Biorimozione di patine di solfati su un bassorilievo marmoreo policromo del XV secolo
10.6092/issn.1973-9494/6177
Il presente caso di studio riguarda l’impiego di cellule batteriche vitali per la rimozione di alterazioni dalla superficie lapidea di un bassorilievo policromo esposto alla Galleria Interdisciplinare della Regione Sicilia di Palazzo Abatellis (Palermo). Le indagini diagnostiche effettuate durante il restauro conservativo sono state finalizzate all’approfondimento della tecnica esecutiva e alla comprensione dei processi degradativi in atto. I risultati delle indagini XRF e FTIR hanno evidenziato che su estese porzioni della superficie del manufatto erano presenti depositi con rilevante presenza di solfati. La pulitura ha dunque previsto l’impiego di cellule batteriche vitali appartenenti alla specie Desulfovibrio vulgaris, valutando e confrontando l’efficacia della biorimozione con quella degli agenti desolfatanti solitamente impiegati. I risultati dimostrano che questa specie batterica, sfruttando il substrato colonizzato come fonte nutrizionale ed esplicando i naturali processi metabolici, può essere utilizzata ottenendo un’elevata efficienza e selettività d’azione, rappresentando una valida alternativa all’utilizzo di tradizionali agenti chimici.
application/pdf
1973-9494
6177
en
Annalisa Balloi
Conservation Science in Cultural Heritage
sustainable restoration
Bioremoval of Sulphate Layer from a 15th Century Polychrome Marble Artifact
Text.Serial.Journal
https://conservation-science.unibo.it/
15
Articles
2015-12-30
2016-05-31
2015-12-30
2017-09-04
235-252
CSCH
Annalisa Balloi
2015/12/30
10.6092/issn.1973-9494/6177
235
Micro4yoU S.r.l., Milan
1973-9494
sustainable restoration
en
252
BOSH-ROIG P., RANALLI G., 2014, The safety of biocleaning technologies for cultural heritage, Frontiers in Microbiology 5: 155
Bioremoval of Sulphate Layer from a 15th Century Polychrome Marble Artifact
https://conservation-science.unibo.it/article/download/6177/5949
15
B       IOREMOVAL 0F SULPHATE LAYER FROM A 15TH CEN-
        TURY POLYCHROME MARBLE ARTIFACT

      Manuela Martino, Salvo Schiavone, Franco Palla*
      Laboratorio di Biologia e Biotecnologie per i Beni Culturali, Dipartimento di Scienze
      e Tecnologie Biologiche, Chimiche e farmaceutiche (STEBICEF)
      Università degli Studi di Palermo, Italy

      Lorella Pellegrino
      Centro Regionale per la Progettazione e il Restauro (CRPR), Palermo, Italy

      Evelina De Castro
      Galleria Regionale della Sicilia di Palazzo Abatellis, Palermo, Italy

      Annalisa Balloi
      Micro4yoU S.r.l., Milan, Italy

      Keywords: biocleaning, bacteria, desulfovibrio vulgaris, sustainable restoration


      1. Introduction

    In recent decades, biotechnology has provided numerous technological innovations
for the detection of microbial consortia able to induce the biodeterioration of artifacts
[1-6], or of those that are present in the aerosol of confined spaces for cultural heri-
tage [7, 8]. Although many bacterial species represent a threat to the conservation of




