Effectiveness of granite cleaning procedures in cultural heritage: A review

STOTEN-20472; No of Pages 12 Science of the Total Environment xxx (2016) xxx–xxx

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Review

Effectiveness of granite cleaning procedures in cultural heritage: A review J.S. Pozo-Antonio a,⁎, T. Rivas a, A.J. López b, M.P. Fiorucci b, A. Ramil b a

Departamento de Enxeñaría dos Recursos Naturais e Medio Ambiente, Escola de Minas, Universidade de Vigo, 36310 Vigo, Spain Laboratorio de Aplicacións Industriais do Láser, Centro de Investigacións Tecnolóxicas, Departamento de Enxeñaría Industrial II, Escola Politécnica Superior, Universidade da Coruña, Campus Ferrol, 15471 Ferrol, Spain

b

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Most of the Cultural Heritage built in NW Iberian Peninsula is made of granite • Biological colonization, black crusts and graffiti are found on granitic stones. • A review centred on granite deterioration forms and their cleaning procedures. • Study on traditional cleaning procedures (chemical and mechanical) and laser ablation. • Global cleaning effectiveness performed considering the coating extraction and the damage on the granite.

a r t i c l e

i n f o

Article history: Received 11 May 2016 Received in revised form 12 July 2016 Accepted 13 July 2016 Available online xxxx Editor: D. Barcelo Keywords: Stone cleaning Granite Cleaning effectiveness Cultural heritage Black crust Graffiti Biological colonization

a b s t r a c t Most of the Cultural Heritage built in NW Iberian Peninsula is made of granite which exposition to the environment leads to the formation of deposits and coatings, mainly two types: biological colonization and sulphated black crusts. Nowadays, another form of alteration derives from graffiti paints when these are applied as an act of vandalism. A deep revision needs to be addressed considering the severity of these deterioration forms on granite and the different cleaning effectiveness achieved by cleaning procedures used to remove them. The scientific literature about these topics on granite is scarcer than on sedimentary carbonate stones and marbles, but the importance of the granite in NW Iberian Peninsula Cultural Heritage claims this review centred on biological colonization, sulphated black crusts and graffiti on granite and their effectiveness of the common cleaning procedures. Furthermore, this paper carried out a review of the knowledge about those three alteration forms on granite, as well as bringing together all the major studies in the field of the granite cleaning with traditional procedures (chemical and mechanical) and with the recent developed technique based on the laser ablation. Findings concerning the effectiveness evaluation of these cleaning procedures, considering the coating extraction ability and the damage induced on the granite surface, are described. Finally, some futures research lines are pointed out. © 2016 Elsevier B.V. All rights reserved.

⁎ Corresponding author at: Institute of Electronic Structure and Laser (IESL), Foundation for Research and Technology (FORTH), Heraklion 71110, Greece. E-mail address: [email protected] (J.S. Pozo-Antonio).

http://dx.doi.org/10.1016/j.scitotenv.2016.07.090 0048-9697/© 2016 Elsevier B.V. All rights reserved.

Please cite this article as: Pozo-Antonio, J.S., et al., Effectiveness of granite cleaning procedures in cultural heritage: A review, Sci Total Environ (2016), http://dx.doi.org/10.1016/j.scitotenv.2016.07.090

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J.S. Pozo-Antonio et al. / Science of the Total Environment xxx (2016) xxx–xxx

Contents 1. 2.

The processes of granite deterioration . . . . . . . Cleaning techniques applied to granitic monuments 2.1. Mechanical cleaning techniques . . . . . . 2.2. Chemical cleaning techniques . . . . . . . 2.3. Laser cleaning . . . . . . . . . . . . . . 3. Conclusions and future research lines . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . .

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1. The processes of granite deterioration Granite is the most common stone in the Cultural Heritage (CH) of NW Iberian Peninsula. Despite of being a silicate rock, granite is as susceptible as carbonate stones to alterations caused by certain agents, which can cause a significant deterioration within a short period of time (Silva et al., 2003; Venice Charter, 1964). The CH built of granite is affected by some deterioration forms that compromise its durability (Silva et al., 2003; Schiavon et al., 1995).

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In 2008, the Illustrated Glossary on Stone Deterioration Patterns (ICOMOS, 2008) was published in order to standardise the terms of the most common deterioration forms in the stone used in CH. Following this Glossary, ones of the more important forms of granite deterioration are black crusts, biological colonization and nowadays graffiti (ICOMOS, 2008). Black crusts are the coherent accumulation of materials on the surface frequently dark coloured. Crusts may have a homogeneous thickness, and thus replicate the stone surface, or have irregular thickness

Fig. 1. A–B: Sulphated black crusts on the walls of the former Convento de Hermanitas de los Ancianos Desamparados in the center of Vigo (Spain). C: Micrograph with Scanning Electronic Microscopy in Secondary Electrons mode of a superficial sulphated black crust (×300) showing the acicular shape of the gypsum crystals. D: Sculpture with black crust in the Cathedral of Porto (Portugal). Sources: A–C: The authors; D: Milagros Abal.

Please cite this article as: Pozo-Antonio, J.S., et al., Effectiveness of granite cleaning procedures in cultural heritage: A review, Sci Total Environ (2016), http://dx.doi.org/10.1016/j.scitotenv.2016.07.090

