Damage control, preservation procedures and durability studies: an investigation approach through the Milan Duomo Cathedral archives

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G. Milani, A. Taliercio, S. Garrity (Eds)

PROCEEDINGS

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i 10th International Masonry Conference Proceedings of the 10th International Masonry Conference Held in Milan, Italy 9‐11 July 2018 Edited by: Gabriele Milani Politecnico di Milano, Italy Alberto Taliercio Politecnico di Milano, Italy Stephen Garrity University of Leeds, UK A publication of:

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10th IMC ‐ Proceedings of the 10th International Masonry Conference G. Milani, A. Taliercio, S. Garrity (Eds.)

First Edition, July 2018

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10th_IMC

10th_{International Masonry Conference}

G. Milani, A. Taliercio and S. Garrity (eds.) Milan, Italy, July 9-11, 2018

DAMAGE CONTROL, PRESERVATION PROCEDURES AND

DURABILITY STUDIES: AN INVESTIGATION APPROACH

THROUGH THE MILAN DUOMO CATHEDRAL ARCHIVES

Stefano Della Torre1, and Lorenzo Cantini2

1

Full Professor, Dept. ABC – Architecture, Built environment, Construction engineering, Politecnico di Milano, piazza Leonardo da Vinci 32, 20133 Milano (Italy)

e-mail: [email protected]

2

Contract Professor, Dept. ABC – Architecture, Built environment, Construction Engineering, Politecnico di Milano, piazza Leonardo da Vinci 32, 20133 Milano (Italy)

e-mail: [email protected]

Keywords: Cathedral of Milan; historical archives, Ferrari da Passano, stone durability,

spe-cial resins.

Abstract. Since the long collaboration between Veneranda Fabbrica del Duomo and Politec-nico di Milano (VFD) was renewed, the Duomo Cathedral has been investigated from differ-ent points of view in order to enhance the continuous care procedures, which can be understood in the theoretical frame of preventive and planned conservation [1].

Among the different study approaches, the collaboration with the historical archive of the VFD showed the potentialities for recording the main pathologies afflicting the building dur-ing different periods and for classifydur-ing the intervention criteria set for facdur-ing specific prob-lems affecting the main loadbearing structures and their materials.

The archive documents provided precious indications concerning the restoration works and strategical measures adopted for the common decay problems presented by the building ma-terials [2], [3]. A turning point in the intervention philosophy, observed in the archives doc-uments, occurred in the beginning of the 1960s, when the Cathedral was concluded, and the VFD had to move from construction and maintenance activities to the conservation issues [4]. On behalf of the large amount of interventions that became a constant care for the building, from the second half of the last century, the VFD promoted interesting efforts in the mainte-nance policy by experimenting new technological solutions [5]. With the aim to protect the different features of the Cathedral, new special mixtures were introduced in the common in-tervention criteria, experimenting on site new resins for repairing. At an impressive extension, these practices produced new systems made up of different materials, namely marble and stones, iron, mortars and resins, whose compatibility on the long run is still to be evaluated. This work focuses on the consequences observed after the development of a clear common practice for the care requested by such a complex building.

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Stefano. Della Torre and Lorenzo Cantini

1 INTRODUCTION

The cathedral of Milan represented for centuries the cultural and economic engine of the city due to the impact that its realization, before, and its maintenance, later, had on the entire Lombardy Region. By the definition “quella gran macchina” (that grand machinery), Ales-sandro Manzoni compared the construction to an innovative propeller, collecting old and new technical competences, able to influence an entire territory by the introduction of new infra-structural solutions (the water channels system from the Maggiore Lake to the city core for the materials transport survived for centuries).

The Veneranda Fabbrica, the institution that followed the realization of the temple, can be considered an ancient vanguard terrain, using an image given by Eugenio Battisti (art histori-an histori-and professor at Politecnico di Milhistori-ano). The Venerhistori-anda Fabbrica provided the resources, the workers and the designers for the realization of the building. The innovative solutions adopted by the Veneranda Fabbrica, for overpassing several technical limits, showed that the experimentation has always been promoted and encouraged for the identification of the cor-rect conservation methodology for preserving the cathedral.

During its long history, the Veneranda Fabbrica collaborated also with the Politecnico di Milano for improving the knowledge of the building characteristics and setting new interven-tion solutions. The procedures for the conservation of the cathedral, in its multiple aspects, were calibrated along the time forming those set of practices that are now consider one of the most important heritage owned by the Veneranda Fabbrica.