                                                                                                     CONSERVATION SCIENCE IN CULTURAL HERITAGE
cultural heritage and / or for operators, some can be used as agents for “biorestoring”.
The related literature reports that certain bacterial strains (Pseudomonas stutzeri, P.
aeruginosa, Desulfovibrio vulgaris, D. desulfuricans) have been used for the removal
of undesired layers from the surface of works of art. These microorganisms exploit
their naturally occurring metabolic processes by transforming the surface layer com-
ponents into non-toxic gases dispersed into the atmosphere [9].
    Currently, bacteria exist that are able to transform organic substances (e.g. Pseu-
domonas stutzeri, nitrate reducers - NRB) or sulphates (e.g: bacteria sulfate reduc-
ers - SRB). Other bacterial species are able to trigger the process of biocalcification
on stone substrates [10 - 12]. Bacteria such as Desulfovibrio desulfuricans and D.
vulgaris have been successfully applied for the removal of black crust from a lunette
in the Cathedral of Milan [13], for the removal of plaster from the basement of Michel-
angelo’s Rondanini Pietà [14] and for cleaning the polychrome marbles of the Florence
Cathedral [15].
    In this study, Desulfovibrio vulgaris viable cells were utilized for the removal of sul-
fate crusts from the surface of a polychrome marble bas-relief depicting the Eternal
Father in the act of blessing, exhibited at the Regional Gallery of Palazzo Abatellis
(Palermo, Italy).


*
    Corresponding author: franco.palla@unipa.it


                                                                                               235
                                                                                                                 The protocol developed for bioremoval in this case study is part of a complex in-
                                                                                                              tervention which, after a preliminary phase of art-historical research and assessment
                                                                                                              of the state of conservation, resulted in the restoration of the artifact, supported by
                                                                                                              specific technical-scientific investigations.

                                                                                                                2. The case study
M. Martino, S. Schiavone, F. Palla - L. Pellegrino, E. De Castro, A. Balloi - Bioremoval 0f sulphate layer




                                                                                                                2.1 The artifact

                                                                                                                 Made by an anonymous Lombard sculptor of the fifteenth century, it has a frame
                                                                                                              with phytomorphic decorations that delimits a pentagonal space depicting the Eternal
                                                                                                              Father with a halo and his right hand raised in blessing while the other holds the world.
                                                                                                              The shape and decorative patterns are realized using a technique of low and high relief
                                                                                                              on a single marble slab of variable thickness. Small traces of colors, visible in limited
                                                                                                              areas of the surface, have led to the hypothesis that the work may originally have been
                                                                                                              a polychrome artifact. To confirm this hypothesis and to collect information on the tech-
                                                                                                              nique of execution used, the artifact was investigated by in situ X-Ray Fluorescence,
                                                                                                              particularly on some areas characterized by the colors red, brown and green-blue. The
                                                                                                              materials found are those widely used during the Renaissance period, in accordance
                                                                                                              with the historical and artistic information regarding the use of specific colors for the
                                                                                                              decoration of marble surfaces [16,17]. The results of the analysis performed on the
                                                                                                              green-blue colored area revealed the presence of copper, to be correlated to the pig-
                                                                                                              ment azurite [2CuCO3∙Cu (OH)2]; in some points altered to malachite [CuCO3∙Cu (OH)2]
                                                                                                              by the action of moisture and / or other agents. In points presenting a red-brown color
                                                                                                              high levels of iron were recorded, related to the use of iron oxide-based pigments such
                                                                                                              as ochre or earths.

                                                                                                                2.2 Conservation state

                                                                                                                 The state of conservation, mapped according to NorMal Recommendations 1/88
                                                                                                              (established standards for the conservation of stone materials), showed that the deg-
                                                                                                              radation was mainly due to the presence of incoherent and coherent deposits. This
                                                                                                              very dark compact deposit forms an uneven layer on the surface, altering the correct
                                                                                                              reading of the sculptural details and colors.
                                                                                                                 In order to perform a selective and effective cleaning operation, the deposit was
                                                                                                              chemically characterized by diagnostic tests, which included X-Ray Fluorescence
                                                                                                              (XRF) and Fourier Transform Infrared Spectroscopy (FTIR).