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and disturb the reading of the stone surface detail (ICOMOS, 2008). They are generally developed on places protected against direct rainfall or water runoff within an urban environment (Fig. 1A, B and D) and they usually adhere firmly to the substrate. They are mainly composed of airborne particles, which are trapped into a gypsum matrix (Fig. 1C). Currently, the studies about the crust formation have been focused almost exclusively on sedimentary carbonate stones and marbles (Brimblecombe and Grossi, 2007 and reference therein) while the studies on granite stones are scarcer (Simão et al., 2006; Rivas et al., 2014). According to the literature, the black crusts on carbonate sedimentary stones and marbles derive from the interaction of the bedrock with air pollution, particularly sulphur dioxide, carbon dioxide and mono-nitrogen oxides (ICOMOS, 2008; Brimblecombe and Grossi, 2007; Schiavon, 2000). The final product of the reaction of sulphur dioxide in a calcium-rich carbonate stone is gypsum, with different physicochemical properties than the substrate. In the case of granite, it has been determined that a formation of black crusts is also possible despite its low calcium content (Simão et al., 2006; Rivas et al., 2014; Silva et al., 2010; Rivas et al., 1997). Studies in an artificial atmosphere determined that a sulphating process on granite is possible in a relatively sulphur dioxide-depleted atmosphere (10 ppm); this compound reacts with the calcium from joint mortars (Rivas et al., 1997). In a study by Simão et al., the presence of organic particles from petrol and diesel in atmospheres with high sulphur dioxide levels (100 ppm) led the formation of black crusts on several igneous stones (gabbro, syenite and granite); in these cases, calcium derived through acid dissolution of the calcium plagioclase (Simão et al., 2006). Silva et al. have differentiated between black crusts in urban environments and biological colonization; the last preferably developed in rural areas (not polluted environments) (Silva et al., 2009). Although in the studied urban black crusts, the burning of fossil fuels (by industry and road traffic) was the most probable source of sulphur, the contribution of other construction materials that carry sulphur in their composition to the whole process was proved (Rivas et al., 2014; Sanjurjo-Sánchez et al., 2011). McAlister et al. have indicated that the material from fossil fuel burning processes has particles that accelerate or catalyse the formation of crusts (McAlister et al., 2008). Biological colonization is considered the growth of organisms (i.e. algae, fungi, lichen, bacteria and cyanobacteria) on the stone surface, reducing the value of the monument (ICOMOS, 2008). This biological growth can compromise the artistic and historical values of the CH object and it will contribute to the physical and chemical decay (Piervittori et al., 2004; Silva et al., 2009; Gaylarde et al., 2012). Fig. 2A–C, shows digital photographs of different biological colonization on granite surfaces: algae patina (Fig. 2A), lichenic crust (Fig. 2B), biological colonization with plants (Fig. 2C). In Fig. 2D, a SEM micrograph of a patina composed by filamentous green algae is shown. The physical damage is caused by the penetration of various biological structures (e.g. the fungal hyphae) through the fractures, holes and cracks of the stone and the subsequent expansion and contraction under changes of humidity levels (Prieto et al., 2002; Prieto and Silva, 2005). Regarding the chemical damage, organisms can generate different types of acids, e.g. oxalic acid, capable of chelating ions such as calcium, leading to mineralogical transformations or mineral neoformations (such as oxalates or gypsum) (Mohammadi and Krumbein, 2008). Biological colonization is mainly found in rural and low polluted environments (Prieto and Silva, 2005). This deterioration form is generally called biodeterioration, firstly proposed by Hueck (Hueck, 1965), indicating a scientifically verifiable change in the physicochemical properties of the stone due to the action of the organisms (Hueck, 1965; ICOMOS, 2008; Miller et al., 2012). The biological colonization can take place more or less easily, depending on the bioreceptivity of the substrate (Urzı̀ and Realini, 1998; Miller et al., 2012). Although granite shows scarce contents in alkali and alkalineearth elements and its hydrolysis process (principal weathering process) is slow, it is a bioreceptive stone (Prieto and Silva, 2005). The pH of abrasion and certain physical properties (porosity, capillary, absorption, etc.) influence on the degree of the colonization (Prieto and Silva,

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2005). The scientific contribution about the biodeterioration on carbonate stone is more abundant than on granite (Warscheid and Braams, 2000 and references therein). For all kind of stones (carbonate and silicates stones), the most common biological colonization is caused by the growth of green algae and the adherence of dust and atmospheric particles (Grant, 1982; Macedo et al., 2009 and references therein). The chlorophyta constitute the most common group of algae colonizing CH stones, being identified on granite monuments the following taxa: Apatococcus sp., Chlamydocapsa sp., Chlorella sp., Chlorococcum sp., Chlorosarcinopsis sp., Coccomyxa sp., Desmococcus sp., Klebsormidium sp., Monoraphidium sp., Muriella terrestris sp., Scenedesmus sp., Stichococcus sp., Trentepohlia sp., Tetracystis sp. (Macedo et al., 2009 and references therein). Generally, algae and cyanobacteria are the first organisms to colonize the stone surface because they only require light, water and carbon dioxide taken from the atmosphere. The most common cyanobacteria found on granite have been: Chroococcidiopsis sp., Gleocapsa sp., Phormidium sp., Plectonema sp., Plectonema sp., Pleurocapsa sp., Synechococcus sp. (Macedo et al., 2009 and references therein). They are responsible for the patinas of intense greenish or reddish colour covering buildings in zones with humid climate (Crispim et al., 2003; Prieto and Silva, 2005; Gaylarde et al., 2007; Silva et al., 2009; Macedo et al., 2009). Fungi (heterotrophic organisms) are other organisms found in the biological colonization of stones. It was reported that fungi develop different metabolic pathways to survive, such as the generation of Extracellular Polymeric Substances (EPS) and dark pigments, e.g. melanin (Gorbushina et al., 1993; Gaylarde et al., 2007). In a study focused on marble, Exidia sp. produce during the expansion of their hyphae some substances that cause the pH drops, leading to the dissolution of carbonate materials and the formation of black crusts (Sarró et al., 2006). Regarding granite stones, Gaylarde et al. (2007) have evaluated the deterioration processes due to the fungi action in some granite forming minerals. As for carbonate stones, the mucilage of fungi gives to the granite surface a very dark colour, which prevents the appreciation of the artistic value of the artwork (Sabbioni et al., 2001, 2003; Bonazza et al., 2005; Gaylarde et al., 2007). Another group of colonizing organisms is lichens, whose deteriorating action has also been widely studied in carbonate stones (Griffin et al., 1991 and references therein). Considering the scientific literature about deterioration processes on granite, lichens have been reported as agents of physical changes, e.g. the increase of the cracking by growth and penetration of biological structures through the fissure system (Prieto and Silva, 2005). They also can affect the chemical stability of some minerals, e.g. mineral transformation and neoformation (Prieto and Silva, 2005). Bacteria, such as sulphur cycle bacteria were also identified on stones (Flores et al., 1997). They participate in the neoformation of gypsum through the oxidation of reduced sulphur compounds present in the surface, transforming them into sulphates (Flores et al., 1997). The activity of these bacteria is higher in carbonate stones than in granite, because the calcium is easily releasable to the environment in an acidic medium. Regarding the composition of biological colonization on granitic surfaces, Silva et al. (2009) provided a very detailed study of samples from natural outcrops and CH monuments. The colonizing organisms were divided in two groups considering the substrate: on the outcrops, there were plentiful of algae of the genus Chlorella and cyanobacteria of various genera (Xenococus of the Pleurocapsales order, Pseudophormidium of the Oscillatoriales order, Stigonema ocellatum of Nostocales order). On the monuments, the authors mainly reported bacteria from Bacteroidetes and Proteobacteria orders, related to animal feces. It was also stated that the carbon present in the colonization of the outcrops was mainly of biological origin, while in the monuments, this carbon was derived from human activities, e.g. the contamination caused by traffic.

