Restoring fragmented marble epistyles: Some critical points

The mechanical behavior of fragmented marble epistyles restored with titanium reinforcing bars is studied numerically using the Finite Element Method. The study is focused to the behavior of restored epistyles subjected to bending under uniform load along the free span of the structural element. The restoration method simulated is the one introduced a few years ago by the scientists working for the conservation of the Parthenon Temple on the Acropolis of Athens and is still under development. Attention is focused to the influence of the geometric features of the reinforcing bars as well as to the role of the cementitious material interposed between the marble and the bar. In addition, the contact properties of the marble-titanium, marble-cement and cement-titanium interfaces are also examined. To achieve the goals of the study, six numerical models are constructed considering centrally fractured prismatic marble epistyles of rectangular cross section restored with a single titanium bar, either cylindrical or threaded. The analysis reveals the critical regions, where the maximum stresses and the strain discontinuities appear and makes clear the influence of the geometrical characteristics of the reinforcing bar. Also, the crucial role of the constitutive law governing the mechanical behavior of the intermediate layer of cementitious material is enlightened.     

Laser cleaning: a study on greyish alteration induced on non-patinated marbles

During a conservative operation, a restorer noted a grey shade on a ‘fresh’ (recently fractured) marble surface during laser cleaning. Following this report, the authors began a study on ten Carrara white marble specimens polished on the surface and treated in the same restoration conditions using two types of laser equipment: the first one a Q-switched laser (20-ns pulse), the second one a short free-running laser with a medium pulsewidth of 20 μs. In a few specimens a grey shade appears on the surface. These specimens were investigated by optical microscopy in reflected and raking light and by SEM/EDX. After that, the same samples were embedded in resin and polished to obtain cross sections to be analysed in the same way. Results show that the white marble of the greyish samples contains very small fragments of pyrite (iron sulphide) of dark shiny aspect and that after cleaning the aspect of the fragment appreciably changes. The morphological studies allow two different behaviours of dark inclusion to be observed following the different laser pulses: the first one occurs after short-pulse laser treatment in the form of micro-explosion surrounding the pyrite grains (mechanical effect); the second one (medium pulsewidth) appears like a fusion and spread of particles on the surface. Results are reported showing microphotographs and SEM/EDS documentation.     

Principal Component Analysis in monument conservation: Three application examples

Multivariate statistics is a well-known and invaluable tool in archaeological science but its use is limited in monument restoration. The aim of this work is to demonstrate the effectiveness of Principal Component Analysis (PCA) on the characterization, technology and weathering condition investigation of building materials from historical monuments.Towards this aim, three examples are given:The first one is a provenance and technology investigation of the Aghia Sophia (Istanbul, Turkey) bricks, some of which had to be replaced due to weathering, during recent restoration works. It was proved by PCA that the original clay, used for the construction of the bricks, is not similar to the clay of other contemporary constructions in Istanbul but presents high similarity to the raw material of the bricks from a contemporary church in the island of Rhodes (Dodecanese, Greece). Additionally, the technology of the bricks was studied by mercury intrusion porosimetry, strength tests and Scanning Electron Microscopy. The use of PCA gives a very comprehensive way to present the difference in the technology of the dome bricks.The second presents a classification of mortars from medieval (Byzantine) monasteries, based on their microstructural characteristics (porosity, reverse hydraulicity ratio) and strength measurements. The PCA grouping gives an illustrative diagram depicting the correlation between mortar syntheses and resulting characteristics.The third case shows an example of the correlation between environmental pollution data and data from the weathering layers of marble surfaces (patina composition, orientation of the monument surface, etc.).     

Assessment of synthetic polymeric coatings for the protection and preservation of stone monuments

The performance of five synthetic coatings for the protection of stone monuments of Hellenistic and Byzantine period has been evaluated. The selected coatings included four commercially available siloxane-, siloxane/acrylic- and perfluoroether-based compositions, as well as a new composition based on newly synthesized fluoro-organosilane. The coatings were applied onto petrologically different stone substrates, such as marble, travertine, sandstone and a newly baked brick compatible with Roman period bricks, used for the restoration processes in Galerius Palace, Thessaloniki, Greece. The coatings' protective efficiency was investigated by measurement of water–stone contact angles, water vapor permeability, and water absorption by capillarity. The optical properties of the applied coatings were also investigated and they were also ranked with regard to their optical characteristics.     

