CONDITION ASSESSMENT OF MARBLE SCULPTURES ON POLYCHROMED AND GILDED DECORATIONS OF THE ARCA SCALIGERA OF CANSIGNORIO DELLA SCALA IN VERONA

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1 11th International Congress on Deterioration and Conservation of Stone September 2008, Torun, Poland CONDITION ASSESSMENT OF MARBLE SCULPTURES ON POLYCHROMED AND GILDED DECORATIONS OF THE ARCA SCALIGERA OF CANSIGNORIO DELLA SCALA IN VERONA Vasco Fassina 1, Gianna Gaudini 2, Romano Cavaletti 3 1 Soprintendenza per i Beni Storici Artistici ed Etnoantropologici per le province di Venezia, Padova, Belluno e Treviso, Venezia, Italy, vasco.fassina@gmail.com 2 Soprintendenza ai Beni Architettonici e il Paesaggio di Verona, Vicenza, e Rovigo, Verona, Italy 3 RWS, Padova, Italy Abstract The funerary monument of Cansignorio della Scala was completely covered by an intense blackening which strongly reduced the enjoyment of the sculptures. During a careful inspection of the surface some traces of polychromy and gilding were visible behind the extensive black stratification. This surprising findings driven us to investigate about the extension of these remains before to start any cleaning process in order to maintain as much as possible the polychromy. As formerly the restoration project forecasted to adopt cleaning techniques typically used to remove the black crusts a special care was adopted to prevent any undesirable removal of the original colours. Closely connected with the correct cleaning approach was the survey on the mechanism and the extent of marble decay. To this purpose a detailed study on the different decay patterns was carried out. Finally an extensive survey on the statues and on the bas-relieves revealed the existence of previous thin surface coating layer as well as maintenance organic-based treatment. Keywords: Marble decay, black crust, polychromy and gilding, laser cleaning, 1. Introduction The marked decay observed on the Cansignorio della Scala s funerary monument has driven the Verona Superintendence to Monuments care to plan a restoration work to stop the continuous decay of the monument and to adopt any preventive measure for a future better conservation of the monument itself. The Cansignorio della Scala s funerary monument was built between 1374 and 1376 by Bonino da Campione by using red ammonitic stone on the columns of the aedicules and on the architrave elements and a white marble on the statues, the pinnacles, the aedicules, the niches and the main load-bearing columns (Figs 1 & 2). As 1

2 the white marble showed different features it was necessary to identify them by using petrographical analysis. A careful inspection carried out during preliminary investigations allowed us to discover some traces of polychromes and gilding almost completely covered by the black crusts and grey airborne particulate deposits. Preliminary investigations included a careful mapping of the various forms of alteration affecting the marble surface of the monument primarily to study the causes of alteration and consequently to adopt the appropriate methodologies for the removal of decay products without damaging the marble substrate (Amoroso & Fassina 1983). Figure 1. Cansignorio della Scala funerary monument (2006) Figure 2. Litta s drawings of Cansignorio della Scala (1826) Our investigation was mainly focused on the: i) new formation products present in areas characterised by different morphologies of dacay; ii) existence of different types of polychromy and gilding, iii) identification of old treatment carried out on the surface as maintenance operation, iv) thin yellow patina widely present on the surface. 2. Materials and methods The condition assessment and the decay processes were investigated by taking surface samples of decayed stone from different parts according to the following criteria: i) the degree of decay through a macroscopic observation, ii) the orientation of the individual architectural elements, iii) the exposure to direct rainfall (white areas), iv) the degree of shelter from rainwater (black areas) To assess the different alteration products the following analytical methods were used: 2

3 i) Optical Microscopic (OM) observation of cross-sections to identify the different layers on the surface stratification, ii) Environmental Scanning Electron Microscopy (ESEM) providing morphological information on crystals, iii) Energy Dispersive x-ray Spectroscopy (EDS) for achieving elemental composition of atmospheric pollutants deposited on the surface as well as the decay products, iv) Ion Chromatography (IC) to determine qualitatively and quantitatively the water-soluble anions most harmful for stone decay. 3. Results 3.1 Decay patterns The decay patterns o 1 f the monument are strongly affected by rain and wind which cause visible streaking especially on the north-east side which is the prevailing wind direction. The wind associated to rain enhanced washed areas, while on the opposite south and west side, the accumulation of dirt and the formation of black crusts were observed. On the top, the equestrian statue of Cansignorio della Scala was completely exposed to weathering agents, such as strong winds and rainwater flow, that have caused two main decay forms: a marked biological infestation due to the moisture stagnation and significant erosion phenomena due to the combined action of wind, water and thermal changes which have contributed to accelerate the ageing of the white marble. On the lantern, which is sustaining the equestrian Cansignorio statue, six couples of Apostles inside niches were strongly exposed to weathering agents especially those on the north-east side. On the gables, under the lantern, the six virtues: Fortress, Temperance, Prudence, Charity, Hope and Justice presented different decay forms mainly ascribed to the combined rain and wind action. All the six statues, located at the vertices of the exhagon, being inside the aedicules, are sheltered with respect to the action of vertical water flow and consequently wide 3