                                                                                                                2.3 Characterization of deposits

                                                                                                                  The XRF analysis was performed in situ on specific sample points (Figure 1). Re-
                                                                                                              sults revealed high levels of sulfur, suggesting the deposits, which cover a large area
                                                                                                              of the bas-relief surface, consist mainly of sulfates (the main components of gypsum
                                                                                                              being CaSO4×2H2O). Gypsum has a marked tendency to accumulate on surfaces,
                                                                                                              forming crusts which thicken by including and cementing atmospheric particulates
                                                                                                              (coal, silicates and other salts), giving rise to “black crusts” [18 - 21].
                                                                                                                  The presence of gypsum was further confirmed by the FTIR spectra, which showed
                                                                                                              all the characteristic bands of this compound.


                                                                                                        236
Figure 1. XRF analysis: A) Set point (in red) on artifact surface; B) corresponding XRF profile
(relevant Sulphur amount).



   3. Bioremoval
   The restoration process involved different phases, one of the most important being
the removal of the black crust, which was performed respecting the physical-chemical
nature of the constitutive materials.
   Considering that extensive portions of the surface were characterized by deposits
with a significant presence of sulfates, a number of sulfate removal strategies were
examined and their different peculiarities assessed.




                                                                                                        CONSERVATION SCIENCE IN CULTURAL HERITAGE
   3.1 Viable bacterial cells

    Desulfovibrio vulgaris viable bacterial cells were utilized for the removal of the black
crusts. The viable bacteria, available as lyophilized cells (Micro4Art solfati© produced
by the company Micro4you S.r.l.), were hydrated and applied to the surface of the ar-
tifact as a gelled solution (modified polyacrylic acid gel - Carbogel) [22]. Bacterial cells
carry out a desulfating action according to the following reaction: 6CaSO4+ 4H2O +
6CO2 → 6CaCO3 + 4H2S + 2S + 11O2 [9].
    Tests were performed on the consistent uniform deposit present on the surface of
the marble artifact by comparing the results after application of the Bacteria + Carbo-
gel solution with the control solution, consisting only of Carbogel in aqueous solution
(Figure 2).
    Both applications were performed by inserting a sheet of Japanese paper between
the artifact surface and the aqueous gelled solutions and subsequently covered with
a polyethylene film (Figure 3) so as to maintain conditions of both humidity and partial
anaerobiosis, necessary for bacterial metabolism [23].
    Compared to the Control solution, the effectiveness of the bioremoval was evident
from the first application. The desired level of cleaning, however, was only achieved
after three applications.
    Successful removal of the sulfate crusts was confirmed by XRF analysis, which



                                                                                                  237
                                                                                                              showed a very low sulfur content in the cleaned area when compared to the untreated
                                                                                                              area.
                                                                                                                 In view of these results an ad hoc protocol was defined for each area of the artifact
                                                                                                              characterized by a chalky coherent deposit, applying the solution once or twice in rela-
                                                                                                              tion to the thickness (Fig. 4).
M. Martino, S. Schiavone, F. Palla - L. Pellegrino, E. De Castro, A. Balloi - Bioremoval 0f sulphate layer




                                                                                                              Figure 2. Preliminary test performed by Desulfovibrio vulgaris viable cells solution, gelled by
                                                                                                              Carbogel (A); Control, Carbogel alone (B). The gelled solutions were covered with a transparent
                                                                                                              polyethylene sheet, in order to keep adequate humidity and partial anaerobic conditions, needed
                                                                                                              for bacterial metabolism.




                                                                                                              Figure 3. Bioremoval: (from left) before processing; Japanese paper and gelled solution
                                                                                                              application; covering by transparent polyethylene sheet; black crust removal.




                                                                                                              Figure 4. Bioremoval, detail: (from left) before; during; after cleaning.


                                                                                                        238
   At times, the bacterial solution gelled by Carbogel proved difficult to apply, mainly
due to the fact that the artifact was positioned vertically in a ventilated space and had
small sections with complex geometries. In addition, tests were carried out by support-
ing the viable bacterial cells with Agar, which unlike Carbogel, is not a “direct” gelling
agent requiring cycles of heating and cooling [24]. Using the Agar gelling agent, the
result of the cleaning operation was much poorer than that performed with Carbogel.