Volubilis (Meknes,Morocco): Archaeometric study of the white and coloured marbles imported in the Roman age

The ruins of the Roman town of Volubilis, located in the Middle Atlas, near Meknes, between Fez and Rabat, constitute the most important Roman archaeological site in Morocco and are included in the UNESCO World Heritage List. Volubilis became the headquarters of the Mauritania Tingitana limes after the annexation of the kingdom of Mauritania to the Roman Empire in AD 42. Evacuated by the Romans after the second half of the 3rd century, the town became the capital of the kingdom of Idriss I in AD 789.The results of an archaeometric study of the marbles imported into Volubilis during the Roman age are reported herein the sixth framework program of the European Union.Of the few coloured lithotypes that have been found, all are of Greek origin with the exception of the so-called “Portuguese pink”, a pale pink marble exploited by the Romans near Vilaviçosa in the Lusitania which, till now, has not yet been identified outside Iberia. As regards the white marbles employed in the statuary, petrographic study in thin section and the δ¹³C and δ¹⁸O isotopic data emphasize the frequent use of Lunense and Pentelic marbles. The latter also features among the white marbles used for crustae and architectural elements together with the dolomitic variety of Thasian marble, Parian marble from Lakkoi, Proconnesian marble from the island of Marmara, an unknown marble most probably of local origin, and a variety of the so-called “greco scritto” whose provenance still remains unclear. In fact, its petrographic and geochemical features do not match those known for the classical “greco scritto” from Cap de Garde, near Annaba (Algeria).     

Three-dimensional modelling of statues: the Minerva of Arezzo

The Minerva of Arezzo is an ancient bronze statue located at the Museo Archeologico in Florence and currently under repair at the Restoration Centre of the Soprintendenza Archeologica of the Tuscany Region. We assembled a complete three-dimensional (3D) digital model of the Minerva before the restoration started. More 3D models will be produced to keep track of the variations that occurred during the restoration process, up to the final acquisition of the form of the restored artwork. The modelling of the Minerva will be the focal point of an ambitious “Minerva Project” that involves the integration of data from other sources in a 3D digital model of the object. Besides this, the project is aimed at showing how 3D techniques can be used to design useful and easily manageable new tools for the diagnostics of archaeological objects. 3D measurements have been realized by means of a high-resolution laser scanner developed at National Institute for Applied Optics (INOA). The instrument is composed of commercial low-cost components in order to be competitive with the very expensive commercial devices. Besides this, our scanner is supported by an efficient and flexible software developed by Consiglio Nazionale delle Ricerche (CNR) that supports all the post-processing phases of a 3D scanning session (range data alignment, merge and simplification).     

Painting technique and state of conservation of wall paintings at Qusayr Amra,Amman – Jordan

The state of conservation of the frescoes at Qusayr Amra was investigated by integrated physico-chemical measurements, particularly optical and scanning electron microscopy, μ-infrared spectroscopy, mass spectrometry and X-ray powder diffraction.The frescoes appear darkened and severely damaged owing to the deterioration of surface treatments and to the widespread presence of different salts. These caused significant detachments of the painting layers.The materials used in the painting and preparation layers and in the wall plaster were characterized in order to clarify the painting technique. Different pigments have been identified on the pictorial layers: the original ones correspond to green earth, yellow and red ochre, realgar, bone black and lapis lazuli and others, such as titanium white and cobalt blue, ascribed to recent restoration works. Proteinaceous materials (egg or yolk) have been identified as binders for the pigments, indicating a tempera painting technique.Three different products, belonging to restoration interventions, have been also identified: the natural product shellac, ascribable to next-to-last restoration and scarcely removed during last restoration also because of its insolubility, the synthetic vinyl acetate and acrylic polymers. Particularly the latter one indicates an on-going degradation of the applied fixatives.The extensive deterioration of the surfaces is also due to the widespread presence of salts, such as chloride, nitrate and sulphate clearly identified by XRD measurements.The obtained results give a comprehensive overview on the employed painting technique and its state of preservation and on the causes of the painting deterioration. Therefore, they are a fundamental tool to develop durable and compatible materials and methodologies for a future conservation strategy of this site.     

