Structural detachments in wall paintings: Causes, methods of identification and decision-making – Case study of Maria Church, Nisse.

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STRUCTURAL DETACHMENTS IN LIME-BASED WALL PAINTINGS: CAUSES, IDENTIFICATION METHODS AND

DECISION-MAKING – CASE STUDY OF MARIA CHURCH, NISSE

MSc Thesis in Conservation and Restoration of Cultural Heritage | Historic Interior Name: Valentina Gatto | Student No°: 11445351 | Date: 08/2020 Contact Email Address: valentina.gatto@student.uva.nl Supervisor: Merel Schrojenstein Lantman MA|PDRes Second Reader: Dr. Herman den Otter External RCE Advisors: Bernice Crijns, Rutger Morelissen

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STRUCTURAL DETACHMENTS IN WALL PAINTINGS: CAUSES, METHODS OF IDENTIFICATION AND DECISION-MAKING – CASE STUDY OF MARIA CHURCH, NISSE.

M.Sc. Thesis

Conservation and Restoration of Cultural Heritage, Historic Interiors

Author: Valentina Gatto Student No°: 11445351

Contact Email Address: valentina.gatto@student.uva.nl Thesis Supervisor: Merel Schrojenstein Lantman MA|PDRes Second Reader: Dr. Herman den Otter

External RCE Advisors: Bernice Crijns, Rutger Morelissen

Word Count (pp. 6-79 and 88), excluding figure captions and tables: 17997 University of Amsterdam

Rijksdienst voor het Cultureel Erfgoed | Cultural Heritage Agency of the Netherlands Original version submitted on: 24/08/2020

Cover: Visible light photography of the wall paintings representing the coronation (centre) and the annunciation of Mary (left) and the Tree of Jesse (right) on the triumphal arch in Maria Church. Photo Credit: The Cultural Heritage Agency of the Netherlands (RCE) and Jorien Duivenvoorden.

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Structural Detachments in Wall Paintings: Causes, Methods of Identification and Decision-Making. TABLE OF CONTENTS A C K N O W L E D G M E N T S _____________________________________________________________________________ 5 A B S T R A C T ______________________________________________________________________________________________ 6 S A M E N V A T T I N G _____________________________________________________________________________________ 6 1. Introducution ________________________________________________________________________________________ 7 1.1. Research focus __________________________________________________________________________________ 7 1.2. Relevance to the field___________________________________________________________________________ 8

2. Case study: maria church and the wall paintings _____________________________________________ 10

2.1. Brief history and iconography _______________________________________________________________ 12 2.2. Composite materials and techniques ________________________________________________________ 18 2.3. Previous conservation treatments___________________________________________________________ 18 2.4. Current condition of the wall paintings _____________________________________________________ 19 2.5. Environmental parameters in maria church ________________________________________________ 24

3. Causes and description of structural detachments in wall paintings ______________________ 25

3.1. Description ___________________________________________________________________________________ 25 3.2. Intrinsic causes _______________________________________________________________________________ 28 3.2.1. Instability of materials ____________________________________________________________________ 29 3.2.2. Defective application of materials and technique________________________________________ 30 3.2.3. Geological condition and composition of the ground ____________________________________ 31 3.3. Environmental factors _______________________________________________________________________ 32 3.3.1. Frost damage ______________________________________________________________________________ 33 3.3.2. Migration and re-crystallisation of soluble salts _________________________________________ 33 3.3.3. Chemical deterioration ____________________________________________________________________ 39 3.3.4. Fungi and biodeterioration _______________________________________________________________ 41 3.4. Extrinsic causes _______________________________________________________________________________ 42 3.4.1. Adverse effects of previous conservation treatments ___________________________________ 43 3.4.2. The influence of vibrations________________________________________________________________ 44 3.5. Conclusion ____________________________________________________________________________________ 45

4. Methods of identification of structural detachments ________________________________________ 46

4.1. Visual approach and percussion method ____________________________________________________ 46 4.1.1. Discussion _________________________________________________________________________________ 47

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Structural Detachments in Wall Paintings: Causes, Methods of Identification and Decision-Making.

4.2. Infrared thermography (IRT) ________________________________________________________________ 49 4.2.1. Passive thermography ____________________________________________________________________ 49 4.2.2. Active thermography ______________________________________________________________________ 49 4.2.3. Discussion _________________________________________________________________________________ 50 4.3. Acoustic methods _____________________________________________________________________________ 51 4.3.1. Ultrasonic pulse velocity __________________________________________________________________ 53 4.3.2. Ultrasonic pulse echo______________________________________________________________________ 53 4.3.3. Ultrasonic tomography ____________________________________________________________________ 53 4.3.4. Discussion _________________________________________________________________________________ 54 4.4. Electromagnetic imaging techniques ________________________________________________________ 55 4.4.1. Ground penetrating radar _________________________________________________________________ 56 4.4.2. Therahertz imaging _______________________________________________________________________ 56 4.4.3. Discussion _________________________________________________________________________________ 56 4.5. Digital speckle pattern interferometry (DSPI) ______________________________________________ 58 4.5.1. Discussion _________________________________________________________________________________ 58 4.6. Conclusion ____________________________________________________________________________________ 58

5. Decision-making: determining treatment needs _____________________________________________ 62 5.1. Conservation methodology of structural delamination _____________________________________ 62 5.2. The establishmente of need for structural treatment _______________________________________ 62 5.2.1. Injection grouting: risks and limitations _______________________________________________ 62 5.3. Development of decision-making model_____________________________________________________ 62 5.4 Prototype of decision-making model ________________________________________________________ 62

6. Maria Church and the wall paintings: a revision ______________________________________________ 73

6.1. Potential causes of structural delamination ________________________________________________ 73 6.2. Suitable methods for the identification of structural delamination ________________________ 75 6.3. Structural delamination: How to determine treatment needs? ____________________________ 77

7. Conclusion and further research________________________________________________________________ 78 8. Reference list ______________________________________________________________________________________ 80

8.1. Books, chapters of edited books, journal articles, conference papers and dissertations _ 80 8.2. Website contents and unpublished documents _____________________________________________ 86 S U M M A R Y _____________________________________________________________________________________________ 88 Appendix I: Iconography of the wall paintings in Maria Church _______________________________________ 89

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Appendix II: Local newspaper ‘Provinciale Zeeuwse Courant’ _________________________________________ 91 Appendix III: History and restoration history of Maria Church ________________________________________ 93 Appendix IV: SEM-EDX of samples no 1,2 and 3 _______________________________________________________ 97 Appendix V: Introduction to composite materials in wall paintings __________________________________ 104 Appendix VI: Moisture passage through porous building materials __________________________________ 106 Appendix VII: Questions of the prototype of decision-making model ________________________________ 109 Appendix VIII: Glossary _________________________________________________________________________________ 113

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A C K N O W L E D G M E N T S

Desperate times call for exceptionally long acknowledgments; writing a thesis during a pandemic is a remarkably challenging task that can only be combined with a list of thanks just as significant. This thesis would not have been possible without the guidance and supports of many supervisors, colleagues and friends: I would like to extend my gratitude to everyone who helped me and listened to me. I would like to start by thanking my supervisor, Merel Schrojenstein Lantman for helping me finalising the subject of this thesis both the first time and also post COVID-19; her suggestions on how to re-establish a meaningful research focus for this thesis have been unique and her through guidance during this whole process has been just priceless.