Please cite this article as: Pozo-Antonio, J.S., et al., Effectiveness of granite cleaning procedures in cultural heritage: A review, Sci Total Environ (2016), http://dx.doi.org/10.1016/j.scitotenv.2016.07.090

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Fig. 2. A: Biological colonization on the Fountain of the Cathedral of Porto (Portugal). B: Biological colonization on a column and the wall of the Cathedral of Santiago de Compostela (Spain). C: Lateral façade of Cathedral of Porto (Portugal). D: Micrograph with Scanning Electronic Microscopy in Secondary Electrons mode of a biological colonization (×1000) showing the filamentous and globular biological structures. Sources: A and C: Milagros Abal; B and D: The authors.

Nowadays, it is impossible to ignore graffiti paints and their unfavourable impacts on CH monuments (Fig. 3). Illustrated Glossary on Stone Deterioration Patterns defined graffiti as engraving, scratching, cutting or application of paint, ink or similar matter on the stone surface

(ICOMOS, 2008). Their uncontrolled application is not just an aesthetic problem; it is a serious threat to the conservation management (ICOMOS, 2008). As other deterioration forms, the graffiti impact has been mostly studied for carbonate sedimentary stones and marbles

Fig. 3. A: Graffiti at the lower part of the XVIII century dockyard gate (made by granite) in Ferrol (NW Spain). B–D: Graffiti in granitic facades and walls of the historic centre of different cities at NW Spain. A: Pontevedra, C: Vigo, D: Ferrol. Source: the authors.

Please cite this article as: Pozo-Antonio, J.S., et al., Effectiveness of granite cleaning procedures in cultural heritage: A review, Sci Total Environ (2016), http://dx.doi.org/10.1016/j.scitotenv.2016.07.090

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(Sanjeevan et al., 2007; Sanmartín et al., 2014; Carvalhão and Dionísio, 2015), while for granite, only a few studies have been published (Fiorucci et al., 2013; Rivas et al., 2012; Pozo-Antonio et al., 2016a, 2016b). These researches were mainly focused on the evaluation of different cleaning procedures. Graffiti is made of aerosol or spray, markers and pens on substrates of different nature. Graffiti paints are composed by pigments, e.g. iron oxide, carbon black or aluminium (inorganic compounds) that provide colour, mixed with synthetic resins (thermoplastic or thermoset) (Sanmartín et al., 2014; Carvalhão and Dionísio, 2015; Fiorucci et al., 2013; Rivas et al., 2012). Solvents (most commonly acetone, alone or in combination with alcohols or esters) allow the pigment-resin mixture to flow (Segalini et al., 2000). Pigments and resins are the responsible components of the penetration of graffiti into the stone (Dubin, 2002). A summary of the main types of graffiti paints, their bulk composition, the more common surface finishes where they are applied and the difficulty level to be extracted, is shown in Table 1. 2. Cleaning techniques applied to granitic monuments The basis of Conservation and Restoration of CH is focused on the preservation of the appearance, the material and the cultural integrity of the monuments. Since the 1960s, several agencies and commissions were created in order to safeguard the CH, e.g. ICOMOS, RILEM, ICCROM-UNESCO, different departments of the Italian CNR, such as ICVBC, ITABC, IBAM, etc. Also, the international charters for the conservation and restoration of monuments and sites, e.g. Venice charter, stated the modern concept of Restoration as a synonymous of Conservation (Venice charter, 1964). Its objective started to be the protection of the work and its historical witness set, in order to provide an integral transmission to the future generations (Venice charter, 1964). Cleaning must be a smooth and delicate procedure destined to remove surface-dirt (deposits and coatings) without damaging the stone substrate and avoiding the possible by-product formation and cleaner remains. Moreover its results do not have to be extrapolated to other types of stone and surface deterioration forms (Smith et al., 2008; Doehne and Price, 2010). In general terms, the cleaning of a CH monument should fulfil the following requirements (Lazarini and Tabasso, 1986; Bellmunt et al., 2002; Doehne and Price, 2010): – No damages on the original material, such as abrasion, micro-fracturing, porosity increases, dissolutions, mineral transformations or colour changes. – It should be gradual (the operator must control the intensity of the Table 1 Graffiti types more often found in Cultural Heritage objects. Their compositions, the more common surface finishes where the paints are applied and the difficulty level to be removed, are also shown in the table (Bellmunt et al., 2002). Type of graffiti

Common surface finish

Level of difficulty to remove

Spray or aerosol (polyurethanes, lacquers and smalts)

All types of surface finishes (polished, bush-hammered, cutting disc, etc.) All types of surface finishes (polished, bush-hammered, cutting disc, etc.) Flat surfaces

High on rough surfaces

Brushed painting (oils and synthetic resins: vinyl, acrylic, acetates) Permanent markers (solvent-based)

No-permanent marker (water-based) Pens

Flat surfaces Flat surfaces

High on rough surfaces

Low on non-permeable surfaces (non-porous) and high on permeable (porous) surfaces Low on all surfaces Low on non-permeable (non-porous) surfaces and high on permeable (porous) surfaces.