文萃

别人的教训 一位雕塑家得到一块质地非常好的大理石。他觉得大理石非常适合雕刻一个人像,于是,他拿起凿子。不知是因为紧张还是用力过猛,只那么一凿,他就敲下了一大块碎片。     

Electronic paramagnetic resonance as a tool for studying the blackening of Carrara marble due to irradiation by a Q-switched YAG laser

Several authors or practitioners have remarked that Q-switched lasers have an immediate blackening effect on mineral pigments such as vermilion (HgS), minium (Pb₃O₄), white lead [2PbCO₃. Pb(OH)₂], malachite [Cu₂CO₃.(OH)₂] and yellow ochre (FeOH₂). A similar blackening effect can also be obtained on veined Carrara marble. This stone contains several trace elements, particularly iron and manganese. The latter is responsible for a particular response in electronic paramagnetic resonance spectroscopy (EPRS). We investigated the effect of a Q-switched Nd:YAG laser irradiation on the EPRS trace of Carrara marble. Further experiments were performed with the same analytical tool on synthetic calcite powders that had been doped with iron or manganese. It appeared that no significant change of EPRS trace could be put into evidence after laser irradiation of both bulk Carrara marble or doped calcite powders, although some samples had actually blackened. It is concluded that the blackening effect is not due to an oxidation of manganese ions. Colour measurements were also performed within the framework of this investigation. They showed that the colour shift to black is proportional to the fluence applied to the stone surface     

Low-cost sensor system for online monitoring during laser cleaning

The self-limiting effect during laser cleaning only occurs in a limited amount of specific applications in restoration (e.g. removal of black crust from white marble). In all the other cases, a control of the removal process will be necessary either by the operator himself or by the employment of sensor equipment. Various methods, mainly spectroscopic (e.g. LIBS), have been investigated and proposed by others. Despite the fact that these have been shown to be promising, they all have in common rather high investment cost close to that of the cleaning equipment. Furthermore, this highly sophisticated control equipment is not easy to handle by conservators in practice. As an alternative low-cost method, we employed a simple photodiode to detect the scattered light from the irradiation area on the artwork surface. In many cases, this signal contains several pieces of information on the layer just being removed. The scattered radiation detected by the photodiode originates from the laser-induced plasma as well as reflected laser radiation. A separation, if necessary in order to separate the information, is possible by spectral filters. First applications during laser cleaning of corroded metal, encrusted glass and stone were promising. It has turned out that there is a distinct influence on the scattered light amplitude or even the pulse-bandwidth once the laser has removed the encrustation completely. The corresponding signal can be used in a closed loop control or for online monitoring.     

Enzymatic decolorization of bacterial pigments from culturally significant marble

Marble monuments and facades are susceptible to microbial colonization. Microbial growth on a marble surface can develop into unsightly red stains whose removal has proven problematic. The purpose of this study was to determine if the red-brown stains on Isamu Noguchi's marble sculpture Slide Mantra (1991) could be caused by pigment-producing microorganisms and to assess the potential of enzymatic stain remediation. Traditional cell culture methods were used to isolate a pigmented bacterium from a stained area of the sculpture. Sequencing and analysis of the 16S rRNA gene identified the organism as a strain of Serratia marcescens, and FT-IR spectroscopy demonstrated that the pigment produced by the bacteria was most likely a prodigiosin. Decolorization of the pigment in solution demonstrated that the enzyme laccase from the fungus Trametes versicolor has potential as a decolorizing agent. This study suggests that enzymatic decolorization may be applicable to stains on culturally significant marble caused by microbial colonization.     

Geometric characterization of a cylinder-shaped structure from laser scanner data: Development of an analysis tool and its use on a leaning bell tower

The romanesque-byzantine style, 1000 year old leaning bell tower of Caorle (Venice Province, Italy) is a unique masonry structure, characterized by single and double lancet windows harmonically distributed on a cylinder-shaped shaft surmounted by a conic cusp. A terrestrial laser scanning (TLS) survey was carried out in 2011 and some analyses were performed on the resulting point cloud to provide the following: bell tower leaning angle, wall inclination/tapering and radius, local deviation from circular shape, and local curvature. Emphasis was placed on the changes of these quantities with elevation. In order to perform these analyses, a MATLAB/Octave toolbox was developed and is available as supplementary material of this paper. In this way, a reliable picture of the current geometry of the bell tower was obtained. In particular, a correlation between leaning angle (average value 1.4° towards East-South-East) and some surface deformations and damage (bulges, brick displacements or also material loss) was found. These results are useful for cultural heritage preservation purposes.     