Furthermore, I would like to express my gratitude to my external RCE advisor, Bernice Crijns, for introducing me to the complex subject of structural delamination in wall paintings and for presenting me the suitable case study of the wall paintings in Maria Church that allowed me the opportunity to place this challenging subject into a realistic context. To Roger Groves from TU Delft, who accepted to collaborate with me on the future assessment of DSPI and ultrasonic techniques to document structural delamination in Maria Church and who has always been available whenever I had technical questions. Gratitude is also due to Rutger Morelissen for his support during the initial phase of development of this thesis and to Adri Spruit, volunteer at Maria Church, for allowing us to visit the church..

I would like to thank all tutors, lecturers and professors of the Conservation and Restoration Department at the UvA, who supported me during the shifting of research focus that had to be implemented due to the arrival of COVID-19. In particular, I would like to thank: Rene Peschar, for providing initial feedback and improvements to the structure of this thesis, Miko Vasques Dias, for taking the time to read this thesis, giving me an insightful list of suggestions to apply and for correcting some of the typos on the text, Maarten van Bommel and Ella Hendriks, for their constant encouragement during this project, and Maartje Stols-Witlox for being able to manage the general COVID-19 chaos, organise useful supporting workshops and for being always available.

I would like to express my gratitude to all my friends, colleagues and family for being there for me during this peculiar time. Many thanks: to Jasmijn, Edith, Francesca, Laura, Sarah-Jane, Eugenia, Raffaele, Sara, Antonia and Marjolijn. In conclusion, I would like to thank my mum Rosmarie and my dad Guido for always supporting me during my studies, my brother Claudio, for always being there for me and Angelo, for tolerating me during this stressful time and for always being by my side.

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ABSTRACT

Structural detachments in wall paintings: Causes, methods of identification and decision-making – Case study of Maria Church, Nisse.

Valentina Gatto, University of Amsterdam, August 2020.

Structural detachments in wall paintings are investigated to evaluate potential causes, methods of identification and means of determining their treatments needs. The case study of the wall paintings of Maria Church exemplifies the relevance of analysing this topic. Lastly, a prototype of decision-making model to approach the conservation of this phenomenon is introduced.

SAMENVATTING

Structurele delaminatie van muurschilderingen: Oorzaken, identificatiemethodes en besluitvorming – Casestudy van Mariakerk, Nisse.

Valentina Gatto, Universiteit van Amsterdam, Augustus 2020.

Structurele delaminatie van muurschilderingen zijn onderzocht om potentiële oorzaken, identificatiemethodes en manieren om de benodigdheden voor hun behandeling te evalueren. De casestudy van de muurschilderingen van de Mariakerk illustreert de relevantie van het analyseren van dit onderwerp. Ten slotte is een prototype van het besluitvormingsmodel geïntroduceerd voor het behoud van dit fenomeen.

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1. INTRODUCUTION

Delamination or detachments are terms implemented to indicate the presence of a decaying process that is gradually causing the loss of adhesion and cohesion of heterogeneous layers in the structure of wall paintings, resulting in the partial separation between plaster layers, pictorial layers and their support (Wang 2011, 121; Fricke-Begemann, Gu, and Joost 2000, 538). Currently, common conservation approaches are based on the identification of areas of delamination and their remedial conservation treatment, often involving the injection of either lime-based or other synthetic grouts (Biçer-Şimşir and Rainer 2011; Biçer-Şimşir et al. 2009; Pasian et al. 2018). This procedure is frequently described in the literature as an obvious treatment and very little explanation and justification of such choice are provided (Rainer et al. 2017, 10, 71; Rickerby et al. 2004, 471–77; Tringham et al. 2013, 94-95; Asp 2001, 41-44; Suneson 2001, 45-51). Moreover, the evaluation of the structural integrity of wall paintings is usually based on the localisation of delaminated areas by means of subjective and empirical methods such as the percussion approach (Preusser 1991, 4; Hinsch et al. 2009, 96; Guelker, Hinsch, and Joost 2001, 188; Tornari et al. 2013, 4; More and Philippot 1968, 170). The need for a more scientific method to identify and characterize delamination in wall paintings has been expressed since the 1960’s (Preusser 1991, 4) and generated into an area of research involving the use (sometimes combined) of: IRT (Infrared Thermography), DSPI (Digital Speckle Pattern Interferometry), ultrasound and other forms of non-destructive techniques.

Therefore, interests for the overall assessment of possible causes and identification methods of structural detachment in lime-based wall paintings spawned from the case study of the mural paintings in Maria Church in Nisse: despite injection grouting, the structural condition of the aforementioned wall paintings appear quite poor. This case study is considered to extensively illustrate the current problematics encountered in conservation practice. Usually, the identification of causes, location and progress of detachments is based on approximate methods that can generate conflict results. Consequently, their remedial treatment is rarely justified and their long-term effects need to be evaluated.1

1.1. RESEARCH FOCUS

The main goal of this research project is to obtain an insightful understanding of the process of formation of structural detachments, the current methods of their identification, and how, by exploiting case specific factors (such as; previous conservation treatments, environmental parameters and in situ logistics) specific treatment needs can be determined. Conventionally, conservation

1 For the sake of simplicity, the term lime-based wall paintings has been shortened into wall paintings throughout this thesis. Therefore, whilst lime-based wall paintings are the main core of this research, several general notions can be pertinent to wall paintings of different structural composition. Consult the glossary in Appendix VIII.

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treatments of wall paintings were based on preconceived assumptions about condition and, since some remedial treatments are believed to have caused additional long-term damages, the necessity of their implementation, together with their long-term efficacy are brought into question (Kaszewski 2018; Wong and Rickerby 2013; Cather 2003, 412).