   3.2 Alternative methods

   In this study, the action of Ammonium carbonate added to direct gelling agents,
such as polyacrylic acid (Carbogel) and Xanthan Gum (Vanzan), were evaluated as
biocleaners. Ammonium carbonate performs a desulfating action according to the fol-
lowing reaction: CaSO4 ∙ 2H2O + (NH4) 2CO3 → CaCO3 + (NH4) 2SO4 [25].
   Although both gelling agents gave good results, for the removal of black deposits
from the pigmented areas with copper residues, the use of ion exchange resins (ani-
onic) was preferred. Desulfation was performed according to the following reaction: R2
(OH)2 + CaSO4 ∙ 2H2O → R2SO4 + Ca(OH)2 + 2H2O, followed by: Ca(OH)2 + CO2
→ CaCO3 + H2O [26].


   4. Conclusion

    Among the advantages of using the viable bacterial cells developed by Micro4you,
as well as the speed and ease of use, considerable importance is represented by the
guarantee of a gradual, selective and repeatable activity that fully respects the surface
of the work of art (Figure 5).




                                                                                                   CONSERVATION SCIENCE IN CULTURAL HERITAGE




Figure 5. The artifact before and after restoration.


                                                                                             239
                                                                                                                 Another advantage is that cleaning can be done without the use of polluting sub-
                                                                                                              stances, guaranteeing high safety standards, both for the operator and the environ-
                                                                                                              ment [27].
                                                                                                                 These results confirm that the use of microorganisms satisfies the selective char-
                                                                                                              acteristics of the cleaning, at the same time respecting the constitutive materials of
                                                                                                              the artifact, as required by modern conservative restoration. This method, therefore,
                                                                                                              is a viable alternative to traditional methods, oriented towards sustainable restoration.
M. Martino, S. Schiavone, F. Palla - L. Pellegrino, E. De Castro, A. Balloi - Bioremoval 0f sulphate layer




                                                                                                                 The results of this study, moreover, provide useful information for the controlled
                                                                                                              biocleaning of polychrome stone surfaces.


                                                                                                                Acknowledgements
                                                                                                                The authors thank the Regional Gallery of Palazzo Abatellis, Palermo, for the co-
                                                                                                              operation.


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                                                                                                                     cultural heritage, Frontiers in Microbiology 5: 155


                                                                                                                 Biographical notes

                                                                                                                 Manuela Martino, is a qualified restorer (L.D, 42/2004 - specialization in Stone Ma-
                                                                                                              terials and Products, decorated surfaces of architecture; she has a degree (MRL / 02)
                                                                                                              in Conservation and Restoration of Cultural Heritage from the University of Palermo
                                                                                                              and is now studying a Master’s Degree, “Expert researcher in nano materials and nano
                                                                                                              technologies for Cultural Heritage” at the University of Palermo.

                                                                                                                  Salvo Schiavone, PhD in Applied Geophysics for the Environment and Territory -
                                                                                                              University of Messina and Department of Earth and Marine Sciences (Di.STe M), Uni-
                                                                                                              versity of Palermo. He graduated with honors in “Science and Advanced Technologies
                                                                                                              applied to the Conservation and Restoration of Cultural Heritage” at the University of
                                                                                                              Catania and has carried out ongoing collaborative activities with the Physics Laborato-
                                                                                                              ry of the Regional Centre for the Planning and Restoration of Palermo. He took part as
                                                                                                              a scientific consultant in the group for works supervision in the Recovery and Conser-
                                                                                                              vation of Villa Romana del Casale in Piazza Armerina, Enna. He is director of S.T.Art-
                                                                                                              Test s.a.s., a company providing diagnostic services for art and the territory, performing
                                                                                                              physical investigations of a non-destructive nature for the characterization and moni-
                                                                                                              toring of cultural heritage and conservation environments. He collaborates with the
                                                                                                              Department of Biological, Chemical and Pharmaceutical Sciences and Technologies
                                                                                                              of the University of Palermo. His scientific work is devoted to materials characterization
                                                                                                              through applied physics in the field of Cultural Heritage and Civil Engineering.

