T
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, lgnazio Fragala, Giuseppe Spoto Universita di Catania
Carmelo Di Stefan0 Soprintendenza ai Beni Culturali ed Ambientali di Catania
Geoffrey C. Allen University of Bristol 0003-2700/95/0367-249A/$09.00/0 0 1995 American Chemical Society
areas and crumbling of the stonework. ~ i G ~ ~ a n ~ ~ The y original t i ~coloration ~ 1is, however, present in the lunette, although a 500techniques yield lOO@pm-thickyellow-brownpatina covers all of the brightly painted areas. ~ ~ l ~i n~f ob~ m l ~e t i oA~massive restoration of the portal is in progress, and several investigations are about the ~ ~ t e ~ ibeing ~ carried 1 sout to determine the origin and nature of the superficial coating and and techniques used the chemical composition of the materials used in the coloration process. The fid ~ c ~~ ~ ~~ of t ~rnal goal ~ gis to~restoret theiremains ~ as~closely as possible to their original condition a Reflak"? 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 microscopy (SEM) , energy-dispersive X-ray microanalysis (EDX), secondary ion MS (SIMS) , electron spectroscopyfor chemical analysis (ESCA) ,and X-ray diffraction 0) techniques to study the most significant elements of the portal.
Analytical Chemistry, Vol. 67, No. 7, April 1, 1995 249 A
,Rna/yr
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-
1
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 removed from the wing region of one of the angels while samples of coloring material were taken from other areas of interest. Azure pigment was removed from the angel’s wing, blue pigment from the angel’s clothing, and red pigment from the lunette background and the internal side of the carving of the Viigin’s mantle
(Degrees
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 = PbCO,; a = azurite; Hg = HgS; Et = q-Fe,O,
250 A Analytical Chemistry, Vol. 67, No. 7, April I, 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 l a clearly reveals a pattern characteristic of calcite, with very weak peaks from gypsum and weddellite, the presence of which is indicative of a degradation process caused by weathering or biological action (1). Similar conclusions result from X-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 1s 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 500-1OOOym-thick yellowish patina, probably spread during the recent repairs. The scanning electron micrographs of patina samples (Figures 2a and b) show the dif€erenttextures 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&, calcium (&,J,copper &,PI, lead (La$), and chlorine (&,p). An identical measurement recorded from the outer region (Figure 3b) is completely different, showing only low-intensity X-ray peaks from sulfur and titanium and intense peaks from aluminum, silica, potassium, calcium, and iron, probably derived from airborne particulate contamination. Finally, the presence of copper as a trace element is almost certainly associated with the mobility of Cu(I1) ions
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 studied to identify the nature of the paint mixtures (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 areas 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
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 (9) Armenian bole.
sulfur levels were present (Figure 3c). The presence of calcium and sulfur is almost certainly a consequence of the gyp sum 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 (F@re IC), in which the pattern of q-Fe,O, can be identified.
Analytical Chemistry, Vol. 67, No. 7, April 1, 1995 251 A
The second redcolored region of the lunette, strictly localized on the internal side of the sleeve of the Virgin’s mantle, shows a different elementalcomposition. Here mercury and sulfur are detected in the EDX spectrum Figure 3d), and the XRD spectrum (Figure Id) indicates a pigment based on the mineral cinnabar (which occurs naturally in the hills of Tup cany and in Spain). EDX of samples taken from the azure areas shows peaks from copper (Ku,p)and lead (La,&(Figure 3e). The corresponding XRD pattern (Figure le) indicates azurite [CuCO,Cu(OH),], mixed with white lead [PbCO,Pb(OH),] to fade the color. Despite the presence of lead, evidence of surface darkening was not d e 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 pm) glassy particles (Figure 2c); the EDX spectrum (Figure 3f) of the particles includes lines from cobalt, silicon, arsenic, 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 m u ) and COO- (76 amu). These findings clearly indicate the use of cobalt arsenide as the starting mineral to prepare the enamel. Especially noteworthy 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 K,O’ 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 microscopy showed a red substrate supporting two layers of golden film, placed one over the other and separated by two intermediate translucent layers (Figure 4). The EDX spectrum (Figure 3g) of the red substrate shows that it consists of “Armenian bole,” aluminum silicate-contain-
