UNIVERSITÀ DI PISA
Department of Chemical and Industrial Chemistry
Master Degree Course in Chemistry
Graphic vandalism:
multi-analytical evaluation of chemical and physical
methods for the removal of spray paints
Candidate:
Claudia Giusti
Supervisors:
Prof.ssa Maria Perla Colombini
Dott.ssa Barbara Salvadori
Co-Advisor:
Prof. Valter Castelvetro
SECTION 1. STATE OF THE ART ... 2
CHAPTER 1 Origin of graffiti and graphic vandalism ... 3
CHAPTER 2 Materials used to graffiti by graffiti writers: spray paints ... 4
CHAPTER 3 Graffiti removal methods ... 11
SECTION 2. EXPERIMENTAL SECTION ... 16
CHAPTER 1 Materials ... 17
1.1. Marble samples ... 17
1.2 Spray paints ... 19
1.3 Chemical cleaning reagents ... 20
CHAPTER 2 Methods ... 21
CHAPTER 3 Description of cleaning methods ... 25
3.1 Chemical cleaning method: solvents optimization ... 25
3.2 Laser cleaning method: optimization of laser parameters ... 27
3.3 Combination of cleaning methods: laser followed by chemical cleaning ... 30
SECTION 3. RESULTS AND DISCUSSION ... 31
CHAPTER 1 Characterization of spray paints ... 32
1.1. Chemometric analysis of graffiti paints ... 32
1.2. Characterization of sound Montana Colors 94 alkyd paints ... 36
1.2.1 Spectroscopic characterization: FTIR-ATR, XRF, XRD and Raman spectroscopy ... 36
1.2.2 Chromatographic characterization: PY-GC/MS and GC/MS ... 42
1.3 Characterization of aged Montana Colors 94 alkyd paint... 46
1.3.1 Spectroscopic characterization: FTIR-ATR ... 46
1.3.2 Chromatographic characterization: PY-GC/MS AND GC/MS... 47
CONCLUSIONS for characterization of spray paints ... 48
CHAPTER 2 Comparison between different cleaning methods ... 50
2.1 Laser cleaning method ... 50
2.1.1 Sound sample... 50
Laser effects on unpainted marble ... 50
2.1.3 Cleaning of the aged-paints sample ... 64
CONCLUSIONS for laser cleaning method …. 67
2.2 Chemical cleaning method ... 68
2.2.1 Cleaning of the sound sample ... 68
2.2.2 Cleaning of the sample with aged marble... 74
2.2.3 Cleaning of the sample with aged paints ... 80
CONCLUSIONS for chemical cleaningmethod ... 82
2.3 Laser followed by chemical cleaning method ... 84
2.3.1 Cleaning of the sound sample ... 84
2.3.2 Cleaning of the aged marble sample ... 90
2.3.3 Cleaning of the aged-paints sample ... 95
CONCLUSIONSfor combined cleaningmethod ... 97
2.4 Cleaning comparison ... 99
CONCLUSIONS ... 103
References 105 Acknowledgments
1
INTRODUCTION
Graffiti, as an act of vandalism, is undoubtedly a major danger to stone Cultural Heritage and a risk for the preservation of the historical and modern monuments. Indeed, graffiti paints can severally damage architectural materials, accelerating their decay and lead the monument to important loss in value and significance.
The removal of vandal signs is an essential part of conservation treatments. Different techniques and methods have been studied and used to remove such unwanted marks such as those involving mechanical methods (water jet 1,2, grit-blasting 3,4, atmospheric plasma 5), chemical removal 2-4 and, more recently, laser technology 6-8 and biological methods 3,9,10. However, graffiti cleaning methods are potentially harmful, accelerating stone decay by interacting with the substrate or generating by-products. In such cases irreversible damage happens to the surfaces which are unacceptable in objects with cultural heritage value. An understanding of the principles, effectiveness, harmfulness and nocivity of each cleaning method/technique and its comparison is thought to be essential for its conscientious use.
The most appropriate cleaning method should be chosen according to the chemical composition of graffiti and the chemical-morphological characteristics of the substrate. Among the wide array of commercial products available (e.g. felt-tip markers, ballpoint pens, lipsticks, varnishes,… ), spray paints are the most widespread material used for graphic vandalism. Furthermore, based on their binding composition, a lot of different paints are commercially available. Among them, the most frequently used are alkyd or acrylic polymers. The choice between various cleaning products offered on the market is ample, but it is necessary to individuate appropriate methods to selectively remove the paints without damaging the support. Since the efficiency of removal may be paint composition-dependent, the determination of chemical composition or the identification of the brand name may play an important role in the planning of conservation strategies and restoration treatments.
Besides paint composition, also chemical and morphological characteristics of the substrate may influence the efficiency of removal. Indeed, weathered marbles are rougher and more porous than the new ones: a higher porosity may cause a greater penetration of the paint and, consequentially, influence the cleaning results. In addition, even though conservators’ interventions are often tempestive, sometimes vandal signs are not immediately individuated and removed. In this case, the paints may undergo molecular changes over time, becoming more difficult to remove than fresh ones.
A number of studies in the literature concern marble degradation11 and chemical-physical variations of paints 12,13 caused by the aging process; however, little is known about if and how the aging process influences the effectiveness of graffiti removal methods.
In particular, this study focuses on the systematic assessment of the chemical and laser cleaning methods by investigating all aspects that have a positive or negative impact on the cleaning of alkyd paint-based graffiti on marble. Laser and chemical approaches are based on different mechanisms of action: chemical cleaning exploits the solubility of different compounds in the paints while, on the contrary, laser cleaning is based on the ablation process.
In this research, an innovative approach based on the combination of laser and chemical cleaning was tested. Laser followed by chemical cleaning could lead to substantial advantages14 since the laser action physically removes the majority of the paint layer and the solvent solubilizes the remaining part. By this way the risk of color diffusion into the substrate is minimized.
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This thesis presents a study aimed to solve the above problems based on a multianalytical approach to characterize both supports and spray paints, and to evaluate a proper method of graffiti removal.
For these purposes, Carrara white marble samples with dolomitic inclusions and dark veins were selected and painted with five alkyd spray paints ( black, yellow, green, red and blue) produced by Montana Colours 94, which is one of the most common brands used by graffiti writers. All the samples (painted and unpainted) were also analyzed after a proper ageing procedure.
The work thesis is outlined as follows:
- investigation of the organic chemical composition of paints (solvents, binding medium, pigments, colorants, fillers and other components) was performed by Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR), analytical pyrolysis coupled with gas chromatography/mass spectrometry (Py-GC-MS) and gas chromatography-mass spectrometry (GC-MS) . In particular, GC-MS method was useful to determine the fatty acid profile after a sample treatment entailing fast microwave-assisted saponification.
- Investigation of the inorganic composition (pigments and dyes) of paints and of the substrates was carried out by Raman spectroscopy, X-Ray fluorescence (XRF) and X-Ray Diffraction (XRD). These two last techniques contributed to characterize the fillers and additives from an elementary and mineralogical point of view, respectively.
- Detection of the molecular changes induced by UV aging in alkyd binder, was performed on a
selected painted samples subjected to photochemical by the same spectroscopic and chromatographic methods.