Therefore, three main areas of research and related aims were determined: (I) a description of potential causes associated with structural detachments; (II) an evaluation of traditional and modern methods of identification of delamination with an overview of associated advantages and disadvantages; (III) the need for developing a decision-making model to establish specific treatment needs for structural detachments including risks and limitation of present methods. Additionally, the case study of the wall paintings in Maria Church exemplifies the need of investigating the aforementioned areas of research; Chapter 6 elaborates the probable causes, method of identification of structural detachments and the possibility of implementing a decision-making model to determine their treatment needs.

Consequently, the resulting research questions were framed:

I. What are the possible causes leading to the formation of structural detachments in lime-based wall paintings?

II. What are the methods available for the identification of structural detachments in wall paintings? What are their advantages and disadvantages and are there any possible areas of research/improvement?

III. How can one determine the treatment needs for structural delamination in wall paintings?

1.2. RELEVANCE TO THE FIELD

Structural delamination affecting heterogeneous layer compounds of wall paintings is a commonly observed degradation phenomenon (Hinsch et al. 2009, 184). While the nature of their occurrence is often described, methods for their identification are frequently based on empirical and subjective approaches, which only provides an approximate indication of the location of detached areas (Lasyk et al. 2012, 3–4; Hinsch et al. 2009, 96; Fricke-Begemann, Gu, and Joost 2000, 537). Hitherto, conservation treatments of structural detachments frequently involve the injection (through either existing or pre-drilled holes) of a variety of adhesive mixtures, with several researches focusing on the evaluation of methods and materials (Biçer-Şimşir and Rainer 2011; Biçer-Şimşir et al. 2009; Pasian et al. 2018; Biçer-Şimşir and Rainer 2014). Generally, conservation treatments of this phenomenon are considered to be executed rather ‘blindly’, with a great lack of description of decision-making, reasoning and justification of the choice for injection grouting (Mancinelli 1991, 56-66; Rainer et al. 2017, 10, 71; Asp 2001, 41-44; Suneson 2001, 45-51).

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Since the identification and consequent consolidation of structural delamination are considered the most urgent and difficult problems in the field of wall paintings conservation, further understanding of the formation process of this phenomenon and an evaluation of its specific treatment needs are at the core of this research.

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2. CASE STUDY: MARIA CHURCH AND THE WALL PAINTINGS

Maria Church is a protestant church, built during the first half of the fifteenth century (St.Mary’s church Nisse 2020). The church, a brick building constructed in the shape of a cross, has been registered as a national monument since 1967 (see Table 2.1) (RCE National Monument Register 2020). Maria Church is located in the picturesque village of Nisse, in Zeeland, part of the municipality of Borsele (Fig.2.1,2.2). The church adorns and dominates, with its majestic presence, the square of the small village of Nisse, Dorpsplein conveying an atmosphere of grandeur to its landscape. Nowadays, the church has retained its original religious function; weekly church services occur in collaboration with two other protestant churches (in 's-Heer Abtskerke and in Hoedekenskerke, both in Zeeland) and joint services are held occasionally.

In addition to the already established historical, national and sentimental significance attributed to Maria Church, the discovery of a cycle of wall paintings during a restoration campaign in the 1920s, conveyed further artistic, educational and research values to the church. The study of the iconography and themes depicted in the wall paintings together with their dating, identification of material composition, degradation phenomena and conservation treatments were central subjects of two other conservation campaigns (executed in the 1980s and in 2017 respectively) and a more recent diagnostic investigation, carried out by the RCE in 2019. Therefore, these mural paintings represented an ideal case study for the purpose of this research project; the presence of detachments appeared to be a phenomena affecting the structure of the wall paintings since their uncovering in the 1920s (Crijns, Morelissen, and Duivenvoorden 2020).

Fig.2.1, 2.2: Location of Maria Church in Nisse, Zeeland and its rear elevation. Photo credit: https://commons.wikimedia.org/wiki/File:Mariakerk_(Nisse)6.JPG

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Table 2.1. Building and Monument Identification

Name of Building Maria Church, Dorpsplein 49, Nisse

Address Dorpsplein 49, 4443 AG Nisse

Status of the

Building National monument Monument

Number 10008 Cadastral

Part/Number 1578/77

Description

Gothic cross church; single-aisle nave approx. 1425, choir second half of the 15th century, south transept approx. 1500, northern transept a few decades later. Arched vestry somewhat later than the choir. Interior: between the nave and the choir a low narrow triumphal arch of the older choir can be observed together with carved apostle figures under the ribs of the choir vault. Both ribs and wall studs are painted. Mural on the triumphal arch: The Holy Trinity, Tree of Jesse and Annunciation. Wall painting on the north side of the ship: approx. 1430 St. Christopher. Late gothic carved gate on the north transept approx. 1525. Bench with gothic panel work and shield-bearing lions approx. 1500. Fences in the ship and in the south arm, 1st half 17th century. Richly carved pulpit 1679.

Function and

Current Use Religious building, Protestant church, Dutch Reformed church Owner of the

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2.1. BRIEF HISTORY AND ICONOGRAPHY

Maria Church was built in the fifteenth century, as a replacement of a much older church, which was demolished in 1425 (St.Mary’s church Nisse 2020). Consequently, the construction of Maria Church is known to have started right after 1425, and is believed to have lasted almost one hundred years. The present church, which is the result of several expansions and reconstructions, consists of: (I) a nave, original architectural element dating from a time lapse between 1425 and 1525; (II) a choir, a fifteenth century expansion most likely to have occurred after the completion of the church in 1525, presumably commissioned by Hendrik II Van Borselen, who requested the addition with the aim of creating a more sophisticated church, that would have reflected his high status; (III) a transept, an early sixteenth century addition; (IV) a tower, which underwent invasive modifications during a restoration in 1805 (such as the removal of three pinnacles).

The attribution of a date to the wall painting in Maria Church appears to be somewhat problematic as no historical records of their creation have been found: their origin is assumed to be dated around the second half of the fifteenth century, approximately during the extension of the church commissioned by Van Borselen. Therefore, a speculated connection between the lord and the creation of the wall paintings in Maria Church can be expressed. Moreover, the whitewashing of the paintings presumably occurred during the second half of the sixteenth century, during the Protestant Reformation (Crijns, Morelissen, and Duivenvoorden 2020). The cycle of wall paintings in Maria Church consists of two wall paintings, depicting a total of four religious themes: three scenes portraying the Coronation of Mary, the Tree of Jesse and the Annunciation of Mary are respectively located in the centre, on the lower right and on the lower left side of the triumphal arch (Fig.2.3,2.5, 2.6, 2.7, 2.8, 2.9), whilst the fourth theme represented, St. Christopher, is positioned on the north wall (Fig.2.3,2.4, 2.10) (Crijns, Morelissen, and Duivenvoorden 2020). A more detailed iconography description of these wall paintings can be found in Appendix I.