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application). – The processes must be selective. – The cost of the intervention (skilled workforce, scaffolding, technical equipment and products, etc.) must be affordable. The operator in charge of applying the appropriate cleaning technique must be aware of the following facts (Lazarini and Tabasso, 1986; Bellmunt et al., 2002; Doehne and Price, 2010): – The physical-chemical and mechanical properties of the materials to be cleaned. – The proper application of the cleaning procedure. – Performance of in situ and in laboratory testing trials in order to evaluate the cleaning effectiveness. The operator skills are decisive in the cleaning results. In general, the choice of the suitable cleaning method must be taken according some considerations (Bellmunt et al., 2002): – The diagnosis of the surface to treat (type of stone, extent and severity of the decay alteration). – The area to be cleaned. A wide range of techniques is available for cleaning stone, ranging from those that are intended for use on large facades to those that are intended for meticulous use on finely carved and delicate sculpture. – The accessibility to the work place. There are different cleaning techniques. Water based cleaning techniques are usually applied in rehabilitation of buildings without historical and artistic interests; nevertheless, the Spanish regulations do not recommend the use of water-based cleaning methods in CH buildings, since the water supply can endanger other sensitive materials (paintings) or, in the case of deterioration related to soluble salts, favours their mobilization towards other unaffected structures. In this meaningless, the water based cleaning methods have not been considered in this article. Accordingly, the cleaning methods considered in this work are those usually applied in real equity interest which are mechanical and chemical procedures (Lazarini and Tabasso, 1986; Cooper et al., 1995; Rodríguez-Navarro et al., 2003; Sanmartín et al., 2014). Concerning the granite, the scientific evaluation of cleaning procedures is once again limited in comparison with carbonate stones. Cleaning results are undoubtedly affected by the properties of granite, especially the fissure-type porous system and the polymineral character as was reported in other types of interventions, e.g. consolidation and waterproofing (Mosquera et al., 2000a, 2000b; Rivas et al., 2001, 2003). These characteristics (pore size distribution, texture, structure and mineralogical composition) are highly variable among varieties of granite; in consequence, the extrapolation to granite of the effectiveness results of cleaning performed on other type of stone is not recommended. And so, fortunately, studies that focus on defining the specific cleaning conditions for each particular case and stone type are becoming more numerous since 90s. The possible procedures to clean a specific deposit or coating should be tested on little areas to evaluate their global cleaning effectiveness, considering the deposit or coating extraction and also, the damage caused on the stone showing the harmfulness of the procedure (Kapsalas et al., 2007; Iglesias et al., 2008; Delegou et al., 2008). Both evaluations (extraction level and harmfulness) are usually determined in laboratory, using destructive techniques. For granite cleaning, the evaluation of the extraction level has been properly performed with polarized light microscopy, scanning electron microscopy coupled with energy dispersive X-ray spectrometry, Fourier transform infrared spectroscopy and X-ray diffraction (Rivas et al., 2012, 2013; Pozo-Antonio et al., 2016a, 2016b, 2016c, 2016d). In order to assess the risk associated with granite cleaning, besides some of the techniques

Please cite this article as: Pozo-Antonio, J.S., et al., Effectiveness of granite cleaning procedures in cultural heritage: A review, Sci Total Environ (2016), http://dx.doi.org/10.1016/j.scitotenv.2016.07.090

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showed above, it is possible to use X-ray fluorescence to find some residues or by-products on the surface and profilometry and interferometry microscopy to detect surface modifications (Pozo-Antonio et al., 2016b). However, currently, micro- or non-destructive techniques are being in situ applied in order to determine the global cleaning effectiveness without taking samples to analyse in the laboratory. Considering the scientific literature based on granite, the in situ techniques that have been used to evaluate the cleaning effectiveness are colour spectrophotometry (Brimblecombe and Grossi, 2007; Rivas et al., 2012) and hyperspectral imaging techniques (Pozo-Antonio et al., 2015). Usually, the use of more than one method is required to achieve better results (Bellmunt et al., 2002). The evaluation of the different procedures to extract a specific deterioration form allowed to avoid extra costs of cleaning in order to correct choice mistakes. For example, graffiti cleaning is an expensive procedure and a complex task to achieve. Nowadays, a lot of money is invested in graffiti cleaning campaigns by local authorities. For instance, Berlin and London annually spend several million dollars to clean the walls of their buildings and wagons of their rail networks (Scheerder et al., 2005; Slaton and Freedland, 2012). The cleaning has to be performed as quickly as possible in order to prevent the graffiti hardening. (Smith et al., 2008). 2.1. Mechanical cleaning techniques Mechanical cleaning procedures are based on the removal of alteration forms on the stone surface by abrading the surfaces. Among the mechanical methods, there are those based on brushing and others based on projection. Firstly, the classic brush with metal tips and the rotating disc of metal wire of different compositions, e.g. flint carbide. Scalpel is one of the most usual procedures to remove biological colonization in statues and monuments built with different stones, but it can induce damage if it is applied in the wrong way (Caneva et al., 2009). In a recent work, the scalpel was applied to clean Pertusaria amara lichen from a fine grained granite, but it did not achieve a satisfactory result as in comparison with the cleaning performed with laser or the combination of the scalpel followed by laser (Pozo-Antonio et al., 2016c). Regarding the mechanical procedures based on particle projection, they derive from the old method of sandblasting which is only used under certain circumstances (e.g. large areas without any artistic and historic interest) due to its high aggressiveness because of the high pressure and the high hardness of the projected sand. The development of new pneumatic and electrical systems (which can regulate the pressure, incorporate dust extraction and particle recycling systems and project dry particles or particles with a certain level of moisture) and the possibility to use projection particles of different composition and hardness (see Table 2) have allowed to fulfil the CH conservation and restoration requirements (Bellmunt et al., 2002). Different mechanical procedures can be used, e.g. brushing with water, hydro pneumatic cleaning, low pressure water projection, low pressure water spray and soft abrasive projection, being the last one the most common used in CH cleaning (Lazarini and Tabasso, 1986; Bellmunt et al., 2002). As was reported by Sanmartín et al. (2014) the scientific publications about these procedures are very scarce, mainly for granite. The most of the studies were focused on marble or limestones and the most common mechanical procedures are the soft-abrasive projection and the hydrocleaning with pressurized water; however, they induced notable surface damages as roughness increases (Ortiz et al., 2013; Carvalhão and Dionísio, 2015). In recent articles, Hydrogommage® (Fig. 4) which is based on the circular projection of a mixture of air-water-micro grained abrasive (99% silica content, 0.5–0.1 mm grain size) at low-pressure (0.5–1.5 bars) (www.clinarte.com) showed satisfactory results in the extraction of biological colonization and graffiti (red, blue, black and silver coloured paints) on a coarse grained granite but an increase of the surface