                                                                                                                  Annalisa Balloi, graduated in Biological Sciences at the University of Cagliari with
                                                                                                              a thesis carried out at the research center in Alnarp, Sweden and a PhD at the Fac-
                                                                                                              ulty of Agriculture of the University of Milan. In 2010, with some colleagues, she par-
                                                                                                              ticipated in the university spin-off “Micro4yoU Srl”, of which she is the CEO. Since
                                                                                                              its establishment, Micro4yoU has received numerous awards for its commitment and
                                                                                                              achievements in Technology Transfer in the field of environmental biotechnology. In
                                                                                                              addition to continuing her work in Micro4yoU, she is currently a professor at the Acad-
                                                                                                              emy of Fine Arts of Brera where she is responsible for the course, “Elements of Biology
                                                                                                              Applied to Restoration.”




                                                                                                        242
   Lorella Pellegrino, qualified as a Restorer of Cultural Heritage, with a Specializa-
tion in Restoration of Stone Artifacts at the OPD in Florence, and in Restoration of Wall
Paintings at the ICR in Rome. She has an executive role as Restorer of Cultural Herit-
age and head of the Laboratory of Restoration of Artifacts of Inorganic Origin at the
Centro Regionale per la Progettazione e Restauro of Sicily. She coordinates ordinary
and extraordinary restoration of movable, architectural, archaeological, historical and
artistic heritage. Chief Operating Officer and Director of works in complex interven-
tions on sculptures and on sites such as the Greek archaeological areas of Halaesa
Arcidiaconea, the Greek theater in Taormina, the Villa Romana del Casale in Piazza
Armerina and Patti. She carries out educational activities, which include theoretical,
technical and practical aspects for internships of the degree course “Cultural Herit-
age Restorer” at the University of Palermo. She coordinates senior and junior Italian
and Cambodian restorers, who carry out restoration work at the archaeological site of
ANGKOR, Cambodia, on sculptures and architectural complexes of Khmer temples.

   Evelina De Castro, has a PhD in History of Medieval and modern art in Sicily from
the University of Palermo, and a degree in Medieval and modern history from the Sapi-
enza University of Rome. She is Executive manager of the collections at the Regional
Gallery of Palazzo Abatellis, Palermo. Her research is mainly devoted to painting in
western Sicily, specifically from the Gothic to Renaissance periods and from Man-
nerism and Baroque to Realism. In relation to the collections of Palazzo Abatellis she
has studied in depth Renaissance marble sculpture and problems of attribution and
historiography related to Domenico Gagini and Francesco Laurana. She collaborates
with the Sopraintendenza of Palermo and Agrigento and the University of Palermo, in
relation to painting and stucco productions from late Mannerism to Serpotta.

   Franco Palla is Associate Professor of Applied and Environmental Botany at the
University of Palermo, Italy. He is Professor and Vice-Coordinator of the Five-Year




                                                                                                  CONSERVATION SCIENCE IN CULTURAL HERITAGE
Degree in Conservation and Restoration of Cultural Property (MRL 02, certified pro-
fessional restorer). He is a member of the Permanent Commission for the Protection
of Cultural Heritage of the National Order of Biologists. Scientific head of UNIPA Re-
search Unit, for the Research Project PON01_00625, It@cha (Italian technology for
advanced applications in Cultural Heritage). He was one of the members of the work-
ing groups in the cooperation project Italy-Cambogia for Training Experts of Cultural
Heritage, University of Palermo-Royal University of Fine Arts and the Ministry of Cul-
ture and Fine Arts, Angkor, Cambodia. He is Coordinator of the Laboratory of Biology
and Biotechnology for Cultural Heritage at the Department of Biological, Chemical, and
Pharmaceutical Sciences and Technologies (STEBICEF) of the University of Palermo.




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