- Multivariate statistical analysis exploiting Principal Component Analysis (PCA) and Hierarchical Cluster Analysis (HCG) was applied to FTIR-ATR and FTIR-reflection spectra of alkyd, (pure or modified) acrylic spray paints and felt-tip pens, to highlight the differences and similarities between various paint brands. This method allows us to easily recognize the binder.
- The evaluation of removal methods was performed by comparing three different cleaning approaches in terms of their efficacy and invasiveness. For the chemical cleaning, a comparison between the action of commercial remover (Art-Shield 4) and Methylethylketone supported on an agar-based commercial gel (Nevek) was made; instead, for the physical approach a nano-second Q-switch Nd:YAG laser was employed with different wavelength radiation (1064 nm and 532 nm) and fluence values (0.26 and 0.29 J/cm2). Finally, the condition optimized for both cleaning methods was applied also in the combined approach. Optical and spectroscopic analyses were performed to evaluate the cleaning effectiveness and to detect the presence of paint remains and/or sub-products generated on the surface, as well as to control the harmfulness of the laser approach to the substrate. FTIR spectroscopy was used to assess the removal of organic components, especially the binding medium, while XRF analyses were useful to monitor the removal of pigments and fillers not detectable with the other techniques, through the presence/depletion of typical elements (for example, titanium dioxide). In addition, colorimetric analyses and photographic documentation were used to compare the surface chromatic evolution and ascertain the presence of remnants. In addition, the paint thickness, penetration depth and the presence of residues underneath the surface were estimated from the observation of the cross sections at the optical microscope in reflected visible light before and after cleaning. Finally, the efficiency and invasiveness of laser cleaning on sound marble was evaluated by Laser Scanner microprofilometry.
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SECTION 1
State of the art
3
CHAPTER 1
Origin of graffiti and graphic vandalism
The term “graffiti” is defined as an engraving, scratching, cutting or application of paint, ink or similar matter on the surface [1]. According to this general definition, graffiti plays an extremely important role in archeology, in fact, it was considered the earliest non-literary and popular written testimony. The first artistic demonstrations were very old and date back to the Paleolithic era. The first rocky graffiti depicted bison, reindeer, mammoth and deer; they had primarily a communicative function and they were considered as a propitiatory act, a rite that anticipates essential events such as the hunting of animals. The art of graffiti wasn’t a typical and exclusive technique of the Paleolithic age, but it was widely used in the Neolithic, although with a different conception: the designs helped the Neolithic man to document an event or concept and became the forerunners of writing. In this era, the graffiti technique was very simple, in fact, the images were obtained scratching the rock with hard stones. The images were then colored with earth and coal. Even the Egyptians, the Greeks and the Romans used graffiti art, but it was widely used in the Renaissance, especially for decoration of the facades of the palaces. Graffiti were considered a true form of art only from the early 20th century.
With the hip-pop culture, graffiti writing developed in the Unites State of America, specifically in Philadelphia and in New York, at the end of the 1960s and it was born as a practice of urban rewriting, a medium with which subcultural communities affirm their own existence inside societies from which they feel excluded. From its origin, in fact, it was a phenomenon of existential and political appropriation of an urban context perceived as dehumanizing and depersonalizing. The first writings essentially depict “Tag”, i.e. the signature of the writer; instead, the first paintings were represented between 1972 and 1975. It was thanks to the realization of the documentary about graffiti in New York undergrounds, Styles Wars, and to the film Wild Style, that graffiti spread to Europe 2.
In particular, the phenomenon of writing graffiti in Italy developed towards the end of the 1980s and it evolved mainly on the Milan-Bologna-Rome axis. Parallel to the phenomenon called "trainbombing", graffiti writers showed their art on the city walls, both illegally and legally through the authorized Hall of Fame. Spray paints on building walls and on subway cars, define the urban landscape3.
In the urban centre, besides the diffusion of the Street Art, the graphic vandalism also developed in the 20th century. In this case, graffiti is an unwanted painting on an urban or monumental surface. It is considered the result of a criminalist act ( art. 639, Italian Criminal Code). Graphic vandalism, particularly strong in urban centers, is an escalating problem and today it is visible in most town and cities worldwide.
Graffiti, as an act of vandalism, is undoubtedly a major danger to stone Cultural Heritage and a risk for the preservation of the historical or modern monuments. In addition to the decay caused by atmospheric component and pollutants, graffiti can severally damage architectural materials, accelerating their decay and lead the monument to important loss in value and significance. Moreover, undesired graffiti have negative social connotations: the affected neighborhoods or communities become stigmatized and labeled as poor socio-economic areas. In conclusion, graffiti can still be considered artwork but, if used in inappropriate places, it can become a means of degradation4.
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CHAPTER 2
Materials used to graffiti by graffiti writer: spray paints
Some of the most common materials for graffiti materials include a wide range of materials such as paint (applied by brush or aerosol), felt-tip markers, ballpoint pens, waxy substances such as crayons and lipstick, chalk, adhesive labels and posters.
Despite the wide array of commercial products available, spray paints are the most frequently used materials because they can be quickly and easily applied to any type of substrate. They are used for valuable contemporary murals by famous artists such as Keith Haring, Bansky, but also for graphic vandalism. Determination of the chemical composition and study of the degradation mechanisms of modern (spray) paints may play an important role in the planning of conservation strategies and restoration treatments. On the other hand, when spray paint graffiti are due to vandalism, the knowledge of their chemical composition may be essential for selecting the appropriate cleaning method in order to efficiently remove the materials without harm to the operators and to the substrate. Spray paint is a combination of paint and a gas propellant. Until the 1987 (Montreal Protocol), the chlorofluorocarbons (CFCs) were also used in order to lower the ignition temperature. Studies by Rowland, Molina and Crutzen demonstrated that the chloride atoms originated by the photoscission of chlorofluorocarbons were responsible of the scission of atmospheric ozone molecules and, consequently, they most contribute to the depletion of the ozone layer. After the Montreal protocol, fluorocarbons (FC), perfluorocarbons (PFC) and hydrofluorocarbons were used as gas propellants because they are safer to the environment, in fact, they not contain potentially harmful elements (e.g. chlorine). However, they aren’t environmental friendly, but they contribute to the increase of the greenhouse effect. Today, hydrocarbons such as butane, methane and isopropanol are commonly used, although they contribute to the atmospheric pollution.
The main ingredients of graffiti spray paint are:
binding medium;
pigment or colorant;
solventFinally, spray paints contain additional material (extenders) that have a significant and specific effect on paint properties, even if present in very small quantities. There are plasticizers, surfactants, wetting agents, thickeners, pH buffer, anti-foaming agents, biocides, freeze-thaw agents and sequestering agents.
Binding medium
According to Sanmartin et al 4, a binding medium represents a film-forming transparent material
in which the pigment particles are dispersed and that hardens and binds the pigments on the painted surface. Binding media can be broadly divided into natural products, synthetic polymers and semi-synthetic products. Natural resins derive from plants or animal sources (e.g. oil, egg, animal glue and plant gum); synthetic resins are chemically synthesized in laboratory and, finally, semi-synthetic products are prepared by chemical modification of natural materials (nitrocellulose). Generally, synthetic and semi synthetic polymers are preferred for their apparently better physical and chemical features as compared with natural binders. In particular, the main advantage of the synthetic resins is that it is possible to modify their chemical structures
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to obtain the physic and functional desired properties. Two important classes of synthetic resins have been frequently used as main binding media in spray paint: alkyds and acrylics. Furthermore, other minor binders are used in the spray paints formulations and they include polyvinyl acetate, polystyrene, polyurethanes, epoxy resins and polycyclohexanone.