roughness was detected in all the cases (Pozo et al., 2013; Pozo-Antonio et al., 2016a, 2016b). 2.2. Chemical cleaning techniques Chemical methods are based on the application of solutions, generally alkalis and acids, which react with the deposits or coatings and dissolve them (Fig. 5) (Lazarini and Tabasso, 1986). However, these substances also attack the stone and generate soluble salts that can be harmful to the stone (Bellmunt et al., 2002) and theses products even may cause health risks (Langworth et al., 2001). Chemical products, e.g. benzalkonium chloride can be applied directly on the stone surface but usually some solvents are applied in a poultice (Fig. 5C) in order to prevent their penetration in the stone. These poultices are made by cellulosic queues, such as methylcellulose or carboxymethylcellulose with clays that have a strong adsorptive power, e.g. sepiolites. These poultices allow a satisfactory adhesion to vertical surfaces, a control of the solution penetration from the surface and the disaggregation of the deposit or coating by dissolution. Cellulose poultices are preferable because they are inexpensive, but they show problems in their extraction as was reported by Pozo-Antonio et al. (2016d) and it is shown in Fig. 5D. Before their application of chemical agent, surfaces should be wetted first (Lazarini and Tabasso, 1986; Doehne and Price, 2010). The products have to be homogeneously dispersed over the entire surfaces with a slow brushing. After a reaction as short as possible, the product is extracted by brush and the surface is cleaned with water to neutralize the solution. The operator can control the concentration of the product and the site of the application. For black crusts, the most commonly used acids are: hydrochloric acid, hydrofluoric acid, phosphoric acid and acetic acid (Hiscox and Hopkins, 1994). As basic-pH substances, caustic soda (NaOH) and caustic potash are also used (KOH) (Hiscox and Hopkins, 1994). The Papetta AB57® (Table 3·), developed by the Istituto Superiore per la Conservazione ed il Restoration (ISCR) in Rome, is an excellent cleaning agent for sulphated black crusts (Mora et al., 1984; Lazarini and Tabasso, 1986; Stavroudis et al., 2005). However, in a recent study about the cleaning of gypsum black crust on a fine grained granite, the Papetta AB57 did not achieve a complete removal (Pozo-Antonio et al., 2016d). The addition of sodium bicarbonate facilitates the detachment of the crust but its use should be avoided as far as possible in order to prevent the infiltration of soluble salts in the stone. The application time of the Papetta depends on the deterioration form characteristics; Pozo-Antonio et al. stated the drying time required for one application in 2 h (Pozo-Antonio et al., 2016d). After each application the surface has to be washed and brushed in order to remove any remaining pulp. El-Gohary (2009) used EDTA poultices to remove weathering red crusts on granites, finding a high level of efficiency. The application of chemical preparations is an effective method, but slower than the mechanical method; also, in many cases, the cleaning depends on the amount of necessary chemicals, which can result in higher operation costs. Generally speaking, it has to be highlight the need to properly remove the residues generated after the application of chemicals and thickeners, e.g. with water steam projection (Pozo-Antonio et al., 2016d). Biocides are the most common chemical products to remove biological colonization. The studies about biocide effectiveness are more abundant on carbonate stones, as limestone (Eyssautier-Chuine et al., 2015). Quaternary ammonium compounds are the most used biocides for their good efficacy against green algae in calcarenite stone (Nugari et al., 2009). However, treatments with quaternary ammonium have to be considered carefully because on the limestone walls of Lascaux Cave was proved the development of fungi due to their ability for growing using quaternary ammonium degradation products (Bastian et al., 2009; Martin-Sanchez et al., 2012; Saiz-Jimenez et al., 2012). Other biocides containing silver nanoparticles, showed good algaecide

Please cite this article as: Pozo-Antonio, J.S., et al., Effectiveness of granite cleaning procedures in cultural heritage: A review, Sci Total Environ (2016), http://dx.doi.org/10.1016/j.scitotenv.2016.07.090

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Table 2 Properties of the materials currently used in soft projection cleaning procedure (Bellmunt et al., 2002). Product

Shape

pH

Mohs hardness

Application surface

Calcium and magnesium carbonate Slag

Angular

9.9

3.8

Soft stones and bricks Dry and wet

Angular

7.5

7

Silicon Silicate glass Calcium carbonate Aluminium silicate

Angular Angular Angular Angular

Neutral 9.2 9 9.2

7 6 3 7

Aluminium oxide (corundum) Siliceous-sodium-calcium glass Siliceous-sodium-calcium glass Quartz powder

Angular Spherical Angular Angular

Neutral Neutral Neutral Neutral

9 5.6 5.6 6

Hard stones and metals General Structural elements Soft stones Hard stones and metals Hard stone Soft stones Soft stones Hard stones

performance in mortars (MacMullen et al., 2014). Satisfactory results about the degradation of organic matter and the inhibition of recolonization in marble and mortars by products with anatase (TiO2) due to its photocatalytic effect were recently reported (Fonseca et al., 2010; La Russa et al., 2014). Most of the research on the effectiveness and risks of biocides have been carried out in situ (Urzì and De Leo, 2007). However, the majority of the researches about the application of these novel products have been performed on carbonate stones and mortars, no works on granite were found. Regarding the application of biocides on granite, biological colonization was cleaned with ethanol, benzalkonium chloride, hydrochloric acid-solutions and commercial products as the commercial biocides Hyvar X® and LimpiaFachadas1® obtained satisfactory biological colonization extraction rates, but some of these procedures achieved deposition of soluble salts, even after the neutralization (Pozo et al., 2013). In contrast, Santamaría et al. (1996) concluded that the chemical cleaning does not cause a significant mineralogical alteration to weathered granite and that salts remaining after cleaning are remnant weathering products and not reaction products. The difference between those researches is that Santamaría et al. (1996) were focused on salt crystallization cleaning, while Pozo et al. (2013) on biological colonization cleaning. Many authors advocate the maintenance of a cleaning with the use of retreats in order to prevent biological growth or the formation of new patinas and crusts (Cuzman et al., 2008). Regarding graffiti, the cleaning is performed with water and neutral or non-ionic detergents which are applied directly or as poultices in order to prevent graffiti penetration into the substrate causing a change in colour (Amoroso and Fassina, 1983; Grimmer, 1988; Whitford, 1990; Dubin, 2002) There are a remarkable number of commercial products that are manufactured in form of pastes or gels, so that their application on surface is quite simple. For granite, scarce works were found about the chemical cleaning of graffiti in comparison with carbonate stones,

Observations

Its hardness allows a low pressure projection, and therefore, a maintained performance Wet Dry Wet Dry Dry Deep cleaning of soft coatings Dry and wet Dry and wet

e.g. an alkaline cleaner based on a solution of potassium hydroxide was satisfactory tested to remove red, blue and black graffiti on two carbonate stones, however some dissolution of grain boundaries was observed (Carvalhão and Dionísio, 2015). For granite, the extraction of red, blue, black and silver graffiti with commercial chemicals, e.g. the commercial products QuitaGraffi 200®-QuitaSombras 60® (Fig. 5A, B), was performed without a total extraction of the paints (Pozo-Antonio et al., 2016a; López et al., 2011). Also, it was registered in those cleanings, an important chemical contamination in the stone.