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Fig.2.3,2.4, 2.5: Floor plan of Maria Church, showing the location of the wall paintings in the arch (red) and the wall painting on the north wall (blue). Photo credit: Rob Crevecouer and Jorien Duivenvoorden.

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Fig.2.6, 2.7: Wall paintings on the triumphal arch in Maria Church in Nisse in 2019 (colour photo) and in 1987 (black and white). Photo credit: Bernice Crijns and Rutger Morelissen, edited by Valentina Gatto.

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Fig.2.8 (A-D): Wall paintings on the triumphal arch, located on the lower right depicting the Annunciation of Mary. A and B are details of the stained glass window and chequered floor, while C and D are details of Mary and the Archangel Gabriel with remnants of letters on a nameplate next to his shoulder. Photo credit: Jorien Duivenvoorden, edited by Valentina Gatto.

edited by Valentina Gatto.

B A

D C

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Fig.2.9 (A-D): Wall paintings on the triumphal arch, located on the lower left depicting the Tree of Jesse, each coloured area correspond to the respective detail picture with matching coloured letter. D shows the representation of King David. B and C portrays two other ancestors within the Tree of Jesse, together with a burning candle (highlighted in B). A depicts Mary within a pointed oval, at the top of the tree. Photo credit: Jorien Duivenvoorden, edited by Valentina Gatto.

A

B

C

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Fig.2.10 (A-D): Wall paintings on the north wall representing St. Christopher, more clearly visible in picture A. The palm trees on the background and the hermit exiting from a dome building, are visible in B and C. D depicts a detail of a pictorial element at the bottom of the wall painting. Photo credit: Jorien Duivenvoorden, edited by Valentina Gatto.

A

B C

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2.2. COMPOSITE MATERIALS AND TECHNIQUES

The exact material composition and painting technique of the wall paintings in Maria Church are still somewhat unclear; further research and investigation are necessary to obtain a more in-depth understanding of the material composition and properties of the structural support of the murals and to allow the full comprehension of the formation process of structural delamination.

The initial investigation carried out by the RCE in 2019 determined the presence of lime for the structural support of the wall paintings, although the number of layers, differences in composition and their respective thickness is still unknown (Crijns, Morelissen, and Duivenvoorden 2020). Analysis of the samples collected during the first phase of investigation revealed the presence of several layers of lime plaster with different aggregates, such as sand and, possibly, plant fibres; the last two layers of plaster appeared to be relatively thinner than the underlying ones, therefore suggesting the presence of a conventional layered structure of arriccio and intonachino, typical of lime-based wall paintings (see Chapter 3 and Appendix V)(Mora, Mora and Philippot 1999, 12). The painting technique involved for the creation of these wall paintings is also unknown. Generally, the presence of frescoes in the Northern countries is somewhat limited and it can be assumed that the wall paintings in Maria Church were executed in secco technique. Furthermore, the presence of common soluble salts was detected on all samples analysed.

2.3. PREVIOUS CONSERVATION TREATMENTS

The wall paintings in Maria Church were discovered by restorer Jacob Por in 1920 and have since endured two further restoration campaigns, in 1984 and in 2017, and a recent diagnostic investigation in 2019.

The uncovering of the mural paintings executed by Por presumably entailed the mechanical removal of concealing whitewash together with the filling of losses and their subsequent pictorial reintegration. The documentation of this conservation campaign is considered too general and lacking in useful descriptive details of the treatment executed. However, according to Por, the wall paintings were ‘treated and preserved’ during restoration. Moreover, water infiltrations on the north wall, on the roof above the wall painting of St. Christopher and on both sides of the triumphal arch, deemed to presumably cause damages to both the structure and the pictorial layers of the wall paintings, were documented and repaired (Crevecoeur 2017).

The second main conservation phase occurred between 1984 and 1986 (Crijns, Morelissen, and Duivenvoorden 2020). This conservation campaign was executed by conservator John Post and a team of long-term unemployed, untrained people and is considered the most invasive one.

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Conservation treatments involved extensive reintegration of losses by means of tratteggio with the casein/oil emulsion paint ‘ETA’ from Royal Talens, filling and cleaning with acetic acid (80% in deionised water). Furthermore, structural detachments were treated by drilling small holes in the painted surface and the successive injection with a two-component synthetic adhesive. The final application, over the entire surface of both wall paintings, of a polyvinyl acetate resin named DupaGrund was carried out with the aim of securing both the original paint and the pictorial reintegration. This PVA resin is currently sold as an ideal primer for outdoor porous paint systems, plasters, cement and mortars (Technical data sheet Dupa-grund, n.d.). The overall restoration project lasted approximately 6 months, and as described on the local newspaper ‘Provinciale Zeeuwse Courant’ (see Appendix III), a vast number of holes was drilled to allow the injection of the two-component synthetic adhesive. The identification of such holes results rather difficult, potentially, due to the current poor state of preservation of the pictorial layers of the murals.

The poor condition of the wall paintings spawned the need for further investigation of the causes of decay and tests for conservation treatments in 2017. These were executed by Rob Crevecoeur who documented the presence of structural detachments on both wall paintings by means of percussion approach. This was executed by implementing a self-made equipment consisting of a flexible rod and a small steel ball, attached to the rod with a silicone tubing (Rob Crevecoeur, email to author, June 3, 2020). The approach relied on the documentation of the different sounds generated to identify delaminated areas, after carefully striking the surface of the wall painting with the steel ball (see 4.3). The additional test of a remedial treatment, which involved the injection of an acrylic adhesive (thickened Plextol B500 in xylene) through pre-drilled holes was also implemented (Crevecoeur 2017).

Because of the presence of paint flakes and plaster debris found nearby both wall paintings, the RCE begun an in investigation into the current condition of the murals, the environmental parameters of the church, the possible causes of decay and whether an effective and sustainable treatment plan could be developed (Crijns, Morelissen, and Duivenvoorden 2020). A more exhaustive description of the history and restoration history of Maria Church and its cycle of wall paintings can be found in Appendix III.