Alkyd resins
The alkyds were the first synthetic resins to be used in solvent-based paint. Introduced in 1927, alkyd resins are oil-modified polyester. They are obtained by the condensation polymerization between a polybasic carboxylic acid and a polyalcohol which is previously modified through transesterification reactions with fatty acids. Phthalic anhydride is the acidic reagent most commonly used in the preparation of alkyd resins, and glycerol, pentaerythritol and, sometimes, sorbitol are the main polyalcohols used. When alkyd resins are used as paint binders, they are usually modified with fatty acids to make them sufficiently flexible. Figure 1.1 shows the chemical structure of a typical alkyd resin. Depending on their ‘oil length’, i.e. the weight percent of oil (triglycerides or fatty acids) in the resin, alkyds are classified as short oil (35-45 wt%), medium oil (45-55 wt %) or long oil (above 56 wt %). The oil length has a great impact on the final properties: long oils are prevalently applied by brush and thinned in aliphatic solvents, while the short oils are employed in the industrial spray paint and require aromatic hydrocarbons for thinning 5. The amount and type of oil(s) used in their synthesis is extremely important in order to define many important properties to the resin, including, cross-linking potential, flexibility, compatibility with different solvents, control of solubility and, finally, its tendency to dry and drying speed 6. As regards the drying characteristics, if siccative oils are added, the nature of the fatty acids determines the final drying characteristics of the paint material. Drying and semidrying oils, such as linseed, soybean, sunflower and castor oil are the most commonly oils used for the production of alkyd resins. Most industrial paints incorporate alkyd resins as the main binder because, thanks to the high molecular weight and to the presence of drying and semidrying oils in its composition, this type of paint dries faster, it is more durable and harder than the traditional oil-based paint7.
Figure 1.1 Chemical structure of a typical alkyd resin containing phtalic anidride, glycerol and linoleic acid 6
Alkyd paint is often modified further to improve certain properties (e.g. reduce the drying time, increase the film hardness and yield better water resistance). For example, these modifications involve the addition of nitrocellulose, styrene, acrylic, vinyl toluene, isocyanates, epoxy or silicone compounds 4. Modification with nitrocellulose increases the gloss, adhesion and hardness of the paint film 7. Few quantities of styrene (15-20%) may also be present in some alkyd paint formulations and, specifically, styrene modification imparts to the resin quick drying and alkali resistance8.
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Acrylic resins
Acrylic resins were first employed as a water-paint medium after the second world war as an alternative to alkyd resins and drying oils. Acrylic polymers belong to the polyaddition polymers class and they are obtained through a free-radical polymerization reaction between alkyl acrylate or alkyl methacrylate monomers (Figure 1.2) or, more in general, their derivates. These monomer units are formed by esterification reactions of the acrylic acid or methacrylic acid with alcohols, generally ethanol or n-butanol. The polymerization of the monomers is caused by radical initiators. The properties of the resulting acrylic polymers vary depending on the nature of the alkyl groups both on the alcohol and the acrylic acid.
The monomer units used for the production of the acrylic polymer can be equal (homopolymer) or different (copolymer or terpolymer if formed by two or three different units, respectively). Acrylic media usually used as binder in paint are: polymethylmethacrylate (PMMA), poly n-butyl methacrylate (nBMA) and polyisobutylmethacrylate (isoBMA) between homopolymers; ethyl methacrylate–methyl acrylate (EMA–MA), and methyl methacrylate–ethyl acrylate (MMA–EA) between copolymers. This last medium ( MMA-EA) was frequently used by the artists from the 20th century due to its properties of reduced drying time and low yellowing. In addition, this synthetic paint may be purely acrylic or copolymerized with other species, such as styrene, vinyl toluene or vinyl acetate. Examples of acrylic polymers frequently used as paint binder in commercial spray paint are shown in Figure 1.3.
Figure 1.3 Chemical structure of three different acrylic polymers:
poly(butyl methacrylate) homopolymer, poly(ethyl acrylate) homopolymer and poly(styrene-co-methyl methacrylate)copolymers
As regards the binder, an overview of the great variety of commercial paints was shown in the study by La Nasa research group about the chemical characterization of the organic material in three different Keith Haring murals, Tuttomondo in Pisa, the Necker hospital murals in Paris and the Melbourne mural. According to the Pie-GC/MS characterization, a styrene/n-butylacrylate copolymer was found in paint samples from Tuttomondo; Necker Children's Hospital mural painting was characterized by an emulsion vinyl resin and, finally, the Collingwood mural painting (Melbourne) was characterized by different paint materials and, in particular, an alkyd resin in the degraded red paint layer and an acrylic polymer in the green paint; both vinyl and acrylic media were also found in the background 9.
Figure 1.2 Ethyl acrylate (left) and ethyl methacrylate (right) monomers involved in the synthesis of the acrylic polymers.
7 Pigments and colorants
Colorants are classified as either pigments or dyes and they are responsible of the color and opacity of the paints. Pigments are inorganic or organic, colored, white or black; they are practically insoluble in the paint medium. Dyes, unlike pigments, dissolve during their application and in the process lose their crystal or particulate structure. Physical characteristics rather than the chemical composition permit to differentiate the pigments from dyes.
Inorganic pigments
Inorganic pigments are further classified as colored, white and metallic. Table 1.1 shows the most common inorganic pigments used in the commercial composition of spray paints. Colored pigments can be natural or synthetic. They are mainly based on oxides and sulfides of iron and chromium and, to a lesser extent, of zinc, molybdenum and cadmium. White pigments can be used to lighten coloured paints or as pigments; so, they are present in all paints, not only in the white ones. Because of the high hiding power of some pigments, they can be partially replaced by cheaper inert pigments or pigment extenders. These are white inorganic minerals that are deficient in both color and opacity and include calcium carbonate or calcite (CaCO3, the most widely used), silicon dioxide or silica (SiO2), kaolin or china clay (rock rich in kaolinite Al2Si2O5(OH)4), talc (Mg3Si4O10(OH)2), barite (BaSO4) and titanium dioxide or rutile (TiO2)[4] . They can be classified in four categories on its elemental composition and, specifically: titanium-based pigments, zinc-based pigments, antimony-based pigments and lead-based pigments. Finally, the metallic pigments, which can be found in the composition of gold, silver and bronze spray paints, are achieved adding aluminum, zinc, bronze or steel; more in detail, Gomez et al.10 and Costela
et al. 11 identified zinc and copper in a gold paint and aluminum in a silver paint.
Inorganic pigments frequently present applicative problems: ultramarine blue, for instance, is not stable in acid condition; prussian blue must not be exposed to alkalis and molybdate reds and chrome yellows are nevertheless sensitive to acids and light. Consequently, the inorganic pigments are frequently used in combination with organic pigments.