This paper presents the last experience matured by a new coordinated collaboration be-tween the Milan Cathedral Veneranda Fabbrica and the Politecnico di Milano. After the dev-astations of the Second World War, an intensive activity for the restoration of the Cathedral was carried out along the 1960s under the guide of the Architect of the Duomo, engineer Car-lo Ferrari da Passano. His activities and the relationships with Politecnico di Milano are well documented but new studies carried out into the archives of the Veneranda Fabbrica provided important additional information on common procedures and new strategies applied on the building.

2 DUOMO CATHEDRAL: DECAYS AND DAMAGES ISSUE

Since the building of the transept, the realization of the cathedral had to face to several technical problems. The construction of the dome (named “tiburio” in the Milan architectural tradition), was largely debated among Italian and international experts in 15th century and the problem connected to the correct dimensions of the main pillars caused a long stop in the building process. The adoption of very original solutions demonstrated the presence of inno-vative competences working to the building. From a structural point of view, the balance of the loads was mainly depending on the realization of barrel vaults contrasting the thrusts of the undelaying cross-vaults of the naves; the accessibility for the periodical maintenance of the higher parts of the cathedral was obtained by realizing a flat cover instead the common roofing system. Moreover, the vertical connections from the plain cover of the naves and the reverse arches of the main spire (at the top of the dome) was guaranteed by four towers, cor-responding to the main four pillars of the tiburio. The contrast to the lateral façades overturn-ing was partially provided by huge rampant arches but the box-like behavior of the buildoverturn-ing was mainly supported by the tie-rods system at the base of the gothic arches used in the naves. All these aspects, and many others, constitute the complexity of this building that since its origin has always been monitored by the institution responsible of its realization, the Ven-eranda Fabbrica del Duomo, created by the Duke of Milan, Gian Galeazzo Visconti, in 1387. This private authority, during the last 630 years, dealt with the design of the various structures

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Stefano Della Torre and Lorenzo Cantini

of the cathedral, the works for their completion and the interventions for repairing and con-serving them.

The complexity of this building has always required a multidisciplinary approach to the different problems connected to the mechanical behavior of its structures and to their material components. Among the various structural problems, the most dangerous can be summarized as follows:

 Water infiltration from the very original walkable plain covering and further wors-ening of the state of conservation of the decorations of the vaults intrados;

 Tie-rods system loss of efficiency due to soil settlements and material decay;

 Decay process of the Candoglia Marble, the stone used for the facades and the inte-riors, due to aggressive urban atmosphere exposure;

The frequent interventions for ordinary maintenance and the constant demands for urgent works concerning structural aspects drove to define a set of roles for the so-called “good prac-tices” that the Veneranda Fabbrica used and still recommends for the conservation of the building. These practices became codified procedures periodically applied on specific struc-tures. Other structural important interventions were carried out for some elements of the building presenting deep damages: here the solutions constituted a sort of case by case re-sponse, in some cases never experienced before, contributing to collect new conservation and repair strategies plans into the archive of the Veneranda Fabbrica.

Considering the history of this building, the main turning points for its realization and its conservation are referred to the serious damages occurred to the main pillars of the dome, the mechanical problems observed on the 4 dome towers and the main spire. Following the chronological order, in 1844-45 a first important conservation design was carried out for the main spire, demonstrating that a correct analysis of the damage, driving the interpretation of the mechanical behavior of the structure, could be useful for preserving the original element, avoiding a complete reconstruction. The analyses carried out by count Ambrogio Nava showed that the main problem was due to the interaction between the stone structure and the huge components of metal devices (figure 1.), which are characteristic of this building, so that the problem was to clean, to passivate, and to seal the joints [6] [7].

Figure 1: Representations provided by Nava of the metal devices supporting the main spire (available in [6]).

More recently, during the 1960s, Carlo Ferrari da Passano had to deal with several struc-tural problems presented by the building, starting from the main spire. In addition, he faced

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also the strengthening intervention for the vaults, the roof, the main spire, the dome towers and the pillars of the dome. The interventions followed by Ferrari da Passano proposed par-tially the practices carried out since centuries by the workers of the Veneranda Fabbrica, but for the main serious mechanical problems, he introduced new contemporary techniques and innovative materials with the aim to provide a solution to different problems concerning the durability of the materials and the reliability of the structures.

Nowadays, the Veneranda Fabbrica is dealing with similar issues, considering the lessons from the past interventions left by Ferrari da Passano. In this sense, the documents conserved in the archive of this institution became an invaluable source of information for interpreting the evolution of some decays processes and the development of refined and innovative inter-vention procedures.