2.4. CURRENT CONDITION OF THE WALL PAINTINGS

The stability of the wall paintings in Maria Church has been the subject of the previous restoration and diagnostic campaigns carried out since the 1920’s. According to the most recent investigation performed by the RCE, the structure of the wall paintings is considered rather stable (Crijns, Morelissen, and Duivenvoorden 2020), whereas opposite opinions have been expressed in the condition report executed by Crevecoeur (2017). Whilst both documentations agree on the presence

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of active deterioration affecting the pictorial layer of the paintings, these seem to differ on the degree of structural delamination present. It is striking to observe that the study executed by Crevecoeur (2017) determined the presence of ninety-six locations of structurally delaminated areas on the wall paintings on the triumphal arch, whereas only forty-four were detected during the RCE research in 2019 (Crijns, Morelissen, and Duivenvoorden 2020). This remarkable difference was caused by the implementation of two different percussion methods to identify delaminated areas; the first one involved the use of a small steel ball whilst the second one was executed by gently tapping the painted surface with the finger tips. In both cases the acoustic response to the tapping was evaluated as indicative of the presence or lack of structural detachments. Therefore, due to the presence of conflicting results, it is currently difficult to determine the level of stability of the wall paintings. This dilemma is extremely representative of the present problematics encountered in wall painting conservation practice: the identification of structural detachments is relied on empirical methods, their conservation approach is hardly ever justified and it usually aims at consolidating damages without relating these to their possible causes or evaluating the long-term effects on the preservation of the wall paintings.

The surface appearance of the wall paintings in Maria Church appears heavily affected by previous restorations: extensive retouching executed in the form of tratteggio are visible, often overlapping with the original paint (Fig.2.11, 2.12) (Crijns, Morelissen, and Duivenvoorden 2020). Moreover, numerous infills can be observed, possibly applied to cover previous losses and pre-drilled holes carried out during the injection grouting of the 1980s. The material composition of this type of infills was analysed during the RCE investigation and resulted to be gypsum based (Crijns, Morelissen, and Duivenvoorden 2020).2 The hardness and brittleness of this filling material did not match with the softness usually associated with gypsum; the addition of an organic adhesive must have been implemented. The presence of these fillings is considered somewhat problematic: their hardness is found to have an adverse effect on the original plaster, creating cracks and the loss of original material. The different reactions of the two materials to fluctuations of relative humidity and moisture passage, due to differences in porosity and density, might be the associated cause of this phenomenon (see 3.4.1.).

Generally, it could be argued that the current condition of the wall paintings has been severely affected by both environmental conditions and by previous restorations treatments. Table 2.2 summarises the different damages and associated triggering cause of the wall paintings on the triumphal arch.

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Table 2.2. Damages and Decay of the Wall Paintings on the Triumphal Arch

Damage/Decay Area Description Cause

Cracks

Over the entire arch

The number of large cracks (larger than 3 mm) is limited, most major cracks were repaired in the 1920s

Environmental conditions

Centre (Coronation of Mary)

Vertical crack following historical filling from 1920 on the right, causing instability in the plaster at the bottom of the arch, probably once caused by sinking in the foundations

Environmental conditions

Cracks/Loss of plaster, paint

Low right side (Annunciation of Mary)

Cracks between filling and original surface, a recent loss of original plaster is visible in this area, the filling material is more brittle and stiffer than the original plaster, leading to the formation of cracks and losses

Environmental conditions/ Previous restorations

Delamination Over the entire arch

Unstable around the tip of the arch, cracks and wooden end of the vaulted ribs Environmental conditions/ Previous restorations/ Inherent faults

Blistering and loss of plaster, paint

Left side and centre (Coronation of Mary, Tree of Jesse)

Causes severe deformation of the surface topography and losses, only visible in the original plaster, not on fillings, probably caused by salt activity Environmental conditions/ Previous restorations/ Inherent faults Surface

deformation Over the entire arch

The entire surface appears three-dimensional and very rough, possibly connected to salt activity and structural detachments

Environmental conditions/ Previous restorations/ Inherent faults Salt efflorescence

Left side and centre (Coronation of Mary, Tree of Jesse)

Not widespread, visible on original and filling material

Environmental conditions/ Previous restorations/ Inherent faults Surface Gloss Over the entire arch Widespread, not on fillings, caused

by fixative applied in the 1980’s

Previous restorations

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Fig.2.11 (A-H): Wall paintings on the triumphal arch, showing some degradation phenomena such as: deformed surface topography (A, C and F normal light, B, D and E raking light) and visible extensive retouching in form of tratteggio (G, H). Photo credit: Jorien Duivenvoorden, edited by Valentina Gatto.

A B C D E F G H

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Fig.2.12 (A-F): Wall paintings on the north wall, showing some degradation phenomena such as: widespread surface gloss (A), large areas of loss and retouching (B, C) extensive retouching and delamination (D, E black area and F). Photo credit: Jorien Duivenvoorden, edited by Valentina Gatto. A

B

C _F

E D

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2.5. ENVIRONMENTAL PARAMETERS IN MARIA CHURCH

Because of the strong vulnerability to environmental conditions of both wall paintings and structure of the church, an initial assessment of the hygrothermal conditions of Maria Church was executed by the RCE in November 2019 (Crijns, Morelissen, and Duivenvoorden 2020). The air temperature (T, in C) and relative humidity (RH) were measured before and after the instalment of a portable heather usually implemented during religious functions in the church. The main purpose was to obtain an indication of the prevailing indoor climate conditions and its potential changes induced by the heating system. Moreover, an infrared thermal camera was used to record the thermal distribution over the surface of the wall paintings. This diagnostic technique is particularly useful to obtain a starting indication of the potential weak areas of the fabric of the church and to determine the likelihood of condensation forming over a specific surface (see 4.2.1).3

The initial climate measurement revealed the presence of a fairly constant RH at 70%, despite an increase in temperature from 8 C to 12 C (Crijns, Morelissen, and Duivenvoorden 2020). The occurrence of an unchanged RH can be explained by observing the mechanism of heating involved: a gas burning heather which releases steam, therefore increasing the overall absolute humidity (AH). Eventually, the AH increased due to the heating system but, since also the T increased, the RH remained unchanged. Therefore, condensation would have been prevented by the presence of temperatures higher than the dew points, which were allocated between 2.9 C and 6.7 C (TIS Climate/Humidity Table, n.d.). According to measurements recorded by the infrared camera, it was possible to document the following surface temperatures: on the triumphal arch initial temperatures of 6-7 C were observed with an increase up to 11 C after the heather was started and similarly, temperatures on the wall painting on the north wall were recorded approximately at 6 C with an increase up to 8 C after the use of the heather. This initial climate monitoring demonstrated that the overall internal air and surface temperatures in Maria Church are just above the dew point temperatures, although the presence of condensation is a documented phenomenon: the relationship of the outdoor and indoor climate is in need of further monitoring to understand both the yearly fluctuations of T and RH and the buffering function of the church.

3By recording the temperature of the surface and the relative humidity of the environment the dew point temperature, temperatures below which water will start to condensate, can be calculated.