Organic pigments
The rapid development of synthetic chemistry during the nineteenth century led to the development of synthetic organic pigments. The term “synthetic” organic pigment refers specially to those pigments that are synthesized in a laboratory and must be distinguished from natural organic pigments obtained from plants or animal sources 12. Synthetic organic pigments now
dominate the colourant market and have partially replaced traditional natural organic colourants
Table 1.1 Most common inorganic pigments used in spray paints.
Pigment group Chemical composition Pigment colour
White
TiO, TiO2
White ZnO, ZnS, lithopone (a mixture of ZnS and BaSO4)
Sb2O3
PbSO4, white lead (2PbCO3·Pb(OH)2)
Coloured
FeO(OH), PbCrO4, ZnCrO4, CdS Yellow
Indian red (Fe2O3 ), red lead (Pb3O4) Red
Ultramarine blue (Na8-10Al6Si6O24S2-4), Prussian blue ( KFe(Fe(CN)6)
Lead chrome green (PbCrO4KFeFe(CN)6, Cr2O3
Blue Green
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and inorganic pigments because their main advantage is their capability to impart hues not accessible using inorganic pigments alone. A lot of publications proposed several classification systems for organic pigments. Basically, it seems appropriate to adopt a classification system by grouping pigments either by chemical constitution or by coloristic properties. A rough distinction can be made between azo and non-azo pigments; the latter are also known as polycyclic pigments. The commercially important group of azo pigments can be further classified according to structural characteristics, such as by the number of azo groups. Polycyclic pigments, on the other hand, may be identified by the number and the type of rings that constitute the aromatic structure. The azopigments that were encountered in the spray paints belong to the monoazo and disazo pigments and they have the azo group (-N=N-) in common. Monoazo pigments are
represented by acetoacetic arylide yellow, -naphtol, BON, naphtol AS and benzimidazolone; on
the other hand disazo pigments with disazo condensation include diarylide, bisacetoacetarylide, pyrazolone. Azo dyes are able to form stable azo–metal chelate complexes, but only a few azo metal complexes are available as pigments. The chelating metal is usually nickel, and less commonly, cobalt or iron(II). The azo group (-N=N-) may be replaced by the analogous (-CH=N-) moiety to form an azomethine complex pigment, usually with copper as a chelating metal 13.
Examples of azopigment are shown in Table 1.2.
Table 1.2 Representative chemical structure of each class of azo-pigments 14 .Blue rectangle: functional group or
element which characterized the pigment class.
Pigment class, pigment group
Chemical structure ID pigment
Monoazo, -Naphtol R2,R4= H, Cl, NO2, CH3, OCH3, OC2H5 PO2, PO5, PR1,PR3,PR4,PR6 Disazo, Pyrazolone X=Cl, OCH3; R1=CH3, COOC2H5; R2=H,CH3 PO13, PO34, PR37, PR38,PR41, PR111 Metal complex, Azomethine metal complex PY129 Metal complex, Azo metal complex
PG10
Next to the azopigment and the azo-azomethin metal complex, pigments are assigned to polycyclic class on the basis of being non azo pigments. All polycyclic pigments (some examples
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are shown in Table 1.3) are chemically characterized by a condensed aromatic or heterocyclic ring systems. Most of these pigments are structurally based on anthraquinone derivatives such as indanthrone, flavanthrone, pyranthrone, or dibromoanthanthrone. Some examples are represented by phthalocyanines, diketopyrrolo-pyrroles (DPP pigments), perylenes and perinones, quinacridones, isoindolinones, polycarbocyclic anthraquinones and dioxanines. In general, the nature and position of the substituents in the aromatic rings influences the pigment solubility properties: on one hand, long-chain alkyl, alkoxy, alkylamino and sulfonic acid functions tend to increase the colorant solubility; on the other hand, the presence of substituents like carbonamide groups, nitro groups or chlorine in azo pigments or hetero atoms (e.g. nitrogen, chlorine and bromine) in polycyclic pigments, will decrease the pigment solubility.
Table 1.3 Representative chemical structure of pigments belong to the polycyclic class.
Pigment class, pigment group
Chemical structure ID pigment
Polycyclic, Phthalocyanine PB15, PB15:1, PB15:2, PB15:3, PB15:4, PB15:6, PB16 PG36 Polycyclic, Diketopyrrolo-Pyrrole (DPP) R3= CN, H ; R4= H, C(CH3)3, Cl, CH3, C6H5 PO71,PO73,PO81,PR254, PR255,PR264,PR270,PR272 Polycyclic, Quinacridone R2,9= H, CH3, Cl ; R3,4,10,11 = H, Cl PV19, PR122,PR192, PR202, PR207, PR209, PR206, PO48, PO49, PV42
A lot of studies reported in the literature confirm the greater variability of pigment composition in the spray paint formulations. In particular, Germinario et al. outlined the chemical characterization of a large number of commercial spray paints used in street art, graffiti vandalism and for decoration purposes. In this selection of 45 spray paints of various brands, different pigments, extenders and fillers were identified. Inorganic pigments were found for the white (rutile and lead white) and black (carbon black) samples; organic pigments belonged to the azo and polycyclic pigments classes. Among these, monoazo acetoacetyl (PY74), monoazo napthol AS (PR170) and blue phthalocyanines were most frequently identified. In addition, the dioxazine
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pigment PV23 was also present in some blue formulation. Finally, chalk and gypsum, used as extenders and opacifers, were also detected in some formulations 7.
Solvents, additives and other components 4
The solvent is a substance that allows the pigment/binder mixture to flow out from the metallic container. Solvents used in the spray paint industry can be separated into hydrocarbon solvents, oxygenated solvents and water. Hydrocarbon solvents, the most commonly used, are further divided into aliphatic, naphthenic and aromatic (such as toluene and xylene). The principal oxygenated solvents, widely used with synthetic binders, are ketones, esters, glycol esters and alcohols (especially n-butanol). Water, used alone or blended with other solvents, is the main ingredient of the continuous phase of most emulsion paint. Additives include plasticizers or dispersants (which increase the plasticity or fluidity of spray paint), surfactants and wetting agents (which disperse pigments), thickeners, pH buffers (which help to stabilize the pH range), anti-foaming agents (which alter the surface tension of a paint), freeze–thaw agents, biocides and sequestering agents (which remove metal ions). The inclusion of low concentrations of wide-spectrum biocides, fungicides and/or algicides in the paint formulation prevents the growth of a wide range of microorganisms.
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CHAPTER 3
Graffiti removal methods
Cleaning of architectural surfaces is always a challenging task for restorers. The removal of vandal signs is an essential part of conservation treatments, necessary for aesthetical reasons but also to ensure better preservation of stone materials. Different methodologies are developed using mechanical methods, solvents and, more recently, laser. However, graffiti cleaning methods are potentially harmful, accelerating stone decay by interacting with the substrate or generating by-products. A variety of treatments are available. The most appropriate method is chosen according to the type of graffiti and substrate. An appropriate knowledge of the principles, effectiveness and harmfulness of each cleaning method is essential for its conscientious use. In this chapter, a state of the art overview on the most common methods for graffiti removal is presented. The four examined approaches include: mechanical, chemical, laser and biological methods. A description of their main characteristics, the advantages and drawbacks of each method are also highlighted.