The history of the “Virgin with Child” sculpture (Ottaviano,Naples, southern Italy): Hypotheses from archaeometric multi-technique investigations

A life-size whitish marble statue of a “Virgin with Child” has been recently rediscovered in the St. Rosario church located in Ottaviano, a small town near Naples (southern Italy). This artwork shows stylistic features of the Tuscan-Roman school of the 16th century, and is framed in an intriguing historical context. Historical documents testify that the sculpture was a property of the cadet branch of noble Tuscan family of the Medici, the Medici of Ottaviano. A multianalytical approach has been used to try to indicate the supply area of the white marble of the studied sculpture. Considering the whole mineralogical, petrographic and geochemical data, the source rock can be possibly limited to the main classical white marbles of the Mediterranean district, as the classical marble of Aphrodisias. A reuse practice of a former artwork can be also hypothesized.     

A multi analytical approach to discriminate the Italian colored marble “Portasanta” from Portasanta “Marmor Chium” (Greece)

The “marble” Portasanta from Caldana (Grosseto, Italy) takes its name from the strict resemblance to the ancient Marmor Chium (also this called Portasanta) coming from the island of Chios in Greece. In this work, a complete characterization of the Italian Portasanta was made: chemical (X-ray fluorescence and isotopic analyses), mineralogical (X-ray diffraction on the bulk sample and on insoluble residue after acid attack), physical (water accessible porosity and imbibition coefficients), petrographical (optical microscopy) and mechanical (compressive strength test) analyses have been carried out in order to obtain a better distinction between Italian and Greek “marble”, sometimes mistaken. The Portasanta from Caldana (Italy) was quarried surely in the sixteenth century, but a possible use of this material during Roman time is still in doubt. The study of samples from some Tuscan archaeological sites (Torraccia di Chiusi, Siena and Roselle, Grosseto) allows us to confirm the use of Portasanta from Chios for the Torraccia site and that of Caldana for Roselle.     

Efflorescence on thin sections of calcareous stones

Limestone and marble, still frequently used as building materials are especially vulnerable to the destructive effects of efflorescence. The effect of interaction between five different calcareous stones and corrosive atmospheres has been investigated. A novel technique of stone degradation analysis has been used where thin sections of fresh stone materials were exposed in a corrosion chamber under controlled conditions (temperature, relative humidity (RH), SO₂ and NO₂ concentration). Following 1-week’s exposure; observations of the initial crystallisation were studied by light microscopy, scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD). The results obtained from the surface analysis clearly showed sulphation of the samples and formation of gypsum. Observations of the initial corrosion indicated differences in the location of efflorescence and its shape among and within the samples. The mineralogy, grain shape and size, mineral defects and existence of cracks and pores, all influenced the substrate’s reactivity. The most vulnerable areas and the places where the corrosion started on the calcitic stones were the triple grain junctions followed by grain boundaries, and on the dolomitic marble cracks and pores.     

Effect of the addition of hydroxyl-terminated polydimethylsiloxane to TEOS-based stone consolidants

Hybrid stone consolidants prepared from tetraethoxysilane (TEOS) and α,ω-hydroxyl-terminated polydimethylsiloxane (PDMS-OH) have been considered as one of the most promising approaches to improve the effectiveness of traditional alkoxysilane-based formulations. They have emerged as response to the negative reports commonly found in the literature: the resulting silica gel phase (SiO₂) tends to develop fractures and fissures inside the stone as the gel shrinks during the drying stage. In this work, we employed SEM, solid-state ²⁹Si NMR spectroscopy and compressive tests to characterize SiO₂-PDMS hybrid gels. We report the morphological characteristics exhibited by gels prepared in vitro and in situ. It was found an appreciable reduction of gel fracture for hybrids prepared from 5% w/w of PDMS. As TEOS polycondenses, PDMS-OH is chemically incorporated into the gel matrix via Si-O-Si bonds. The inclusion of these elastic chains provides the necessary flexibility to resist the stress imposed by capillary pressure. Additionally, an important hydrophobic character is imparted to the stone.     

Yellowing effect and discoloration of pigments: experimental and theoretical studies

Two issues of great interest in the field of lasers in artwork conservation are the so-called yellowing effect and the discoloration of pigments. We have viewed these issues from a comprehensive point of view, considering all our present experimental results as well as ongoing modeling and theoretical calculations. The first concern to be discussed is the yellowing effect in laser cleaning of marble or stone artifacts. Although, in most cases, a yellowish layer exists underneath the black encrustation, the so-called ‘patina’, it has become clear that there are situations where yellowing cannot be attributed to an existing layer. In the present study, a light scattering model that may account for the yellowing is presented. This model considers a thin absorbent layer and the surface roughness and/or created voids and accounts for the reflectance spectra measured by (i) hyper-spectral imaging and (ii) integrating sphere. Additional experimental data, such as the absence of yellowing when the third harmonic of a Q-swihed Nd:YAG laser is used, support this model. A thorough understanding of the quantitative characteristics of pigment discoloration, on the other hand, has been attempted by means of X-ray diffraction and theoretical studies. The model developed suggests a nucleation process for cinnabar resulting in a structural modification within the volume of a pigment’s crystal or particle close to the ‘ablation front’, which extends for a few nanometers from its surface.     