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3. CAUSES AND DESCRIPTION OF STRUCTURAL DETACHMENTS IN WALL PAINTINGS

Generally, the causes of deterioration affecting mural paintings are multiple and their occurrence is commonly simultaneous and interdependent: the presence of some usually triggers the manifestation of others (Mora 1974, 11). A classification and a connection between fundamental (main triggering factors) and secondary causes (those triggered by the fundamental causes) can be established. Consequently, the successful treatment of any damage greatly relies on the correct classification and identification of its cause; since wall paintings are integral part of the structure of their building, any form of decay affecting the latter is also influencing both aesthetic and stability of the other (Mora, Mora and Philippot 1999, 3). Thus, during the elaboration of any conservation plan this correlation should be retained, especially during the documentation of causes of decay.

Structural detachments are physical forms of damage induced by either external or internal mechanical stresses. Their main causes are the following: (I) intrinsic causes related to faults of structural materials, defective techniques and the geological condition and composition of the ground of the building; (II) environmental factors, primarily associated with the presence of moisture and fluctuations of relative humidity; (III) extrinsic causes related to human factors, mainly connected to the adverse effects of previous conservations and the influence of external vibrations (D’Ossat 1982, 8-24). The secondary causes, decaying reactions spawned by the single or combined action of the main causes, are: frost damage, migration and re-crystallisation of soluble salts and fungi.

3.1. DESCRIPTION

Structural detachments are complex phenomena, considered among the most frequent types of damage found in mural paintings. The most commonly observed types of delamination can affect both structure and pictorial layer of a wall painting: discussions regarding the second type are not included in this research, despite their occurrence being often generated by the same factors responsible for the first type.

Structural delamination, detachment, defect or even disintegration are terminologies used to define damages involving: the loss of cohesion within the same structural layer, the loss of adhesion between two different layers and the loss of adhesion between different applications of the same layer (Fig. 3.1, 3.2, 3.3) (Adams et al. 2005, 525). A categorisation of the different types of detachments that can affect both pictorial layers and structure of lime-based wall paintings with a brief description of their causes is retrieved from the literature (Mora, Mora and Philippot 1999, 254; Calicchia and Cannelli 2005, 116). Table 3.1 summarises these two concepts, providing a linear representation of the different types of delamination and their possible causes (see 3.2., 3.3., and 3.4., for an elaboration of the three

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types of causes described). Furthermore, for clarity reasons, a brief description of the different materials historically used for the structural realisation of wall paintings can be found in Appendix V.

Table 3.1. Types of delamination, description and causes in lime-base wall paintings

Type of delamination Description Causes

Within the same structural layer such as: within the rinzaffo, arriccio, intonaco

Material is friable and almost disintegrated. Delamination occurs in form or relatively small blisters/gaps

Intrinsic/ Environmental

Lack of cohesion between binding agent (lime) and aggregates (sand), different rates of carbonisation between different layers leading to the hardening of the pictorial surface and its consequent lifting, the exposure and direct contact of these structural layers with water/ air/pollution is facilitated

Between different structural layers such as: between the intonaco and arriccio or rinzaffo and the brick wall

Relatively big pockets/gaps

Extrinsic/ Environmental

Exposure to repeated cycles of relative humidity, crystallisation of salts, re-deposition of calcite and vibrations

Between different applications of the same layer: between different application of intonaco or arriccio

Friability of the aggregate material and delamination in form of relatively big

pockets/gaps

Intrinsic/ Extrinsic / Environmental

Lack of adhesion and cohesion between layers of the same material, exposure to repeated cycles of relative humidity, crystallisation of salts, re-deposition of calcite and vibrations

Between pictorial

layer and intonaco Both blisters and larger _{areas of delamination}

Intrinsic/ Extrinsic / Environmental

Lack of cohesion within the paint layer (between pigments and binder) and adhesion to intonaco, exposure to repeated cycles of relative humidity, crystallisation of salts, re-deposition of calcite and vibrations

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Fig.3.1, 3.2, 3.3: Simplified representation of three main types of structural delamination within the layer archaeology of wall paintings. Sizes, shapes of delamination and their effect on the surface topography are only indicative; the aim of this illustrations is to portray locations and highlight differences in space of delamination. Photo credit: Valentina Gatto

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3.2. INTRINSIC CAUSES

Intrinsic causes generate damages strictly connected to the origin and nature of the wall painting. In particular, faulty materials, defective techniques and the geological composition of the ground of the building are elements considered to be directly responsible for the formation of structural delamination (D’Ossat 1982, 11–12).

Mortars and building supports can have inherent defects that might lead to the formation of structural delamination. However, the pictorial layer can also suffer forms of decay that might facilitate the development of detachments. For instance, any loss of the pictorial surface enables the exposure of the intonaco to moisture, air, dust and pollution (see 3.3). Finally, it is important to note that, due the great variation in their composition, each type of lime mortar can exhibit different hydraulic and mechanical properties and that their durability depends on the grain size distribution of their admixtures (Stambolov and van Asperen de Boer 1976, 14).

Fig.3.4: Reiterative diagram of the intrinsic causes connected to the formation of structural detachments. Photo credit: Valentina Gatto

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3.2.1. INSTABILITY OF MATERIALS

A failure of the mechanical properties of both mortar and building support might result in the manifestation of structural delamination. This process might be instigated by two factor: (I) mortars with equally sized small grains; (II) the mechanical failure caused by excessive external stresses (load and thermal expansion) (Torraca 2005, 19–29).

The presence of well-rounded, well-sorted small pores in lime-based mortars can have detrimental effects on the setting process of lime. This occurs by the reaction of lime (calcium hydroxide) with carbon dioxide in the air, leading to the formation of calcium carbonate (Stambolov and van Asperen de Boer 1976, 14) (see Appendix V).

Mortars composed of the admixture of small grains possess large specific surface areas; the amount of binder needed to create structurally stable mortars is larger than that suggested in the literature and risks of creating mortar with weaker mechanical properties might occur (see 3.2.2.). Moreover, an excess of fine grains in mortars might lead to densely carbonised layers at the surface, which could hinder the passage of carbon dioxide within the material and prevent the success of the setting reaction throughout the layered structured of the wall painting. This can result in the formation of weakened areas and, consequently, delamination.

Structural detachments can form due to mechanical behaviours instigated by external stresses imposed by architectural elements like lintels or beams (Torraca 2005, 19–29). Bricks and lime-mortars are defined as brittle, rigid and fragile, however, they partially exhibit plastic behaviours in the form of irreversible deformations. For example, the external load exerted by a lintel can induce tensile stress that would result in the permanent deformation of the structure of the underlying wall painting (Fig.3.6.,3.7.). The heterogeneity of lime-based mortars is responsible for this behaviour: these aggregates are composed of several different crystals and glasses held together by bonds with variable strength. Consequently, these bonds would inevitably break unevenly causing localised fractures (i.e. the rupture of bonds between binder and aggregates), leading to the formation of weak areas prone to be irreversibly deformed. Therefore, tensile stress can eventually generate a lack of cohesion properties of lime-based mortars that would unavoidably result in the formation of structural detachments.

