and intense peaks from aluminum, silica, potassium, calcium, and iron, probably derivedfromairborne particulate contamination. Finally, the presence of copper as a trace element is almost certainly associated with the mobility of Cu(II) ions
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Figure 2. Secondary electron micrographs. (a) Patina inner side, (b) patina outer side, (c) glassy particles in the blue pigment, and (d) inner gold leaf from the angel hair region.
from the azurite pigment identified at the surface of the angel's wings. Lunette. The portal colors were stud ied to identify the nature of the paint mix tures (red, azure, and blue) as well as the golden layer at the surface of the sculp tured model of the angel's hair. EDX spectra of samples taken from the red ar eas indicate that two different chromatic mixtures may have been used. In the case of the red coloration of the lunette background, greater iron, calcium, and
0
2 4
6 8 10 12 14 16 1E Energy (keV)
Figure 3. EDX spectra. (a) Patina inner side, (b) patina outer side, (c) red ocher pigment, (d) cinnabar red pigment, (e) azure pigment, (f) glassy particles in the blue pigment, and (g) Armenian bole.
sulfur levels were present (Figure 3c). The presence of calcium and sulfur is almost certainly a consequence of the gypsum phase identified in the XRD measurements of the patina (Figure lb). Evi-
dence for the presence of an iron-based pigment, probably based on a red ocher, is found in the XRD spectrum (Figure lc), in which the pattern of T|-Fe203 can be identified.
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The second red-colored region of the lunette, strictly localized on the internal side of the sleeve of the Virgin's mantle, shows a different elemental composition. Here mercury and sulfur are detected in the EDX spectrum (Figure 3d), and the XRD spectrum (Figure Id) indicates a pig ment based on the mineral cinnabar (which occurs naturally in the hills of Tus cany and in Spain). EDX of samples taken from the azure areas shows peaks from copper (Κ„ ρ) and lead (La p) (Figure 3e). The correspond ing XRD pattern (Figure le) indicates azurite [CuC03Cu(OH)2], mixed with white lead [PbC03Pb(OH)2] to fade the color. Despite the presence of lead, evi dence of surface darkening was not de tected. The use of oil in the preparation of the pigment, and hence the presence of a thin film of surface oil, might explain this observation. Electron micrographs of the blue areas of the sculpted clothing of an angel show a composite structure consisting of small (10-100 μηι) glassy particles (Figure 2c); the EDX spectrum (Figure 3f) of the parti cles includes lines from cobalt, silicon, ar senic, and bismuth. The pigment recipe, therefore, probably involved the milling of arsenic and cobalt-based glass or smalt. SIMS data obtained from a 100-pmwide glass particle show that the positive ion peaks from Co+ (59 amu), CoO+ and As+ (75 amu), and AsO+ (91 amu) have counterparts in the negative ion signals from As" (75 amu) and CoO~ (76 amu). These findings clearly indicate the use of cobalt arsenide as the starting mineral to prepare the enamel. Especially notewor thy are the SIMS triplet structures at 28, 29, and 30 amu and 44, 45, and 46 amu from the isotopic patterns of Si+ and SiO+, which indicate the presence of silica sand added to the glass paste. Finally, the detection of K\ NaOK+, SiOK+, and K20+ appears to be related to the use of potash as a flux. Analysis of the remains of a golden film found in the angel hair by use of optical mi croscopy showed a red substrate support ing two layers of goldenfilm,placed one over the other and separated by two inter mediate translucent layers (Figure 4). The EDX spectrum (Figure 3g) of the red substrate shows that it consists of "Ar menian bole," aluminum silicate-contain252 A
alate may also have been produced from a complex mixture containing calcium sul fate, silicates, and copper salts. This obser vation supports other measurements Golden lamina from the bronze portal of the Duomo di Loreto (2), which showed the presence of Litharge gypsum, copper, and calcium oxalates in the surface corrosion products. Thick oxa late crusts also have been found on quartzGypsum ites and sandstones in the Australian Northern Territories, where a complex in Golden lamina teraction among rainwater acids, particu late matter, and microorganism activity Armenian bole has been proposed to explain their for mation (3). Our analytical data also provide an in sight into the techniques used to prepare Stone the pigments. Safflorite or skutteridite were almost certainly used as starting minerals for smalt preparation because the Figure 4. Stratification scheme of cobalt, iron, nickel, arsenic, and bismuth gold leaf in the angel hair region. identified in both the EDX and SIMS mea surements are indicative of such miner ing gypsum and iron oxides used as a sub als (4, 5). Early textbooks on chemical technol layer background on which the golden lamina was laid. The translucent layer be ogy (6, 7) record that the