2.3. Laser cleaning Despite the application of lasers in stone cleaning started in the 70s with the pioneer work of Asmus et al. about the cleaning of incrustations in Venetian marble (Asmus et al., 1974), the use of this technique in the field of CH conservation has spread since the late 80s, parallel to a better understanding of the physical processes involved and advances in laser technology. As it was indicated by Siano et al. (2012), various hundreds of Nd:YAG laser systems are nowadays operative in CH conservation laboratories and restoration yards all over Europe and abroad. At the same time, laser technologies for conservation also increased their presence in exhibitions and fairs, as well as in formation and tutorial frameworks. Moreover, case studies of important masterpieces also stimulated the interest of mass media, which gave a big resonance to the present innovation, thus extending its dissemination up to the social level. The laser cleaning technique is based on the ablation process in which surface layers of a substrate are removed when they are irradiated with a laser beam. Laser ablation is a strongly non-linear process that occurs when the irradiation fluence (energy deposited by the laser per unit area) exceeds a critical threshold which is an intrinsic property of

Fig. 4. A–B: Application of the Hydrogommage procedure (mechanical procedure), which is based on the circular projection of a mixture of air-water-micro grained abrasive (99% SiO2 content, 0.5–0.1 mm grain size) at low-pressure (0.5–1.5 bars). Sources: A: www.clinarte.com, B: the authors.

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Fig. 5. A: Application of the products QuitaGraffi 200®-QuitaSombras 60®, a two phase graffiti cleaning solution (http://www.proliser.es) to remove different graffiti on granite. B: The product residues were washed using 120 bar. C: Application of a poultice to remove black sulphated crust. D: Result of a chemical cleaning of black crust and beeswax on northern façade of the cathedral of Ourense (NW Spain). Sources: the authors.

the material under irradiation. In CH, only pulse lasers with light emission in the form of short duration pulses are used for cleaning. The dynamic development of the laser ablation involves complex phenomena of different nature depending on the laser parameters (i.e. pulse width, wavelength, repetition rate, pulse overlap, etc.) and the properties of the material (Fotakis et al., 2006). Depending on the pulse duration, it is possible to distinguish between 1) short-pulse lasers, i.e. nanosecond laser (pulse length in the range of 10−9 s) with photothermal, photochemical and mechanical effects and 2) ultrashort-pulse lasers, i.e. picosecond lasers (1 ps = 10−12 s) and femtosecond lasers (1 fs = 10−15 s) with minor photothermal effects (Gamaly et al., 2002a, 2002b, 2002c; Moreno et al., 2005a, 2005b; Rode et al., 2008; Vázquez de Aldana et al., 2012) and, in consequence, with less harmful effects than nanosecond lasers, e.g. carbonization, cracking and chemical modifications. These last characteristics make ultrashort-pulse lasers an attractive and promising tool for CH conservation (Lippert and Dickinson, 2003; Pouli et al., 2010).

Table 3 Composition of Papetta AB57® by Istituto Superiore per la Conservazione ed il Restauro (ISCR) in Rome. Component

Quantity

H2O Ammonium bicarbonate Sodium bicarbonate Bi or tetrasodium salts EDTA Surfactant agent Carboxymethylcellulose

1L 30 g 50 g 25 g 10 cm3 60 g

Nowadays, the conventional laser systems for CH conservation are nanosecond lasers and, in the case of stone cleaning, the most used sources are the different harmonics (different wavelengths) of neodymium-based systems, i.e. Nd:YAG or Nd: YVO4 (Fig. 6). Regarding ultrashort-pulse lasers, although a number of experiences in CH cleaning have been performed (Gaspar et al., 2000a, 2000b; Castillejo et al., 2002; Gaspar, 2003; Lippert and Dickinson, 2003; Rode et al., 2008; Walczak et al., 2008; Oujja et al., 2011; Rivas et al., 2012), until now, the femtosecond sources consisted on complex and voluminous systems which require expensive equipment. Although in recent years, more affordable femtosecond lasers are available in the market, with promising expectations in CH conservation. In the field of CH, laser ablation of patinas, coatings and crusts provides significant advantages with respect to mechanical and chemical approaches (Cooper et al., 1995; Fotakis et al., 2006): – No mechanical contact with the object which is very important in extremely fragile or poorly cohesive surfaces. – The cleaning does not introduce new substances or generate byproducts. – The cleaning is located and accurate. – The laser can be adapted to control process-systems, even in real time. – It can be applied to clean different materials.

However, the suitability of a laser source as cleaning tool depends on the type of stone, the deposits or coatings to be removed, the adequate selection of laser parameters and the processing method.

Please cite this article as: Pozo-Antonio, J.S., et al., Effectiveness of granite cleaning procedures in cultural heritage: A review, Sci Total Environ (2016), http://dx.doi.org/10.1016/j.scitotenv.2016.07.090

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Fig. 6. A–B. Tests performed with a pulse nanosecond Nd-based laser (Nd:YVO4, Coherent AVIA Ultra 355–2000®) operating at a wavelength of 355 nm using in cleaning tests in LAIL at University of A Coruña (http://investigacion.udc.es/en/Research/Details/G000188). C: Different cleaning test of sulphated black crust on a granitic wall. Sources: the authors.