3 THE INVESTIGATION APPROACH

The Veneranda Fabbrica di Milano began in recent years a general renovation of its ar-chive. This archive knew a first setting during the 18th century, when it was organized in clear sections according to the new illuminist principles. After a fundamental reorganization made by Verga at the beginning of the 20th century, the archive maintained a subdivision into 9 sec-tions. Among them, the materials concerning the activities of the Veneranda Fabbrica for the completion and the conservation of the cathedral are stocked in the “Archivio Storico” (His-torical Archive) and in the “Archivio di Deposito” (Repository Archive). The first one con-serves the documents from the foundation (14th century), whilst the second one collects the documents belonging to the last two centuries. Thanks to the agreement between Veneranda Fabbrica and Politecnico di Milano, the authors had the opportunity to examine also the so-called Ferrari da Passano Archive, a stock of important documents that have not been regis-tered in the Repository Archive.

This still not classified section contains different materials, like reports, drawings, photo albums, a rich section of correspondence and the notebooks about the work activities (includ-ing the day by day build(includ-ing site operations) written by Ferrari da Passano. For a complete recognition of those materials, the investigation approach foresaw the identification of the main arguments subdivided in advance by the archivists into folders and sub-folders. The ref-erence activity of those materials constituted a first analysis of the materials and their contents. According to the archivists, a special electronic sheet was used for recording the main infor-mation on the examined materials.

3.1 The Red Books written by Ferrari da Passano

During his permanence as Architect of the Duomo, Ferrari da Passano carried out different structural interventions that he described in detail into the so-called Red Books series. Ferrari da Passano used to report several notes on his diaries and notebooks (usually formed by sim-ple piece of sheets) about the works he had to follow on the Cathedral, from the common maintenance practices to the special rehabilitation works carried out on important structures. In addition, he used to document the on-site operations by realizing photo albums, indicating the date of the shoot and reporting a short description, directly at the backside of the picture and crating real captions under the photos attached to thick paper.

Thanks to this well-detail work activity, Ferrari da Passano created different books collect-ing the main works he promoted and followed on the Cathedral. Among these works, the res-toration of the vaults, the strengthening intervention on the main spire, the four Tiburio towers and the reconstruction of the pillars of the dome are the main interventions reported in the so-called Red Book series. These documents are available only in the Veneranda Fabbrica

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chive and present typed text, with introduction, descriptions of the works, statistics, list of the interventions, materials used and relative costs. Moreover, the Red Books collect also a large number of commented pictures and graphic schemes documenting the on-site operations and the operative aspects of the different projects.

3.2 The Photo library

Ferrari da Passano used the camera for a constant documentation activity of each interven-tion on the building. The quantity of pictures he took is divided into several albums, classified by argument. Respect to the Red Books series, where the contents were codified by text sup-ported by selected pictures, the albums contain all the shoots taken during the daily control of the on-going activities. The albums are divided in specific arguments. Each album contains the pictures in chronologic order and for the most complicated and long works, the albums are divided in more than one volume.

These albums are useful for evaluating the work process documented in detail for many structural interventions occurred on different parts of the Cathedral. For some specific task, Ferrari da Passano prepared a special collection of pictures, displayed on A4 paper, adding even the necessary explanations for driving the correct comprehension of the documented op-erations.

3.3 The day by day notebooks

The personal agendas, simple sheets and pages coming from different size notebooks con-stitute this section of the archive. The hand-write papers follow only the chronological order of the day-by-day activities of the Veneranda Fabbrica. The identification of the information is hard, due to the different activities that are also not always consecutive. Some additional details on the works documented in the Red Books series and by the photo-albums can even be present, but only a preliminary check of the periods recorded in the Red Books allows find-ing the correspondfind-ing hand notes reported in the diaries.

4 DURABILITY AND MONITORING ISSUES

After important interventions on different structures, now the main issues for the preserva-tion of the Cathedral are connected to compatibility and durability of the new materials used for strengthening and preservation interventions. Thanks to previous studies [8] [9], the decay process of the Candoglia marble, used for coating all the vertical masonry elements of the Ca-thedral, is documented in detail. The analysis of the effects of the mechanical workout of the surfaces of the Candoglia marble, with a consistent increasing of the porosity of the stone, drove to a more proper finishing of this material, in order to avoid the decays connected to moisture content in a city characterized by a polluted atmosphere. These negative effects are erosion due to acid rain; scaling of the surface due to frost cycles; deposit and compaction of the detritus coming from the surface into black crusts, due to the chemical action of the acid rain and the urban smog.