4 Fig. 3. The six statues are located at the vertices of the exhagon. The north-east prevailing winds show that San Valentinus statue is more exposed to the action of natural weathering agents such as water and wind. blackening areas are observed. In addition to this feature, even if the statues is sheltered by the aedicules, many washing areas are present due to the inclined water action caused by the contemporaneous presence of rainwater and the wind flowing. The sheltered areas present different blackening of surface deposits as well as the growing of black crusts (Fig. 3). The San Martinus statue (Fig. 4) is located on the south-west side and as it is the best sheltered from water surface flow it shows the most wide blackened areas. Turning in counter-clockwise towards San Quirinus, which is south oriented, some white areas on the parts of the statue exposed to north-east are observed. San Sigismundus, which is exposed to the prevailing north-east winds, shows more wide white areas due to the enhanced surface water flow action. San Valentinus (Fig. 5) is the most influenced by the combined action of prevailing winds and rainwater and consequently the most wide white areas are observed. The last two statues, San Aloisisus and San Georgius show a blackening similar to San Martinus. Figure 4. San Martinus is S-SW oriented and shows the most wide blackened areas. Figure 5. San Valentinus is N-NE oriented and shows the most wide white areas. The typical decay patterns are clearly visible in figure 6. The right side of San Sigismondus is north-east oriented and is directly interested by the action of driving rain or surface water flow, which associated with prevailing winds coming from north-east, are causing some white areas and these were classified as washing areas (a). On the left 4

5 side, in sheltered areas far from surface water flow, a dirt accumulation (area b) of suspended particulate matter is depositing on the surface and a grey-black deposit is visible. The optical microscopic analysis of samples taken from these areas shows the presence of natural silico-aluminate particles and a low amount of gypsum. In these areas deposition takes place mainly in dry condition and the low amount of gypsum measured is indicating a scarce interaction between the marble and the sulphur-based pollutants (Fassina 1993). As it is known the formation of sulphuric acid is favoured by the contemporaneous presence of sulphur dioxide, carbonaceous or metal oxide particles and moisture. In this case the low amount of moisture is preventing the formation of sulphuric acid and consequently the gypsum formation is slowed down. Under the optical microscope thin layers of black deposits appear to be formed by a very dark external layer which Figure 6. San Sigismondus: different progressively becomes less dark moving decay patterns are visible. towards the inner part of the marble (Fassina et al. 2002). Generally on sheltered areas located in between, washing and dirt accumulation areas, a darker feature (area c) is visible due to a more efficient deposition of black carbonaceous particles (Fig. 7) as a consequence of a higher moisture content coming from the nearby washing areas. The high moisture content is responsible of the more a b Figure 7. Black scale from dirt wetting areas (area c in the previous picture) Figure 8. ESEM-BSE observation shows the outermost layer which is properly called black crust, whereas the underneath layer (a) is ascribed to an intentional application of the past. 5

6 efficient CaSO 4 formation. These areas were classified as dirt wetting. Under the optical microscope a close observation of the black crust shows a carbonaceous particlesenriched outermost layer due to the prolonged exposure, in the last four to five decades, to increasing levels of atmospheric pollutants emitted from combustion processes of fossil and oil sulphur-based compounds (Fig. 8). Granular disaggregation is mainly associated with micro-cracks formation on the boundary areas between sheltered and exposed areas. This typically was observed on the lower part of statue legs where water splash-back is depositing on marble surface. This indirect action of water is cyclically investing the surface. Surface granular disaggregation on the areas directly exposed to rainwater (white washing) is mainly ascribed to natural agents, such as thermal changes, and it is strongly accelerated by the decreasing ph of acid rain of urban polluted environment (Fig. 9). As the white material exposed to washing areas showed different features, sometimes large grain size were visible in contrast to other parts characterized by small grain size, it was decided to take samples from these different textures and to analyze them from a minero-petrographic point of view. Microscopic observations show a compact isotropic metamorphic rock, classified as Candoglia marble, as well as the presence of inter granular micro-cracks which are responsible of the bad state of conservation of the material (Fig. 10). The micro-cracks cause the infiltration of carbonaceous particles as it is clearly visible in fig. 11. Figure 9. Couple of Apostles of southeast side. Washing areas are characterized by an advanced marble dechoesion phenomenon. Figure 10. Thin section shows the typical texture-structure of Candoglia marble. Figure 11. The marked marble decohesion allows carbonaceous particles penetration among marble grains. 6