Any cleaning strategy begins with the selection of the laser wavelength in function of the patina or crust to be removed to ensure the optimal laser-matter coupling/absorption. In this sense, some laser equipment allows the selection of different wavelengths. Then, in order to establish the working fluence, the threshold value should be determined. Different values of fluence can be obtained by adjusting the output power of the laser and the beam diameter at the sample surface. Other important aspects concern the pulse rate which can be varied in some lasers, and the pulse overlap which depends on the relative displacement laser-sample. Controlled overlap of the pulses is extremely important in order to ensure a homogeneous removal of the crust or patina. With regards to the pulse rate, high values allow us to scan the surface in less time and can improve the efficiency of ablation (Brygo et al., 2006) though, there is an upper limit for the pulse rate which is given by the significant interaction between one pulse with the plasma generated by the previous one (Knowles et al., 2007). There are many published papers concerning the parameters of laser systems and the mechanisms involved in surface-cleaning processes, particularly in the case of carbonate stone (Cooper et al., 1995; Maravelaki-Kalaitzaki et al., 2001; Moropoulou and Kefalonitou, 2002; Bromblet et al., 2003; Oujja et al., 2005; Potgieter-Vermaak et al., 2005; Fotakis et al., 2006; Chapoulie et al., 2008; Iglesias et al., 2008; Pouli et al., 2011; Speranza et al., 2013; Osticioli et al., 2014; Sanz et al., 2015). In the case of granite the scientific production is, once again, quite lower than in carbonate rocks. In some of the studies the authors analysed the modifications in the rock surface without patina or crust (Esbert et al., 2003; Ramil et al., 2008). These works showed that the colour of the stone is a feature highly vulnerable to the action of laser; the granite irradiated with the fundamental (1064 nm wavelength) and the third harmonic (355 nm) of a Nd:YAG laser, exhibited a colour change on Rosa Porriño granite: the characteristic pink colour becoming paler; however, no colour changes were observed in grey granites (Esbert et al., 2003; Ramil et al., 2008). These colour changes were

attributed to different causes, e.g. Urones-Garrote et al. (2011) concluded that the main reason for the fading of the pink colour in the Rosa Porriño granite after laser cleaning was the physical elimination of the ZnFe2O4 particles from the feldspars due to the thermal effects. In order to avoid these alterations which were also found in carbonate stones, other harmonics of the Nd:YAG and different laser equipment were evaluated for cleaning procedures (Pouli et al., 2008; Ramil et al., 2008; López et al., 2010). The first harmonic of Nd:YAG (1064 nm) was applied to remove sulphated black crust on granite, but gypsum was not completely eliminated even at the highest fluence used (Pozo et al., 2014). Regarding biological colonization, laser ablation has obtained different results depending on the type of coating (biological colonization patina or lichen crust) and the mineralogical and petrographic characteristics of the stones (Esbert et al., 2003; Siano et al., 2012). The third harmonic (355 nm) of a Nd-based laser was successfully applied to remove a patina composed of filamentous green algae (López et al., 2010; Pozo et al., 2014; Pozo-Antonio et al., 2015). In general, UV lasers seem to be more effective than IR lasers to remove biological colonization, i.e. fungi or lichens (Marakis et al., 2003). For graffiti removal, since Liu and Garmire's paper (1995), some authors have investigated the use of lasers as an alternative method to conventional. Different harmonics of Nd: YAG lasers were used to clean graffiti on carbonate stone (marble and limestone) (Chapman, 2000; Costela et al., 2003; Gómez et al., 2006; Sanjeevan et al., 2007). The first studies about graffiti cleaning on granite were performed with the third harmonic of a Nd: YVO4 laser (355 nm wavelength) and the results showed the strong influence of both graffiti and substrate properties on the cleaning efficiency. The reflectivity of the inorganic pigments of the graffiti (e.g. aluminium particles for silver colour graffiti) derived on a low laser effectiveness. Also, coarse grained granite experimented higher graffiti cleaning rates than fine grained granites (Fiorucci et al., 2013; Rivas et al., 2012; Pozo-Antonio et al., 2016a, 2016b).

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Moreover, it is important to highlight that most of the works cited above are performed in surfaces with low roughness. The difficulty of cleaning rough granite surfaces by means of lasers has to be considered; rough surfaces generally define the finished surface of architectural elements of high historical interest which were manually carved. 3. Conclusions and future research lines The optimization of the cleaning process for CH objects must be performed in order to achieve the maximum extraction of the deposit, patina or crust and the minimum damage to the substrate. In this regard, the information provided in this review demonstrates specifically for granites, that 1) this optimization is especially dependent on the composition of the deposit or crust to be removed and also on the characteristics of the granite, such as mineralogy and grain size, and 2) the effectiveness of the cleanings depends also on the principle of the cleaning method. Following the results presented in this article, all the methods considered in this review were able to satisfactorily extract biogenic black patina. On the contrary, all the methods failed to remove the gypsum of the sulphated black crust; this fact could suggest the existent of a particular interaction sulphate-granitic substrate which perhaps hinders the gypsum removing. The removal of the graffiti, specifically in the case of laser cleaning, is dependent on the chemical characteristics of the paintings that define their capacity of radiation absorption. The grain size of the granite greatly influences on the extraction level e.g. graffiti removals. In the case of the laser cleaning, the high susceptibility of biotite to the laser beam must be considered on the cleaning in granite samples rich in this forming mineral, being necessary to evaluate the consequences on the durability of the of this mineral modification. However, the reduction of the surface roughness in samples subjected to laser cleaning, which has been attributed to the fusion of the biotite, could be considered as an advantage, taking into account that the higher the roughness of the rock, the higher the susceptibility to the rock against physical and chemical deterioration agents. Regarding the principle of the cleaning procedure, the mechanical methods, e.g. Hydrogommage, has achieved satisfactory cleanings of graffiti, being the procedure economically more affordable. In general terms, the laser has achieved good results in the extraction of graffiti and biogenic patinas, but it was ineffective to clean sulphated black crusts. The chemical products applied to clean different coatings or deposits have attained different results dependent on the deposit composition, but sometimes they have contaminated the granite. The cleaning evaluations carried on up to now still lack a more complete approach in order to optimize the cleaning of biological colonization, sulphated black crust and graffiti paintings on granite. Different approaches are in need to be investigated in further researches: 1. Different chemical formulation should be developed in order to remove mainly sulphated black crust because until the moment, none of the chemical product achieve the complete extraction of the gypsum. 2. Low pressure mechanical procedures should be improved in order to diminish the influence on the granite surface, e.g. roughness modifications. 3. Different laser equipment working on different domains and wavelength should be tested to remove all these deterioration forms. 4. Optimize laser cleaning of biological colonization, black crust and graffiti on granite with different degrees of roughness. So, other challenge is to find adapting procedures to remove these deterioration forms from no flat granitic surfaces, through the incorporation of systems to control de laser focal point. 5. The combination of cleaning procedures to remove biological colonization, black crust and graffiti will improve the results of the cleaning, always considering the global effectiveness in terms of coating or deposit extraction and the harmfulness on the granite.