3.1 MECHANICAL CLEANING METHODS
Mechanical cleaning procedures are based on the removal of alteration forms (e.g. graffiti) on the stone surface by abrading the surfaces. The most common mechanical procedures are the soft-abrasive projection and the hydrocleaning with pressurized water. The mechanical method proposed by Careddu et al.[15] is based on plain high-pressure water-jet without abrasive particles or chemical addictives. Any chemical effect doesn’t occur after its application, but its main drawback is that the water-jet can cause excavation on the stone surface. Weaver et al. [16]
underline the invasiveness of the pressurized water and, in particular, they report that the water pressure may damage the historic buildings, removing the natural or artistic patina even at moderate pressures (100-400 psi).
The mechanical procedures based on particle projection derive from the old method of sandblasting; this traditional method was rarely applied in cultural heritage cleaning because it is considered an aggressive method against the stone 16. The development of new pneumatic and electrical systems permitted to reduce the invasiveness of these blasting methods; the possibility to regulate the pressure and to use projection particles of different composition and hardness have allowed to use these methods for conservation and restoration purposes. Hydrogommage® 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). Recent studies showed satisfactory results in the extraction of graffiti (red, blue, black and silver coloured paints) on a stone surface by Hydrogommage®, but an increase of the surface roughness was detected in all cases 18. Weaver et al. suggested the use of a micro-abrasive technique to remove graffiti from fragile masonry surface using low pressure water-jet (35-40 psi) with small abrasive particles and under highly controlled conditions16.The abrasive particles often used as blasting material in the novel blasting methods include sodium bicarbonate crystals, dry ice and soy-oil products. Some examples showed the use of soda blasting in which sodium bicarbonate crystals are applied to a graphitized surface by using compressed air or in a high-velocity air-water mixture. Dry ice and soy-oil product are also recently used to clean graffiti in San Francisco (USA). Dry ice blasting cleaning is advantageous because dry ice turns into a gas and returns into atmosphere as carbon
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dioxide, leaving no residue behind, but it is limited by the problem of thermal shock to the substrate 4. The application of soy-oil products is also interesting: methyl ester, a compound in this vegetable oils, is able to remove certain type of paint when it is mixed with rubbing alchol. A recent innovation in the field of conservation is the use of atmospheric plasma for cleaning application. It represent a good alternative to abrasion methods. Plasma, an ionized gas containing highly reactive species (air, oxygen, hydrogen, etc.), hits the building surface at almost ultrasonic speed and this impact causes the evaporation of the paint. The interaction of plasma cleaning is strongly dependent on the nature of the treated material: the polymeric component of the paint is successfully removed while the inorganic component (e.g. TiO2), used as extender in the paint, are more stable and reluctant to chemically interact with plasma. Plasma cleaning is contactless, precisely controlled due to the reduced diameter of the plasma plume and confined to the very first layers of the surface. The experiments performed by Aibèo et al. [19] showed that the plasma cleaning isn’t much invasive for the substrate; indeed, the macroscopic properties are slightly affected by the cleaning procedure and both static contact angle and water drop absorption reach values close to those of reference stone.
Only few applications using atmospheric plasma can be found in literature and, in particular, a comparison between arc discharge torches and dielectric barrier discharge (DBD) devices were evaluated. The plasma generated by arc discharge was able to remove graffiti paints from marble but it was not suitable for cleaning cultural heritage surfaces because it could cause a darkening effect of the stone surfaces and some thermal effects. More in detail, the first effect was caused by the deposition of metallic particles from the electrode of the torch, while an increase in the temperature of treated surfaces was responsible of chemical or mineralogical transformations in the sample.
On the other hand, dielectric barrier discharge torches exhibit worse performance than the arc discharge ones, but, for long exposure times, satisfactory removal can be achieved without temperature increase and other detrimental effects on the surfaces. According to the EU-PANNA project, a novel plasma design has been developed avoiding the main drawback of metal deposition and keeping the balance between cleaning times and preservation of surfaces.
In conclusion, such as the other mechanical method, plasma cleaning procedure are often used as a preliminary step followed by other cleaning techniques (chemical or physical) to remove spray paint or felt tip pens signs from stone surface.
3.2 CHEMICAL CLEANING METHODS
The chemical cleaning is considered as a traditional technique of graffiti removal. Chemical methods are based on the application of chemical solutions which react with the coatings and dissolve them. A number of solvents and paint strippers are capable of dissolving or breaking down spray paint.
Solvents are chosen according to the type of graffiti and, generally, a mixture of solvents is necessary to dissolve the paint because all different components of the paint (binder, dye…) have to be completely solubilized. Hot water aided by a neutral or non-ionic detergents is able to remove water-soluble graffiti, while in other cases, solutions of weak bases (e.g. ammonia) are necessary to obtain an effective cleaning. Alkaline compounds can be used to remove some oils/greases and waxes from non-alkali sensitive monuments. Application of alkaline compounds should always be followed by a weak acid wash and a water rinse to neutralize or remove all the
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alkaline residues from the substrate. As basic-pH substances, sodium hydroxide (NaOH) and potassium hydroxide (KOH) are often contained in the paint removers and used for graffiti cleaning. These strong alkalis (pH 13 or 14) should not be used because, if they aren’t properly neutralized, they can cause efflorescence and staining on stone surfaces; for example, hydroxide-based paint removers may react with iron compounds to form ferric hydroxide compounds which are very difficult to remove16.
Some commercial compositions are based on volatile organic compounds (VOCs) because they are renowned for their ability to dissolve the paint. Paint formulations can often be removed with white spirit, N-methylpyrrolidone, toluene, isopropyl alcohol, methylethyl ketone, acetone or clorurated solvents 18. The advantage of using this volatile organic solvents is that they evaporate completely, leaving no residual material in the substrate. However, organic solvents are dangerous to environment and workers because they are toxic by ingestion, inhalation and skin contact.
Solvents can be applied directly on the stone surface but, usually, they are applied in a poultice form. Various substances, either mineral or organic, are capable of increasing the viscosity of solvent solutions when combined with them. These materials are usually further divided into two basic categories: those which may be defined as “ supporting” agents, which only mix or swell in the solvent (e.g. kaolin or sepiolite or cellulose pulps), and “gelling” materials completely dissolved in solution 20. In this context, gels have been proposed and developed as cleaning materials because they offer several advantages over free solvents. The use of gel systems ensure a fine control of liquid transport and limit the solvent penetration, a phenomenon that could lead to mobilization of soluble salts inside the substratum. Gels also enable better control of the solvent evaporation. Finally, their transparency ensures a higher control of the treated areas and permits to monitor the sample during the cleaning process17,18. Recent applications show that polymer gels have been successfully used in the restoration practice, specially Agar based gels. Agar is a polysaccharide that has a sugar skeleton consisting of two alternating polysaccharides: agarose and agaropectin (Figure 1.4). Agar gel seems to have an intrinsic capacity to be used as a cleaning tool and not only as a thickener of a liquid phase, in fact, the polysaccharidic chains arrange in a double-helix ordered structure by hydrogen bonding and generate a three-dimensional network containing solvents. Furthermore it is not yet clarified if their cleaning capability is due only to the solvent action or also to the polysaccharide structure 21.