Laser cleaning of stone artefacts: a substitute or alternative method?

Laser cleaning of ancient marble and stone monuments is nowadays a well-accepted procedure in conservation. Operators and specialists are presently divided between those giving maximum confidence to this method of cleaning and others who, in contrast, are strongly critical and emphasise possible drawbacks and damages caused by laser. As any other method of cleaning, laser also offers advantages but at the same time is limited by drawbacks. Most conveniently, it should be integrated with the other methods. On the basis of the above considerations, the authors are proposing a combined procedure using laser cleaning as a first step – in this way removing the relevant part of black gypsum encrustation – followed by chemical methods as a final step. The latter, conveniently, could be accomplished by means of anion exchange resins, better than ammonium carbonate. An experimental check-up was made of four different methods applied on three different lithotypes on the façade of the Florentine Cathedral. Laser, micro-sandblasting, ammonium carbonate and sodium EDTA were the cleaning methods compared, whilst Carrara marble, green Serpentine (Verde di Prato) and a red limestone were the three lithotypes. Results are commented by showing differences in the advantages and drawbacks among the different methods on the various lithotypes.     

Innovative uses of 3D digital technologies to assist the restoration of a fragmented terracotta statue

This paper describes how some innovative methodologies have been designed and employed to support the restoration of the Madonna of Pietranico, a terracotta statue severely damaged in the 2009 earthquake. The statue, fragmented in many pieces, has undergone a complex restoration performed by a multidisciplinary working group. The contribution of digital technologies was planned from the very beginning, since the complexity of this restoration originated the design of innovative procedures for managing the reassembly and restoration process. The Madonna test bed was therefore an example of how technology innovation could be pushed by clear application needs. A first important contribution was the study of the recombination hypothesis of the fragments. This initial phase was performed on digitized 3D models of the statue fragments, with the aim of reducing fragments manipulation, preventing further damages and increasing the capabilities to rehearse and evaluate different reassembly options. The accuracy of the 3D scanned models and the new recombination procedure introduced in this paper allowed to manage this phase in the digital domain with successful results. The digital 3D models were also used to design and produce an innovative supporting structure, constructed with a rapid prototyping device. Another important contribution concerned the study and virtual restoration of the polychrome decoration of the statue; our aim was to reproduce and restore in the virtual 3D domain the very complex original polychrome decoration, on the base of the remaining traces. Consequently, new virtual painting functionalities have been designed on the MeshLab platform (an open-source tool for 3D models visualization and manipulation) for reproducing pictorial decorations over digital 3D models and have been assessed on this specific test bed. This allowed us also to investigate the complexity of the virtual repainting process and to identify further technology enhancements. Finally, computer graphics technologies have been also used to produce a video that tells the story of the restoration.     

Tailoring new fluorinated acrylic copolymers as protective coatings for marble

The protective performances of two new fluorinated acrylic copolymers (based on the monomers 1H,1H,2H,2H-perfluorodecyl methacrylate (XFDM) and 1,1,1,3,3,3-hexafluoroisopropyl methacrylate (HFIM) are evaluated and compared with Paraloid B72, a commercial copolymer ethyl methacrylate/methyl acrylate (EM/MA) and its partially fluorinated homologous 2,2,2-trifluoroethyl methacrylate/MA (TFEM/MA). The polymeric materials have been tested on Candoglia marble, a very low open porosity stone (<1%) used in Italian historical architecture (Milan Cathedral). The copolymers were tested according to UNI-Normal protocol. Measurements of capillary water absorption, static contact angles, colour variation and water vapour permeability were made before and after accelerated photo-ageing. FTIR spectroscopy and size exclusion chromatography (SEC) analyses were carried out on samples aged on inert substrate to assess the photostability of the copolymers themselves. The relationship between the protection efficacy and the fluorine content and distribution has been investigated. The results obtained show that the addition of a partially fluorinated co-monomer to the Paraloid B72 copolymer structure did not result in the expected improvement of the coating properties, while the fluorination of the copolymer side chain gave rise to encouraging performance.