6. Concerning the transfer of satisfactory solutions from carbonate to silicate substrates, the two technologies successfully applied to carbonate stone, which might be considered for granite are the biocleaning using sulphate-reducing bacteria (Alfano et al., 2011; Cappitelli et al., 2006) to remove black crusts and the application of microemulsions or gels for the polymers removal (Baglioni et al., 2013) due to its potential use against graffiti binders. 7. More studies centred on the cleaning evaluation for granite in CH are needed in order to preserve our artistic and historical identity. Granite is the most common building stone in NW of Iberian Peninsula; however, as it was reported in this review, the scientific research about the deposits or coatings formation on granitic stones and their cleaning evaluation is less abundant than those for carbonate stones (marble and limestones).

Acknowledgements The authors acknowledge the help of Proliser S.L. (Mr. Aarón Sánchez), Clinarte S.L. (Mr. Alberto Pereira), Kimu 2000 S.L. (Mr. Javier Prieto) and Escuela Superior de Conservación y Restauración de Bienes Culturales de Galicia (specially Ms. Cristina Montojo and PhD Fernando Carrera) for providing useful information, and finally, Ms. Milagros Abal and Mr. Alberto Pereira (Clinarte S.L.) for some of the photographs shown in this article. J.S. Pozo-Antonio was supported by a postdoctoral contract with the University of Vigo within the framework of the 2011– 2015 Galicia Plan for Research, Innovation and Growth (Plan I2C) for 2014. References Alfano, G., Lustrato, G., Belli, C., Zanardini, E., Cappitelli, F., Mello, E., Sorlini, C., Ranalli, G., 2011. The bioremoval of nitrate and sulfate alterations on artistic stonework: the case-study of Matera Cathedral after six years from the treatment. Int. Biodeterior. Biodegrad. 65, 1004–1011. http://dx.doi.org/10.1016/j.ibiod.2011.07.010. Amoroso, G.G., Fassina, V., 1983. Stone Decay and Conservation : Atmospheric Pollution, Cleaning, Consolidation, and Protection, Amsterdam. Elsevier, New York. Asmus, J.F., Murphy, C.G., Munk, W.H., 1974. In: Wuerker, R.F. (Ed.), Studies on the Interaction of Laser Radiation With Art Artifacts. Annu. Tech. Symp., International Society for Optics and Photonics, pp. 19–30 http://dx.doi.org/10.1117/12.953831. Baglioni, P., Chelazzi, D., Giorgi, R., Poggi, G., 2013. Colloid and materials science for the conservation of cultural heritage: cleaning, consolidation, and deacidification. Langmuir 29, 5110–5122. Bastian, F., Alabouvette, C., Jurado, V., Saiz-Jimenez, C., 2009. Impact of biocide treatments on the bacterial communities of the Lascaux Cave. Naturwissenschaften 96, 863–868. http://dx.doi.org/10.1007/s00114-009-0540-y. Bellmunt, N., Paricio, R., Vila, A., 2002. Reconocimiento, diagnosis e intervención en fachadas. Instituto de Tecnología de la Construcción de Cataluña, Barcelona. Bonazza, A., Sabbioni, C., Ghedini, N., 2005. Quantitative data on carbon fractions in interpretation of black crusts and soiling on European built heritage. Atmos. Environ. 39, 2607–2618. http://dx.doi.org/10.1016/j.atmosenv.2005.01.040. Brimblecombe, P., Grossi, C.M., 2007. Damage to buildings from future climate and pollution. APT Bull. 38, 13–18http://www.jstor.org/stable/40004714. Bromblet, P., Laboure, M., Orial, G., 2003. Diversity of the cleaning procedures including laser for the restoration of carved portals in France over the last 10 years. J. Cult. Herit. 4, 17S–26S. Brygo, F., Dutouquet, C., Le Guern, F., Oltra, R., Semerok, A., Weulersse, J.M., 2006. Laser fluence, repetition rate and pulse duration effects on paint ablation. Appl. Surf. Sci. 252, 2131–2138. http://dx.doi.org/10.1016/j.apsusc.2005.02.143. Caneva, G., Nugari, M.P., Salvadori, O. (Eds.), 2009. Plant Biology for Cultural Heritage: Biodeterioration and Conservation. Getty Conservation Institute, Los Ángeles, CA, USA. Cappitelli, F., Zanardini, E., Ranalli, G., Mello, E., Daffonchio, D., Sorlini, C., 2006. Improved methodology for bioremoval of black crusts on historical stone artworks by use of sulfate-reducing bacteria. Appl. Environ. Microbiol. 72, 3733–3737. http://dx.doi. org/10.1128/AEM.72.5.3733-3737.2006. Carvalhão, M., Dionísio, A., 2015. Evaluation of mechanical soft-abrasive blasting and chemical cleaning methods on alkyd-paint graffiti made on calcareous stones. J. Cult. Herit. 16, 579–590. http://dx.doi.org/10.1016/j.culher.2014.10.004. Castillejo, M., Martin, M., Oujia, M., Silva, D., Torres, R., Manousaki, A., Zafiropulos, V., van den Brink, O.F., Heeren, R.M.A., Teule, R., Silva, A., Gouveia, H., 2002. Analytical study of the chemical and physical changes induced by KrF laser cleaning of tempera paints. Anal. Chem. 74, 4662–4671. Chapman, S., 2000. Laser technology for graffiti removal. J. Cult. Herit. 1, S75–S78. http:// dx.doi.org/10.1016/S1296-2074(00)00153-9. Chapoulie, R., Cazenave, S., Duttine, M., 2008. Laser cleaning of historical limestone buildings in Bordeaux appraisal using cathodoluminescence and electron paramagnetic resonance. Environ. Sci. Pollut. Res. Int. 15, 237–243.

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