Figure 1.4 Chemical structure of agarose and of possible repeating disaccharide units of agaropectin with different substituents (i.e. hydroxy, methoxy, sulphate, ketal pyruvate): Agarose is a linear polymer consisting of alternating β-D-galactose and 3,6-anhydro-L-β-D-galactose units linked by glycosidic bonds; Agaropectin is a heterogeneous agarose heavily
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Despite this wide range of chemical products available, the chemical cleaning may be not completely efficient. Indeed, Weaver et al.[16] noted that some chemical solvents may permanently discolor or stain the building surface, and in addition, the remaining paint may become more difficult to remove. Finally, even if the solvents are supported in a gel matrix, chemical solvents may however penetrate into the substrate and cause irreversible damage like the diffusion of the paint on the surface and in depth 4,16 .
3.3 LASER CLEANING METHODS
The use of laser for graffiti removal is a promising alternative or addition to conventional cleaning methods, though irradiation parameters must be carefully selected in order to achieve the effective cleaning without damaging the substrate. The laser cleaning is based on ablation phenomenon and the mayor benefits of this physical method include: non-contact nature of the treatment, selectivity, precision, gradual removal of graffiti, repeatability of the treatment, control of the area processed and low environmental impact 22.
Laser ablation process depends on material properties (i.e. absorption coefficient) and several laser parameters such as: wavelength, fluence, pulse radiation frequency and pulse duration. Various studies have been carried out to compare the effect of the application of the different laser wavelengths, from the IR to the UV.
Gòmez et al. [10] carry out a comparison between the performance of the fundamental radiation of the Nd:YAG laser (1064 nm, IR) and the radiation of xenium chloride (XeCl) excimer laser (308 nm, UV) in the removal of graffiti from construction materials. This enabled a comparison between the photochemical and phototermal mechanisms involved in the ablation process.
In the photochemical process, the absorption of one or more photons causes an electronic excitation and the decomposition of the compound is caused by direct bond breaking.
In the thermal mechanism, the initial electronic excitation produced by the absorption of the laser radiation is converted into vibrational excitation; the relaxation processes cause thermal mechanism of the compound and heating of the material. Probably in the ablation process both mechanism are involved, but one of them may be dominant: IR irradiation at 1064nm acts essentially via a thermal mechanism; on the other hand, the UV photons have a sufficient energy to break the chemical bonds and so, at 308 nm, photochemical process is expected .
In order to evaluate the best laser conditions, Costela et al. 11compare the effectiveness of second (= 532 nm) and third harmonic (= 355 nm) wavelength of a Nd:YAG laser. The ablation rate studies reveal that irradiation at the third harmonic wavelength of 355 nm enables efficient cleaning at the same or lower fluences than those adopted at the second harmonic 532 nm radiation. More in detail, their optical microscope studies demonstrated that the second harmonic left remnants of polymeric base of the paint on the substrate, while the 355nm radiation completely removed both pigments and polymeric binder.
An important parameter to be considering during the cleaning test is the time of pulse of laser radiation. Pulsed laser cleaning techniques have demonstrated a great potential for the removal of painting from different materials like marble, stone and a variety of metals. Considering the pulse duration, two operating modes of laser systems are normally used for these applications: Q-switched mode (10-9 s) and normal mode (10-6 s). A different interaction laser-matter is produced depending on pulse duration: a thermal contribution is involved with long pulse, while, a photomechanical process occurs when the shorter laser pulse is applied with high power density.
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Some studies evaluated the effects of different pulsed lasers on unpainted marble and, in particular, SEM investigation of morphology shows that the short-pulsed laser induces a mechanical effect similar to a micro-explosion that damages (formation of micro fractures) and increases the roughness of the substrate; on the other hand, long-pulsed laser seems to generate a thermal effect causing vitrification and melting of marble particles caused by the excessive heating of the substrate 23.
As concern the effects of laser fluence, Sanjeevan et al. 24 demonstrated that the efficiency of laser cleaning (Nd:YAG first harmonic) decreases with increasing laser fluence: lower laser fluence with greater number of laser pulses is more effective than the higher laser fluence. Fragmentation and disgregation of substrate is observed at higher fluences.
In some work, water-assisted laser irradiation process is evaluated. The addition of water films on the samplesincrease the effectiveness of the laser treatment without the need to increase the fluence of the incident laser radiation 24. The water films don’t participate in the interaction process, but they facilitate the removal process because they reduce the percentage of the reflected radiation (refractive effect) and facilitate the vaporization of the material which doesn't absorb to that radiation wavelength (secondary ablation effect)25. Colorimetric evaluations show that a thin water layer on stone surfaces can also help to diminish the color changes induced by laser radiation 26,27. The main drawback is that the water could act as a medium to carry some paint components inside the marble. The choice of the laser is made on the basis of the spectral range of material absorption. Various laser systems have been used in attempts to remove paint coating, including the following showed in Table 1.4.
Table 1.4 Type of commercial lasers used in the cultural heritage applications 26
Spectral range Type
Ultraviolet 193 nm
248 nm 266 nm 308 nm 355 nm
Excimer laser (ArF) Excimer laser (KrF)
Nd:YAG Excimer laser (XeCl)
Nd:YAG Visible 488 nm 514 nm 532 nm 543 nm 633 nm 694 nm 700-1100 nm Argon Argon Nd:YAG He-Ne He-Ne Rubino Ti:Zaffiro
Near infrared 1064 nm Nd:YAG
Mid-infrared 2940 nm Er:YAG
Far-infrared 10600 nm CO2
Nd:YAG with its different harmonics (first, 1064nm; second, 532nm; third,355nm; fourth, 266nm; and fifth, 213nm) is the most common and efficient high-power laser system for paint removal. Despite the effectiveness of the UV radiation, the fundamental wavelength of a Q-switched Nd:YAG laser (= 1064 nm) is nowadays the most utilized wavelength to clean stone surfaces. The major advantage of the fundamental mode compared to higher modes of Nd:YAG laser is commonly described as a self-limiting phenomenon. However, the irradiation by 1064 nm caused a dramatic color-change of the marble substrate from originally white to yellow. On the contrary, no yellowing was observed on the marble surface when a laser wavelength of 266 nm
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was used 4. Finally, other studies 28 revealed that different colors of spray paint responded differently to the same laser treatment. The authors attributed this finding to the chemical composition and particularly to the type of resin in the spray paint. In particular, it has be found that graffiti in metallic colors (silver, gold and bronze) are very difficult to remove by laser. Laser treatment is less effective for these colors because the metallic particles reflect the incident radiation and, consequently, only a small part of the emitted energy is available for the ablation process. Indeed, the measured reflectance of the paints at 355 nm was low for black blue and red ( 10%) and, conversely, very high for silver (90%). Rivas et al. [4] confirms this consideration. As well as other cleaning methods, the only laser ablation isn’t always sufficient to achieve satisfactory results in the treatment of surfaces affected by graphic vandalism. Generally, therefore, it is used in combination with other techniques, whose chronological sequence is not random, but well-planned.