If the Candoglia marble was studied in depth, the products tested and later applied for bonding and protecting the marble blocks were not subjected to corresponding complete anal-ysis. A specific literature on the mastics applied for joining the stones and on the protective products, used on the stone and iron surfaces, is not available. Even the indications contained in the books written by Ferrari da Passano [3] [4] [5], as in the very detailed Red Books series, are generic and disregard any information concerning the mechanical properties and the dura-bility of the resins.

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4.1 Observations on the new resins applied to Duomo Cathedral

The sources mentioning the substitution of the common lime based mortars for the mason-ry joints are reported in a brief passage of the Red Book describing the repair work carried out on one of the Tibutio tower of the Duomo: the so-called Gugliotto Amodeo, built at the be-ginning of the 16th century. In 1961, Ferrari da Passano reported some short hand notes about the experimentation of a new product for substituting the common mortar used for repointing the joints. It is indicated as “special mastic”, a chemical resin composed by a bonding mixture called REDIL, produced by the society SIBECK [10]. The product is composed by the resin, a catalyst and a special primer is indicated for the preliminary preparation of the joint. Accord-ing to the documents found in the Archive, after a period of testAccord-ing, the new resin was ap-proved and applied for all the interventions on the masonry structure. In 1963, a letter from the sales clerk of SIBECK [11] reported the following order:

_{n. 1 tin of REDIL resin;}  20 Kg of catalyst;

 20 Kg of asbestos dust.

Since 1961, the regular exchange of correspondence testified the increasing interest for those products. Reading the various notes left by Ferrari da Passano, the research on the “spe-cial mastics” mentioned different final aims, briefly listed in table 1.

Resin type Product name Main purpose

Waterproof treatment Silirain Acqua and Silirain 50 by SOGESIL

Tested on the Candoglia blocks and sculptures for waterproof pro-tection in 1961.

Heat varnish Redil A/2 by SIBECK Isolating and thermal protective

for stone blocks and stone slabs.

Sealant stucco RTV 501 by SOGESIL

Positively tests in 1961 for partial reconstructions of decorative ele-ments.

Sealant coating Silicone mixture by SOGESIL

Applied since 1961 as protective mixture on the joints.

Invisible plasticizer

Protar 965-PV-6110 and Protar 888-PV-6111 by TECCO (Technical and Chemical Consultants)

Applied in 1964 on the stones of the roof-terraces as waterproof protection

Table 1: Examples of the main “special mastics” tested by Ferrari da Passano.

During his frequent report activity, Ferrari da Passano provided also the indications for the composition of the heat varnish, containing 1 Kg of Ducotone water-based paint and 200 gr of asbestos dust. The frequent use of these new mixtures imposes three main questions that the new management establishment of the Cathedral should take into account: durability aspects, mechanical properties decay and toxicity of the interventions.

After a phase devoted to the testing of the products, Ferrari da Passano wrote several notes containing observations on some products applied during the restoration campaign of the vaults, the roofing terraces and the stare towers of the tiburio (gugliotti). His comments were mainly related to some incompatibility aspects between the natural stones and the new resins. He noticed the changing of the colors of the new products after the exposure to Sunlight as first alteration. Moreover, he understood the problems connected to the lack of compatibility

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between the physical characteristics of natural stones and the polymeric products. Neverthe-less, the constant alteration followed by deep decay problems of the Candoglia marble re-quired a solution and the research in the field of chemical products aims to calibrate the proper treatment, offering the correct protection to the stone elements.

A study in depth over the conditions of the new applied resins was carried out in 2001 [12], considering the interventions on the cathedral façade from 1972 to 1974. The study focused on the polymeric products used in that period for stone integrations, sealing and joining inter-ventions. The aim of the analyses was to observe the decay process occurred, in 30 years, to these chemical treatments. According to this research, the epoxy resin Erostac EP 2501 was used for three main purposes: strengthening treatment for the crystalline structure of the Can-doglia marble; binder fraction for sealing interventions; integration mortar. The acrylic resin named Surface Clear Preserving Opaco, composed by polyisobutylmethacrylate, was adopted as surface transparent protection film and as white painting coating. After 30 years of direct exposure to the urban atmosphere, these treated surfaces showed decays connected to discol-oration, color changings, exfoliation of the paintings and again color variation and an exces-sive mechanical resistance developed by the epoxy resin. Table 2 reports the results of this research on the resins used on the facade.