7 3.2 Polychromy and gilding During the preliminary restoration operations a detailed mapping of the different forms of decay was carried out in order to plan the appropriate methodology to remove the deterioration products harmful for the conservation of marble. Behind the extensive black stratification the findings of some colour traces attracted our attention to investigate on the composition and distribution of these remains. Starting from the top of the monument some colour remains on the ground niches of the six couples of the Apostles and on the shell of the six Virtues were observed. As above mentioned for the black crust the colour traces were mainly located in sheltered areas, while on the water surface flow areas the colour has completely disappeared. On the dresses and armours of the six Saints some remains of coloured painted surfaces as well as gilded areas were detected. The bas-relieves of the Cansignorio sarcophagus were extensively gilded. The relevant extension of the polychromy as well as of the gilding has driven us to study in depth both on the painting technique and on the elaboration of an appropriate methodology to select the removal of harmful decay products without damaging the delicate colour remains which were weakly adherent to the substrate. From the back of Sigismundus (Fig. 12) a blue painted layer azurite-based strongly decayed is visible in the layer b (Fig. 13). The paint layer was covered by a thick deposit mainly black carbonaceous particles-based and secondarily composed of gypsum, silicates, iron oxides. Figure 12. From the back of Sigismundus Figure 13. Cross section of painted surface. b) azurite paint layer strongly decayed, c) black carbonaceous particles. It was relevant to find gilding layer on the edges of the dresses and of the armours. As for the polychromy some gilded remains were visible by naked eye, but the most parts were completely covered by black deposits of atmospheric pollutants (Fig. 14). Under the optical microscope the gold lamina (b) is applied above a white lead preparation layer (a). What is surprising is the application above the gold layer of a new white lead layer (c). The outermost layer (d) is gypsum-based and it is strongly decayed. 7

8 4. Conclusions According to the different exposure to natural weathering agents as well as to the increasing levels of atmospheric pollutants occurred in the last five decades some different decay patterns were observed. The cross sections OM and ESEM observations associated with EDS and IC analyses allowed us to identify different decay mechanisms dirt accumulation, dirt wetting and washing areas. A careful observation of the funerary monument surfaces showed the presence of some remains of coloured layers on the marble surface of the statues which were partially or completely covered by a wide blackening due to atmospheric pollutants. D C A B Figure 14. The edges of dresses were gilded as it is visible during laser cleaning operations. Figure 15. Gold (b) was applied above a white lead (a) and was successively covered by a white lead layer (c) now completely turned into brown. These unexpected findings has convinced us to take a special care for the choice of the most suitable cleaning methodology in order to selectively remove the black dirt and the decay products without damaging the marble substrate. As reported in another paper (Fassina et al. in press) after some preliminary tests it was found appropriate to use laser techniques which revealed to have a good selectivity in the removal of black crusts without damaging the underlying painted and gilded layers. Acknowledgements The work was funded by the Italian Ministry of Cultural Heritage. The authors wish to thank the Superintendent Sabina Ferrari who gave the permission for the publication of the preliminary results. References Amoroso, G.G. & Fassina V., Stone decay and conservation, Amsterdam, Elsevier. Fassina V., The weathering mechanisms of marble and stone of Venetian monuments in relation to the environment. In Proceedings of the 10 th Triennial meeting of ICOM, Committee for Conservation: , Washington, August,

9 Fassina V., Favaro M. & Naccari A., Principal decay patterns on Venetian monuments. In Siegesmund S., Weiss T. & Vollbrecht A., (eds.) Natural Stone, Weathering Phenomena, Conservation Strategies and case Studies: Special publication 205, London: Geological Society. Fassina V., Gaudini G. & Siano S. in press, The Arca Scaligera of Cansignorio della Scala by Bonino da Campione: cleaning of the polychromy and gilding, Proceedings International Symposium of 7 th Lacona, Madrid, September,