alate may also have been produced from a complex mixture containing calcium sulfate, silicates, and copper salts. This observation 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 oxalate crusts also have been found on quartzites and sandstones in the Australian Northern Territories, where a complex interaction among rainwater acids, particulate matter, and microorganism activity has been proposed to explain their formation (3). Our analytical data also provide an insight into the techniques used to prepare the pigments. Safflorite or skutteridite were almost certainly used as starting minerals for smalt preparationbecause 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 measurements are indicative of such minering gypsum and iron oxides used as a sub- als (4,5). Early textbooks on chemical technollayer background on which the golden ogy (6, 7) record that the process of prelamina was laid. The translucent layer beparing smalt involved two main steps. tween the two golden laminae consists Fiit, 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 O C in a composed of litharge. Each of the two laminae consists of a thin (2-4 pm) 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 inthese separate gold layers and their substrates suggests that this particular area volved the preparation of the glass by adding 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 The smalt remains identified in this hence older, region of gold lamina show study reveal the presence of arsenic in the that it has longitudinal markings. This could indicate the use of an old rolling pro- glassy particles and indicate that either a low-temperature process or a reduced aircess to obtain thin (2-4 pm) gold leaves stream was used to treat the raw, and or a polishing process that involvkd rubprobably badly milled, mineral. The furbing the gold with dog or wolf teeth, ther high-temperaturemelting of the known to be used by ancient gilders. glassy paste to prepare the enamel does not appear to have resulted in a successful What have we learned? This investigation of the Renaissance por- formation of volatile compounds because the added potash encouraged the productal 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 enhancing the opacity and covering power of gions and masked the original beauty of the artwork, is composed primarily of gyp the resultant glassy paste. The technique used to roll the golden sum and may have been a consequence leaves was amazingly effective. The final of recent restoration work. The presence of weddellite in the matrix of the mainly thickness and homogeneity of gold laminae suggest that great care and a very effigypsum patina indicates that calcium ox-
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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 reddish 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 enrich the quality and appearanceof the finished product. The application of modern chemical microanalyticaltechniques does not merely offer a route to improved methods for the conservation of important buildings, monuments, and valuable artifacts. Such approachesalso provide special information about the materials and techniques used by the craftsmen responsible for their construction. Additionally these microanalyticaltechniques permit us to discover the chronological evolution of their technological development.
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References (1) Fassina, V. In Proceedings oflntemational Symposium on the Oxalate Films: Origin and Significance in the Conservation of Works of Art; Ed. Centro “CNR’ Gin0 Bozza: Milan, 1989; pp. 5-22. (2) Mazzeo, R; Chiavari, G.; Morigi, G. In Proceedings of International Symposium on the Oxalate Films: Origin and Significance in the Conservation of Works of Art; Ed. Centro “CNR” Gin0 Bozza: Milan, 1989;pp. 271-79. (3) Watchman, A. L. Studies in Conservation 1991,36,24-32. (4) Roseboom, E. H. Am. Mineral. 1962,47, 31&27. (5) Redcliffe, D.; Berry, L. G. Am. Mineral. 1968,53,1857-81. (6) Wurtz, A. Dictionnaire de Chimie Pure et Appliqude,Tome Second; Ed. Librairie Hachette: Paris, 1876; pp. 1506-7. (7) Selmi, F. Enciclopedia di Chimica Scientifica e Industriale; Ed. L‘Unione TipograticeEdritice Torinese: Torino, 1877; Vol. 11, pp. 693-96.
Enrico Ciliberto is an associate professor of chemistry, Ignazio Fragala is a professor ofgeneral chemistry, and Giuseppe Spot0 is a graduate student at the Universita de Catania (Italy). Geofiey C. Allen is professor of materials science and deputy director of the Inteflace Analysis Center at the University of Bristol (U.K.). Carmelo Di Stefan0 is director of the Beni Architettonici section at the Soprintendenze Beni Culturali of Enna (Sicily). Address correspondence about this article to Ciliberto at Dipartmento di Scienze Chimiche, dell’ Universita d i Catania, viale A . Doria 6, 95125 Catania, Italy.
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