process of pre paring smalt involved two main steps. tween the two golden laminae consists First, a finely divided, milled mixture of of a white layer, which was identified as cobalt minerals, generally cobaltite or gypsum by EDX, and a yellowish layer smaltite, was calcined at 500-600 °C in a composed of litharge. Each of the two laminae consists of a thin (2-4 μηι) layer of stream of air to produce cobalt oxide (zaffera) and to eliminate arsenic sesquioxgold (Figure 2d). The coexistence of ide. The second step in the process in these separate gold layers and their sub strates suggests that this particular area volved the preparation of the glass by add ing potash and quartz sand to the zaffera had been restored. Interestingly, the scanning electron mi and melting the resultant mixture at high temperature. crographs recorded from the inner, and hence older, region of gold lamina show The smalt remains identified in this that it has longitudinal markings. This study reveal the presence of arsenic in the could indicate the use of an old rolling pro glassy particles and indicate that either a cess to obtain thin (2-4 μιη) gold leaves low-temperature process or a reduced airor a polishing process that involved rub stream was used to treat the raw, and bing the gold with dog or wolf teeth, probably badly milled, mineral. The fur known to be used by ancient gilders. ther high-temperature melting of the glassy paste to prepare the enamel does not appear to have resulted in a successful What have w e learned? This investigation of the Renaissance por formation of volatile compounds because the added potash encouraged the produc tal has revealed the use of a variety of pigments painted "a secco" on the surface tion of anionic species. Nevertheless, it is interesting that the presence of arsenic in of the stone forming the low reliefs. The patina, which covered all of the colored re the glass enamel had the advantage of en hancing the opacity and covering power of gions and masked the original beauty of the artwork, is composed primarily of gyp the resultant glassy paste. sum and may have been a consequence The technique used to roll the golden of recent restoration work. The presence leaves was amazingly effective. The final of weddellite in the matrix of the mainly thickness and homogeneity of gold lami gypsum patina indicates that calcium ox nae suggest that great care and a very effi-
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Patina
cient mechanical device were used by the artisan to reduce the total quantity of gold used. The Armenian bole, the red dish complex mixture used as the smooth substrate to support the gold leaves, was chosen because of its color to reduce the transparency of the metal film and to en rich the quality and appearance of the fin ished product. The application of modern chemical microanalytical techniques does not merely offer a route to improved methods for the conservation of important buildings, monuments, and valuable artifacts. Such approaches also provide special informa tion about the materials and techniques used by the craftsmen responsible for their construction. Additionally these microanalytical techniques permit us to dis cover the chronological evolution of their technological development. References (1) Fassina, V. In Proceedings of International Symposium on the Oxalate Films: Origin and Significance in the Conservation of Works of Art; Ed. Centro "CNR" Gino Bozza: Milan, 1989; pp. 5-22. (2) Mazzeo, R; Chiavari, G.; Morigi, G. In Pro ceedings ofInternational Symposium on the Oxalate Films: Origin and Significance in the Conservation of Works of Art; Ed. Cen tro "CNR" Gino Bozza: Milan, 1989; pp. 271-79. (3) Watchman, A. L. Studies in Conservation 1991,36,24-32. (4) Roseboom, Ε. Η. Am. Mineral. 1962, 47, 310-27. (5) Redcliffe, D.; Berry, L. G. Am. Mineral. 1968,53,1857-81. (6) Wurtz, A. Dictionnaire de Chimie Pure et Appliquée, Tome Second; Ed. Librairie Hachette: Paris, 1876; pp. 1506-7. (7) Selmi, F. Enciclopedia di Chimica Scientifica e Industrials, Ed. L'Unione Tipogratico-Edritice Torinese: Torino, 1877; Vol. II, pp. 693-96. Enrico Ciliberto is an associate professor of chemistry, Ignazio Fragala is a professor ofgeneral chemistry, and Giuseppe Spoto is a graduate student at the Universita de Catania (Italy). Geoffrey C. Allen is professor of materials science and deputy director of the Interface Analysis Center at the University of Bristol (U.K.). Carmelo Di Stefano is director of the Beni Architettonici section at the Soprintendenze Beni Culturali ofEnna (Sicily). Address correspondence about this article to Ciliberto at Dipartmento di Scienze Chimiche, dell'Universita di Catania, viale A. Doria 6, 95125 Catania, Italy.
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' Please send me a free brochure describing the ACS Short Course, Supercritical Fluid Fractionation/Extraction/Chromatography, to be held May 14-18, 1995 at Virginia Tech in Blacksburg, VA. ι Name [Title Organization Address
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