3.1 BIOLOGICAL CLEANING METHODS
Biotechnology represents an attractive and sustainable alternative to traditional cleaning in the conservation of cultural heritage materials. Although microorganisms are commonly associated with negative effects on the integrity of buildings, materials and structures, there is growing evidence that they can be used for the purpose of bioremediation. Bioremediation is the use of living organism to remove human or environmental pollutants through biodegradation4. Microorganism are good candidates for bioremediation because of the vast metabolic diversity that allows them to use various organic/inorganic compound for growth. Bioremediation offers several advantages over chemical and physical cleaning methods: it is selective to xenobiotic compounds, generally inexpensive, not harmful to substrates, environmentally friendly and safe for human health. Only few studies were focused on bioremediation of graffiti spray paint, so, possible limitations aren’t still investigated. The only negative aspect reported was that the complete elimination/detachment of the paint layer from the substrate (glass slide) did not occur. Bioremediation provides a novel approach to graffiti removal. Spray paints contain a large variety of biodegradable organic (synthetic polymers) and inorganic components (emulsifiers and thickeners) and, in particular, their high carbon content (above 50% 4) may represent a potential source of C for bacteria and fungi. In this sense, microorganisms can presumably be good candidates for graffiti removal. Currently, few studies are done on this theme. Giacomucci et al. 4 based their studies on degradation of nitrocellulose-based spray paint and, in particular, they proposed that the biodegradation of this paint derived from a mechanism of nitrocellulose degradation, probably ammonification. In this respect, in fact, cellulose derivatives can act as nutrients for fungal cells 29, whereas organic solvents and heavy metals in pigments can adversely affect the cells. In another study, nine bacterial and one fungal strains were selected for their capabilities to degrade spray paint containing alkyd and polyester resins 30. This study revealed that the paint coating showed pinholes, blistering and/or lifting/disruption, visible color fading (with varying intensity) and a loss of gloss after the treatment with microorganisms; on the other hand, biologically untreated painted slides showed no alteration in the adhesive characteristic of the paint. It was also highlighted that selected microorganisms are part of the natural microbiota inhabiting graffiti environments. Nowadays, the biological cleaning method is a matter still open to scientific discussions.
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SECTION 2
17
CHAPTER 1
Materials
1.1. MARBLE SAMPLES
Marble is the most common material offended by graphic vandalism, due to its wide use for statuary and buildings in Italy. In this study, nine Carrara white marble samples (13 cm x 10 cm x 1.4 cm) with dark veins were selected and painted with spray varnishes. Dolomitic inclusions and some pyrite grains (section 3, chapter 2) were also found in these samples. Even though conservators’ interventions are often tempestive, sometimes it may occur that vandal signs are not immediately individuated and removed. In this case, over time the paints may undergo molecular changes, becoming more difficult to remove than fresh one. Thus, it is crucial to understand if the aging process may affect the effectiveness of graffiti removal methods, especially laser and chemical ones. For this purpose, three types of samples treatment were considered:
Sound marble samples;
Aged-marble samples;
Aged-paints samples.
In Table 2.1 all the samples are listed, as well as the cleaning approaches.
Preparation of the sound marble samples.
The surface of each marble sample was divided in five areas (2 cm x 8 cm) by a pencil, as shown in Figure 2.1; each area was painted with a different spray paint color (see the following paragraph of this chapter). Successively, the paints were left to dry on the substrate. The cleaning tests were performed one month later.
Figure 2.1 Example of a sound marble sample (N) before painting (t0).
Table 2.1 List of samples and corresponding cleaning approaches
Sample type Cleaning method Identificative letters
Sound marble samples
Chemical cleaning CN
Laser cleaning LN
Laser followed by chemical cleaning LCN
Aged-marble samples
Chemical cleaning CM
Laser cleaning LM
Laser followed by chemical cleaning LCM
Aged-paints samples
Chemical cleaning CV
Laser cleaning LV
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Preparation of the aged-marble samples.
Three samples were first subjected to thermal shocks, to increase marble porosity, and then to chemical treatments, in order to increase surface roughness. Before aging, five areas (2 cm x 8 cm) were drawn by pencil on the surface of each sample. The complete procedure used for the aging process is described below.
Thermal shocks
The samples were first kept in a bath of demineralized water for about 30 minutes. The wet samples were kept in freezer for about 60 minutes and then dried in oven at 125°C for 90 minutes. After this time, they were left to cool in a bath of demineralized water. Successively, the samples were dried in oven at 125°C for 60 minutes, cooled in the water bath and, finally, they were freezed. These freeze-thaw cycles were repeated for 3 times. The three samples were then kept in a muffle at 500°C for 120 minutes for two times, and then freezed.
Chemical treatment
After thermal shocks, the surface of each sample was treated with a solution of HCl 2.4M. Finally, the samples were rinsed with demineralized water and dried in oven at 50°C. A schematic representation of the aging process procedure is illustrated in Figure 2.2.
Figure 2.2 Schematic representation of the aging process procedure applied to unpainted marble samples (M)
The final result of this aging process is shown in Figure 2.3. Compared to Figure 2.1, yellowing of the white areas of the marble and darkening of the veins is clearly visible.
As a result of aging treatments, the intergranular porosity of the marble increased1
, as well as the roughness of the surface. Water absorption tests by contact sponge were carried out on both sound reference samples and on aged samples, to quantitatively confirm the morphological modifications observed. The water absorption results clearly highlighted that the aged samples
absorbed more water than the sound ones (Figure 2.4).The substrate is clearly degraded because
•Water absorption •Freeze-thaw cycles (200-0°C) •Muffle (500 °C) Thermal treatment HCl 2.4M Chemical treatment
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of the chemical and thermal aging: these macrophotographs highlight an increase of porosity and a color change, specifically, the yellowing of white zone of the marble and darkening of the veins.
Figure 2.4 Results of the water absorption analysis of the three sample subject to the marble aging process (M) in comparison with an sound marble used as reference sample. For each sample, two or three areas (A,B,C)
are tested. On the right the contact sponge is shown during its application on a marble sample.
The three aged-marble samples were then painted. The cleaning tests were performed one month later.
Preparation of the aged-paints samples.
Three sound marble samples were subject to the aging of the paint. The surface of each marble sample was first divided in five areas (2 cm x 8 cm) by a pencil (Figure 2.1) and each area was painted with a different spray paint colors. Successively, the paints were left to dry on the substrate. Photochemical ageing was run using a Solar Box CO.FO.ME.GRA model 3000e equipped with a Xenon-arc lamp and an outdoor type UV filter with cut-off <290 nm to eliminate radiation not present in the external sunlight. According to the ISO 1134/2004 protocol [2, irradiance was
kept at 550 W/m2 and black standard temperature (BST) at 65 ± 2°C.The accelerated UV exposure
of the paint samples was carried out for 37 days (887 hours). The aging process was monitored through stereomicroscope, Fourier transform spectroscopy in reflection mode (FTIR-refl) and color measurements. The cleaning tests were performed one month later.
1.2 SPRAY PAINTS
The spray paints used in this work correspond to trademarks MONTANA COLORS 94 (MTN94). They are low pressure sprays and widely used by graffiti writers. The spray colours investigated in this study are shown in Figure 2.5 and listed inTable 2.2. The chemical composition of the paints was described in detail in the followed section (Section 3, Chapter 1). Consistently with the composition declared in the technical data sheet by the producer, FTIR-ATR and Py-GC/MS analysis confirmed that these paints were characterized by an alkyd binder.
0.00 0.01 0.02 0.03 0.04 0.05 H2 O a b so rp ti o n ( g /c m 2)
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Figure 2.5 Montana Colors 94 spray paints.