Fig.3.5: Carbonation reaction representing the setting of lime-based mortar.

Ca(OH)

2

+ CO

2

Calcium hydroxide + carbon dioxide

CaCO

3

+ H

2

O

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Moreover, stress within a building might not be equally distributed between its components, leading to different areas being differently affected by stress related deformations and detachments. According to Torraca (2005, 19-29), brittle materials which exhibits areas of permanent deformation are likely to have developed micro-cracks. As a consequence, the access of moisture is facilitated triggering further deterioration processes (see 3.3). Thus, due to the heterogeneous nature of lime-based mortars, it is understood how each composite material might endure different deformations. This way, structural delamination both within and between layers can be explained: if two structural layers suffer different deformation, a lack of adhesion between them might occur, whilst the rupture of bonds between lime and sand can generate a lack of their cohesion leading to both detachment and disaggregation within the same structural layer. This phenomenon can be further enhanced by the action of moisture (see 3.3).

3.2.2. DEFECTIVE APPLICATION OF MATERIALS AND TECHNIQUE

Structural detachments might form due to mistakes occurred during the preparation of both building support and rendering (Mora 1974, 13). The classical literature describes the cleaning and subsequent application of abundant water on the building support as a crucial step. Dust and other debris might hinder the adhesion of mortar to the building support whilst a dry support, especially a highly porous one, might eventually absorb water from the mortar and inhibit the carbonation of lime. This way, the adhesion of superimposed layers of mortars is also prevented. The presence of un-set mortar (i.e. calcium hydroxide) within renderings is a somewhat common phenomenon that can be also caused by the presence of small pores within the aggregates.

Fig.3.6, 3.7: Simplified representation of how the external load from two lintels can lead to the deformation of the building support and the consequent detachment to the first structural layer (rinzaffo) of the wall painting. Sizes and shapes of delamination are not realistically representative. Photo credit: Valentina Gatto

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Furthermore, the correct composition of mortar is essential for the long-term preservation of the wall painting (Mora 1974, 13). Both an excess and lack of binder can form structural instabilities: an excess in lime might result in the formation of cracks and detachment between layers whilst a lack of lime might cause a reduced cohesion with its aggregates, leading to powdery disintegrations within the same layer. The choice of inadequate aggregates (such as sand containing salts), the incorrect proportions of the mix and the excessive thickness of layers can contribute to the failure of the adhesive properties of mortars and, hence, delamination (D’Ossat 1982, 16).

3.2.3. GEOLOGICAL CONDITION AND COMPOSITION OF THE GROUND

The stability of a building relies on the condition and nature of the ground on which it is erected (D’Ossat 1982, 15). The presence of geo-topographical elements (such as streams, aquifers or the vicinity of the sea) can influence the composition of the ground and its ability to withstand the load of the building. Arguably, the formation of structural detachments can be connected to three causes related to the ground of a building: (I) the inability of the ground to sustain load transmitted to the foundations of the building resulting in the partial sinking of these with consequent deformation of building supports and renderings; (II) the rising of moisture and water from either a nearby source of water or from a highly hygroscopic ground (see 3.4); (III) the presence of friable or weakened materials (such as a deteriorated clay-based ground) unsuitable to resist the load of the building with subsequent collapse of parts of its foundations.

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3.3. ENVIRONMENTAL FACTORS

The formation of structural delamination in wall paintings can be caused by three secondary reactions induced by the synergetic action of environmental factors: (I) frost action; (II) the migration and re-crystallisation of soluble salts; (III) the chemical deterioration of calcareous materials. Therefore, both physical and chemical mechanisms can be responsible for the development of structural detachment; the first process exposes the porous structure of building materials to large internal stresses, whilst the second one involves the acid corrosion of the calcium carbonate, its dissolution and subsequent re-deposition, creating weak areas more susceptible to external stresses (Torraca 2009, 83-88). The main environmental factors directly responsible for the abovementioned phenomena are the presence of moisture4 and atmospheric pollutants (Mora 1974, 16–22). However, secondary reactions like fungi and biological accretions, might play a fundamental role both on the formation of structural delamination and also on the process of re-crystallisation of salts (Garg, Jain, and Mishra 1995, 263). Therefore, the combined action of several environmental factors can lead to the development of structural detachments, however none of them would occur without the presence of moisture (Mora 1974, 16-22). The mechanism of passage of moisture thorough porous building materials is rather complex and a simplified description is found in Appendix VI.

4 For simplicity reasons, this term is used to define the passage of water in its gaseous and liquid phase through building materials and cycles of relative humidity.

Fig.3.8: Reiterative diagram of the environmental factors connected to the formation of structural detachments. Photo credit: Valentina Gatto

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3.3.1. FROST DAMAGE

The formation of ice crystals within porous building materials is facilitated by low temperatures (below 0C). The mechanical compression exercised by the growing crystals and the subsequent resistance of the materials surrounding them can lead to large internal stresses, resulting in both adhesive and cohesive failures (Torraca 2005, 31).

The development of ice crystals can follow two mechanism: frost heave and water entrapment (Torraca 2005, 31). Generally, the formation of ice crystals occur easily in large pores, whilst their growth in capillaries occurs only in presence of strong pressure. Water molecules inside small pores (capillaries) are strongly attracted to the surrounding hygroscopic building material and, due to their restrained movement, cannot move into the crystallised structure typical of ice. Oppositely, in large pores, the water molecules have more freedom to re-arrange themselves and form ice crystals. During this process, water from the capillaries moves to the growing crystals by means of diffusion: if the crystal has formed and there is still water available in the capillaries, enough pressure is developed to allow the growth of ice crystals also in the small pores. Frost heave damage is endured by materials with a high percentage of small pores: the amount of pressure generated can lead to large internal stresses, resulting in mechanical failures.

The development of ice crystals by means of water entrapment is slightly different, although, eventually, similar internal stresses are formed. Liquid water can remain confined between already frozen areas generating enough stresses to overcome the material tensile strength leading to both adhesive and cohesive failures (Torraca 2005, 31). Interestingly, the increase in volume of water molecules upon freezing is not regarded as the major cause of development of internal stresses: liquids that do not exhibit an increase in volume after freezing resulted in imposing similar stresses as water (Torraca 2009, 84). Therefore, the primary reasons behind the development of structural delamination is the pressure exerted by the crystals to the surrounding material.