Resin product Specific use Observed decays

Epoxy resin (Erostac EP 2501)

Marble consolidant

Breaking due to too high cohe-siveness respect to stone mechan-ical properties

Sealing binder Color yellowing

Integration mortar Color yellowing

Acrylic resin (Surface Clear Pre-serving Opaco)

Surface protection film Color yellowing and detachment White painting coating Exfoliation and fragmentation of

the film

Table 2: Analyses of the polymeric products applied on the Duomo façade in 1972-1974 according to [12].

4.2 Considerations on the polymeric treatments used on stone materials

One of the most relevant aspect underlined after the analysis on the epoxy resin, initially characterized by a certain elasticity and a good adherence with the substrate, is the unexpected development of very high stiffness. As a result, being the characteristics of the epoxy resin not proportioned with the ones of the marble, the presence of high stresses produced breaking ef-fects always in the underlying Candoglia blocks substrate and not in the resin.

The survived layers of the acrylic paintings are responsible for another problem afflicting several areas of the building: the superficial accumulation of the incoherent deposit in particu-late matter, further evolving into greyish-black aspect.

Considering these two aspects, the polymers used on the surfaces showed their limits for preventing possible marble decays. It is a fact that the chemical treatments carried out on the Candoglia marble produced negative effects as oxalate films and sulfated substrates driving to the formation of thick black crusts. Therefore, the Veneranda Fabbrica has to deal with peri-odic cleaning interventions on the surfaces to avoid losing the aesthetic characteristics of the coating materials. In addition, after the application of several polymeric products, nowadays a new issue has to be investigated: the effects of these materials on the natural stones.

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If in 2001 the epoxy resin appeared as well-adherent with the substrate, recent work activi-ties on the cathedral showed that the resin used as binder for the external mortar joints is no more characterized by any elastic property and, due to the different dilatation coefficient, joints appeared often detached from the stones. The defensive role of the joints, against acid rain and other decay sources is not effective (see figure 2).

Moving inside the cathedral, the polymeric resin used for the joints of the pillars appeared yellowed but still adherent with the stones (see figure 3). The point is here related to a lack of information about the mechanical properties of the joints, taking into account that the on-going investigation campaign on the tie rods of the naves and the apse is revealing that some structural element could be subjected to horizontal forces and the masonry pillars should face this condition. The response of the constructive system composed by mortar (in many cases polymeric) joints and stone blocks should investigated in depth.

Figure 2: Decayed samples of polymeric resin used as binder into a mortar joint on an external wall

Figure 3: Color variation of the polymeric resin used into the mortar joints of a pillar and a vault of the cathedral.

The negative effects of the interaction between metal elements and stones were observed during the 2016 restoration works: some arches of the stares towers presented deep cracks due to the presence of metal bars hidden into some marble block. After an on-site inspection, a resin coating on the metal bars was observed. This coating had to work as passivating solution for protecting the bars that appeared oxidized, reviling that the cracks formed in the joints

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lowed water infiltration and the consequent decay process for the resin and the metal bar (see figure 4).

Figure 4: Mechanical damage caused to stone elements by the metal bars passivated by polymeric resin.

Inside the structure, these passivating coatings have been detected as very thick. Therefore the compatibility problem metal-stone turned into a ternary problem, involving also these res-in layers, whose characteristics are defres-initely peculiar. This problem requires a purpose-designed research, involving the analysis of the evolution of border conditions [13], such as for instance the changes in temperature, which are very high on the Cathedral higher parts.

The inspections carried out on different parts of the cathedral showed a general alteration of the color of the epoxy resin when exposed to wet conditions (figure 5a), both in external and internal areas. The most common decays occur on the external areas where loss of adhe-sion and the superficial crumbling are clearly visible (figure 5b and 5c).

Figure 5: Comparison among different decays observed on the polymeric resin.

5 CONCLUSIONS

The research showed how much the physical reality of Milan Cathedral has been modified by 20th c. interventions, aimed at a better conservation, but introducing new materials not yet tested on their long-term behavior.

In the practice these materials are now undergoing general substitution as for surface joints. An open problem is to be found in the thick resin beds inside the structure, just on the connec-tions between metal bars and stones. A novel problem of compatibility here arises, which has

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to be analyzed taking into account the long-term transformation of the applied resins as well as the dynamics of border conditions.

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