Table 2.2 Color spray paints and their Codes.
Color Code
Lemon Yellow (Y) RV 1016
Madrid Red (R) RV 241
Era Green (G) RV 127
Babylon Blue (B) RV 243
MattBlack (K) R 9011
1.3 CHEMICAL CLEANING REAGENTS
The chemical products used for the chemical cleaning procedure were: Art-Shield 4 (AS4) and Methylethylketone (MEK) supported on a gel (Nevek).
ART-SHIELD 4 (CTS, Vicenza) is used as graffiti remover of alkyd spray paints on marble samples. According to its safety data sheet, it is a mixture of glycol ethers and surfactants. The composition of graffiti remover as reported on the safety sheet is summarized in the following table (Table 2.3).
Table 2.3 Art-Shield 4 (CTS, Vicenza) chemical composition.
Compound Composition %
Fatty alcohol ethoxylate 1-3
Oleic amine ethoxylate 1-5
Propylene Carbonate 9-10
Dipropylene glycol monomethylether 50-100
The other chemical method was based on a solution of METHYLETHYLKETONE (Sigma-Aldrich U.S.A) supported on Nevek (33% w/w). NEVEK (CTS, Vicenza) is a novel Agar-based commercial gel; it is used as supporting agent of the organic solvent during the chemical cleaning procedure.
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CHAPTER 2
Methods
The effectiveness of the different cleaning procedures was evaluated through a multi-analytical approach. The spray paints were also preliminarily characterized. The analytical techniques exploited in this study are listed in Table 2.4.
Table 2.4 Analytical techniques used in this study.
Characterization of spray paints Evaluation of cleaning methods
Spectroscopic analysis FTIR-ATR FTIR-reflectance XRF Colorimetry SEM Micro-Raman XRF XRD Colorimetry Chromatographic analysis Py-GC/MS - GC/MS Optical analysis - Steromicroscope Optical microscope Laser Scanner Micro-profilometry Spectroscopic analyses
Reflectance spectra on marble surfaces, before and after cleaning were recorded using a portable
FTIR spectrophotometer Alpha Bruker equipped with a front reflection module and OPUS
software. An integrated video camera allowed to define exactly the sample area to be measured. The instrument operates in the frequency range of near and mid-infrared 7000 – 400 cm-1 with a measurement spot of 6 mm in diameter. 128 scans were acquired at a resolution of 4 cm-1. Kramers-Kronig algorithm was finally applied to the spectra to correct the spectral distortions due to reflection/diffusion effects. FTIR-ATR analysis were performed with the same spectrometer equipped with a front ATR module and a diamond crystal to characterize the powders of spray paints. The resulting spectra were collected and evaluated with the spectrum software OPUS® of Bruker Optics. The infrared absorption bands obtained for the investigated paints were identified by comparing the acquired spectrum of the paint with reference spectra in the literature[3]
.
Micro-Raman analyses were performed using a Renishaw Raman Invia instrument equipped with
a 633 nm HeNe laser and a 532 nm Nd:YAG laser. The proper excitation wavelength was selected according to the color of the analyzed paints, and was focused on the sample using a 50× objective. To avoid the fluorescence of the sample, each spectrum was acquired with energy of 1%. The spectra were recorded with a spectral resolution of 4 cm−1 and they were interpreted according to references in the literature 4-6.
XRF spectra were recorded using a handheld TraCer III-SD spectrometer by Bruker equipped with
a Rhodium X-Ray tube and an SDD detector. Spectra were recorded for 60 s at a voltage of 40 kV and current 12 μA using the S1PXRF dedicated software. A preliminary screening for all elements was done without any filter, to excite all elements from Mg to Pu. Successively, an aluminum and titanium filter (yellow filter) was used to empathize the metallic elements Ti to Ag, W to Bi in the samples..
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X-ray diffraction analyses were carried out using an X-ray diffractometer X’ Pert PRO (PANalytical)
for powders (XRDP), anticathode Cu (λ = 1.54 Å, investigated 2θ 3–70°, step size 0.017°, time per step 50 s), equipped with an X'Celerator multidetector. Data were processed using HighScore software and an ICDD database.
A spectrophotometer CM-2600d Konica Minolta was used for the colorimetric analyses. Reflectance measurements were carried out using a D65 lamp in the range of 360 -740 nm. Calibration was performed against a Spectralon; a 10° Standard Observer was used. The diameter of the measurement spot was 8 mm. The specular reflection component of the radiation was collected but it wasn’t considered in the total colour measurament (E*).
Optical and electron microscopy
A steromicroscope Stemi 2000c Zeiss equipped with a camera ACT1 and a software for image manipulation was employed for the optical investigation of the samples.
A Nikon Eclipse E600 microscope, equipped with a halogen lamp and a mercury lamp (OSRAM HBO, 200 W), to carry out optical observations of the cross sections under visible and UV light. The cross sections were prepared by embedding some sample fragments in a bi-component epoxy resin (Epofix, Struers DK) and polished with silica abrasive papers (grit from p120 to p1200*). Finally, laser scanner micro-profilometry analysis was performed in order to produce a topographic map of the investigated marble surface.. The device used for this analysis was realized by INO (Istituto Nazionale di Ottica). The conoprobe consisted of a video camera coupled with a uniaxial birefringent crystal between two circular polarizers; the light source was a laser diode with a λ = 655 nm. The conoscopic probe had a height resolution of about 1μm, an accuracy better that 6 μm, and a transverse resolution of about 20 μm. The scanning velocity was from 100 to 400 points for second. The instrument worked at a standoff distance of 4 cm with a
measurement range of ± 4mm. Areas of 3x10 cm2 were scanned with a resolution of 50 μm (n°
steps on x-axis: 601; n° steps on y-axis: 2001).
ESEM Quanta200 (FEI/ Philips Electron Optics) electron microscope equipped with an X-ray
spectrometer (EDX) was employed for SEM-EDX analyses. The specimen chamber was maintained in a low vacuum (1 Torr), thus avoiding metallization, and the accelerating voltage was 25 keV at 1–3 × 10−7 A.
Chromatographic analysis
To characterize the paint on the basis of the chemical composition of the binder, chromatographic analyzes were also performed. For Py-GC/MS analysis was used a multi-shot pyrolyzer PY-3030D (Frontier Lab, Japan) coupled with a 6890 N gas chromatography system and combined with a 5973 mass selective single quadrupole mass spectrometer (Agilent Technologies, U.S.A.). The samples (0.2mg) were placed in platinum sample cups on silanized glass wool. The cups were placed on top of the pyrolyzer at ambient temperature and then placed into the furnace. Pyrolysis conditions were optimized as follows: pyrolysis chamber temperature and the interface temperature were kept at 600°C and 280 °C, respectively. The GC injector temperature was 280 °C. The GC injection port was operated in split mode with a split ratio of 1:10. The parameters for the pyrolyzer in the experimental condition without derivatization were: initial temperature 40°C, 20°C/ms up to final temperature of 300°C, final temperature held for 10 seconds (condition used for the red paint). The parameters for the pyrolyzer in the experimental condition with hexamethyldisilazane (HMDS) derivatization were: initial temperature 36°C, 10 min isothermal, 10 C/ms up to 280°C, 2 min isothermal, 20°C/min up to final temperature of 310 °C, final