3.3.2. MIGRATION AND RE-CRYSTALLISATION OF SOLUBLE SALTS

The evaporation of water from porous building materials can cause both structural and pictorial damages due to the re-crystallisation of its dissolved salts (Torraca 2009, 85). The mechanism of salt and ice crystals development is similar: the growth of crystals occurs first within large pores, whilst capillaries diffuse water to the growing crystals, eventually leading to large internal stresses caused by an increase in pressure. Moreover, internal stresses can be caused by an increase in volume of a salt crystal due to either thermal expansions or to the re-dissolution and re-crystallisation into a different hydration state (Doehne and Price 2010, 15). The latter can follow two mechanisms: (I) the hydrated salt re-crystallises into a less hydrated form (anhydrous), thus releasing water molecules

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and increasing its volume (triggered by an increase in temperatures); (II) the anhydrous salt crystal draws water from the surrounding capillaries, becoming a more voluminous hydrated form (caused by an increase in relative humidity) (Torraca 2005, 33). However, the majority of internal stresses are caused by repeated cycles of evaporation, re-dissolution and re-crystallisation of salts and their accumulation at the point of evaporation (Doehne and Price 2010, 16).

Generally, the process of migration and re-crystallisation of salts is quite complex and its destructiveness depends on the location of the evaporation zone (Mora 1974, 18). Two main types of crystalline formations can be classified: (I) efflorescence, where salts re-crystallise over the surface of the mural; (II) sub-efflorescence, where salts crystals develop within the structure of the mural. The latter is considered the most destructive mechanism and also the one associated with the formation of structural detachment. Both phenomena are determined by the mechanism of moisture evaporation from building materials, which is directly influenced by the environmental conditions and the porosity of the materials. During the formation of sub-florescence, the evaporation zone of

Fig.3.9, 3.10: Schematic representation of the increase in volume a salt crystal can undergo, inside a pore: by re-crystallising into a less hydrated form (dehydration) or by re-re-crystallising into a more hydrated form (hydration) . Photo credit: Valentina Gatto

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moisture is located below the surface of the wall painting. Therefore, the water vapour generated has to travel through the remaining porous material, where further chemical reactions can be triggered (see 3.3.4) before it can reach the surface and evaporate. Generally, a fast moisture evaporation is necessary for the formation of sub-florescence, a circumstance which appears to be favoured by the presence of strong wind, relatively warm temperatures and low relative humidity (Torraca 2009, 86). However, the location of the evaporating front below the surface of the wall painting can also be formed by a drop in moisture supply which can be caused by an abrupt increase of high temperature due to seasonal cycles (Mora 1974, 17). When the source of moisture is scarce, its speed of movement decreases leading to a premature evaporation. In this instance, both efflorescence and sub-florescence might occur simultaneously due a shift of the evaporation zone from the surface of the wall painting to its rendering.

Furthermore, the different composition of soluble salts can have diverse destructive effects on lime-base mortars, according to their increase in volume after re-crystallisation and their ability to deliquesce and react with other ions to form other salts of different hygroscopicity (Piqué, Ferroni, and Dei 1992, 217). Table 3.2 summarises the salts most commonly found in wall paintings, their form of crystallisation, provenance and associated damage.

It is important to note that, both crystalline formations of salts can lead to the development of structural detachments: efflorescence can cause pictorial losses which can lead to further weakening of the mortar structure due to its facilitate exposure to the atmosphere, whilst sub-florescence can generate internal stresses, due to the volume expansion of salt crystals and an overall increase in pressure, which can lead to the adhesive and cohesive failure of the mortar (Mora 1974, 21). Finally, the enhancement of the chemical degradation of lime-based mortar exerted by salt activity is discussed in the next paragraph.

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Table 3.2. Composition of soluble salts and common damages affecting lime-mortar

I. Sulphates Salt usual hydration state Chemical Name Mineral Name Origin Form of crystallisation Damage Na2SO4 . 10 H2O Sodium sulphate hydrate Thenardite Derivate of construction materials or their decomposition, from bats/bird excreta Efflorescence Sub-florescence Physical damage, pictorial losses and structural detachments Na2SO4 Sodium sulphate Mirabilite K2MgSO4 . 6H2O Potassium magnesium sulphate hexahydrate Picromerite K2MgSO4 . 4H2O Potassium magnesium sulphate tetrahydrate Leonite K3Na(SO4)2 Potassium sodium sulphate Aphthitalite K2SO4 Potassium sulphate Arcanite MgSO4 . 7H2O Magnesium sulphate heptahydrate Epsomite MgSO4 . 6H2O Magnesium sulphate hexahydrate Hexahydrate CaSO4. 2 H2O Calcium sulphate hydrate Gypsum Derivate of construction materials, from bats/bird excreta, previous conservation treatments, atmospheric pollutants Formed by chemical reaction with CaCO3, can cause pictorial losses and structural detachments

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Structural Detachments in Wall Paintings: Causes, Methods of Identification and Decision-Making. II. Carbonates III. Silicates IV. Chlorides Salt usual hydration state Chemical Name Mineral Name Origin Form of crystallisation Damage CaCO3 Calcium carbonate Calcite/ Aragonite Derivate of construction materials or their decomposition, from bats/bird excreta (surface re-deposition of calcite) Surface incrustations Chemical reaction with CO2 leading to the re-deposition of calcite, which can cause pictorial losses and structural weakening Salt usual hydration state Chemical Name Mineral Name Origin Form of crystallisation Damage CaSiO3 Calcium metasilicate Wollastonite Derivate of construction materials or their decomposition, previous conservation treatments Surface incrustations Formed by chemical reaction with CaCO3, can cause structural weakening and pictorial losses Salt usual hydration state Chemical Name Mineral Name Origin Form of crystallisation Damage NaCL Sodium chloride Halite From the atmosphere (vicinity of the sea), from bats/bird excreta Efflorescence (more common), Sub-florescence No disintegrating effect, can react with other ions to form more damaging salts

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Structural Detachments in Wall Paintings: Causes, Methods of Identification and Decision-Making. V. Nitrates VI. Oxalates Salt usual hydration state Chemical Name Mineral Name Origin Form of crystallisation Damage NaNO3 Sodium

nitrate Soda nitre

From the soil, from bats/bird excreta, atmospheric pollutants Efflorescence Disintegrating action inferior to that of the sulphates, pictorial losses which can lead to structural detachments KNO3 Potassium nitrate Nitre or Saltpetre Ca(NO3)2 . 4H2O Calcium nitrate tetrahydrate Nitrocalcite Salt usual hydration state Chemical Name Mineral Name Origin Form of crystallisation Damage CaC2O4 .H2O Hydrated calcium oxalate Whewellite From fungal metabolic activity Efflorescence Formed by chemical reaction with CaCO3, can lead to pictorial losses and structural detachments CaC2O4 .2H2O Calcium oxalate dihydrate Weddellite