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.
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•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.
•Daylight resistance of alkyd-, PVA- and PMMA-based paints was evaluated.•Type of substrate and paint composition influence on coatings failures.•Colour degradation is related to organic phase ...loss.•Fluorescent PMMA-based paints experimented the highest colour change.
Solar radiation is one of the deterioration factors par excellence of modern urban artworks, as among other manifestations, it causes loss of colour. This deterioration can compromise the durability of the works, although it is not known to what extent this durability can vary depending on the substrate supporting the paint. We evaluated the susceptibility to sunlight of 10 paints of differing compositions as typically used by urban artists, by exposing them, applied to concrete and brick, to artificial sunlight ageing test in laboratory conditions. Colour changes over time were studied, along with mineralogical (x-ray diffraction), chemical (scanning electron microscopy-energy dispersive x-ray spectroscopy) and physical (contact angle and peeling tests) changes. The durability of the paints on these substrates depends mainly on the paint composition (base, pigment and filler) but also on the type of substrate, as this influences paint adhesion capacity. The greatest colour modification was detected in fluorescent paints, which lose colour due to degradation of the organic phase, and with aggravation of defects during drying, eventually detach from the substrate.
We describe the results of sulphur and oxygen isotope analyses used to identify sources of the gypsum present in black crusts that grow on the granite of historical buildings. The crusts were sampled ...at various locations in and near the city of Vigo (NW Spain) and were analysed for their sulphur content and δ34S and δ18O isotope ratios. Sampled crusts had δ34S values of 7.3‰ to 12.9‰ and δ18O values of 6.56‰ to 12.51‰. Sampled as potential sulphur sources were bulk depositions, seawater, foundation, ashlar and construction materials and combustion residues. The results indicated marine and, to a lesser extent, anthropogenic, origins for the sulphur and ruled out the contribution of sub-soil sulphates by capillary rise from building foundations. Isotope analyses would indicate that cement and mortar were enriched in sulphur after their application in buildings. The fact that facade orientation (towards the sea or fossil fuel pollution sources) was correlated with sulphur isotope distribution pointed to various contributions to black crust formation.
•Gypsum-rich black crusts on granite and possible sulphur sources were studied.•Sulphur and oxygen isotope analysis was used to identify sulphate origins.•Sulphur contributions were marine and anthropogenic.•Mortars on buildings were enriched by sulphur on buildings.•Orientation towards different sulphur sources influenced the δ34SCDT and δ18OSMOW ratios.
Laser ablation is an accepted cleaning technology for cultural heritage stonework. Optimized laser ablation may, however, have different side effects depending on the mineralogical composition of the ...stone. In the case of granitic rocks, observed side effects – colour changes, fracturing or/and mineral melting – have been attributed to laser parameter interactions with the patina to be cleaned from this complex rock with a grained texture and polymineral composition. We describe the influence of the weathering degree of a granite on its response to laser irradiation, confirming that the existence of secondary minerals in the rock influences the intensity and typology of side effects. Knowledge of this influence is of special relevance to the conservation of heritage built with Western European Variscan granites, which are already slightly weathered in quarry.
Two specimens of the same Variscan granite taken from the same quarry but with different colours (suggesting different weathering rates) were subjected to nanosecond (ns) Nd:YVO4 laser irradiation, working at 355 nm (UV irradiation) under different fluences. The specimens responded very differently to laser irradiation in terms of colour changes and resistance of the main minerals to laser radiation. The existence on one of the specimens of kaolinite deposits covering feldspar grains and Fe oxyhydroxides filling fissures seemed to be the reason for the different response to laser radiation. Our findings would suggest the need, during laser interventions, to take into account – in addition to texture and porosity – the degree of weathering of this particular kind of granite, widely used in the architectural and archaeological heritage of Western Europe.
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•Two granite samples with different weathering rate were irradiated with a ns 355 nm Nd:YVO4 at four different fluences.•Colour modification and minimun fluence under which minerals melt were different.•Iron forms and kaolinite, minerals related to weathering, increase susceptibility against laser.
•The polymineral characteristic of the granite influences on the laser cleaning effectiveness.•Nd:YVO4 ns laser at 355nm cleaned granite of red, blue and silver graffiti.•The satisfactory cleaning ...shows a complete extraction of the graffiti without damage in the substrate.•Quartz and K-feldspar grains were successfully cleaned with 0.1–0.2J.cm−2.•Plagioclase cleaning depended on the colour of graffiti to be cleaned.•Biotite at 0.06J.cm−1 started to be cleaned but already damaged.
The polymineralic characteristic of granite implies an additional challenge during the laser cleaning of cultural heritage built of this rock, because the forming-minerals experience different behaviours under the laser radiation, leading to damages of variable nature and intensity.
The aim of this work was to identify the ranges of fluence to satisfactorily and safely clean the four main constituent minerals of granite (quartz, K-feldspar, plagioclase and biotite) painted with three different graffiti colours. The cleaning was performed using a nanosecond Nd:YVO4 laser which works at high pulse rate (kilohertz). The evaluation of cleaning effectiveness was performed in terms of graffiti extraction and mineral damage evaluated by optical, electronic and confocal microscopies and hyperspectral imaging technique.
The results showed that, regardless of the graffiti colour to be cleaned, quartz and K-feldspar grains were satisfactorily cleaned at the same fluence range. However, in case of plagioclase, it depends on the colour of graffiti to be removed. In case of biotite the analytical techniques allowed to confirm that it was already cleaned with the lowest fluence tested and it was the mineral with the highest sensitivity to laser radiation.
Therefore, in order to obtain a satisfactory and safe laser cleaning it would be required the prior identification of the granitic forming minerals to adjust the laser fluence.
► Cleaning using NdYVO4 laser at 355nm of four different graffiti colours applied on granites. ► Analysis of the influence of paint composition and properties of rocks on the process. ► Composition ...of the paints and, in turn reflectance, determine the removal effectiveness. ► The laser removal is not conditioned by properties of the stones.
This paper presents the cleaning efficiency results for four differently coloured graffiti paints applied to two types of granitic stone by Nd:YVO4 laser at 355nm. The paints were characterized in terms of mineralogy and chemistry using x-ray fluorescence, X-ray diffraction, Fourier transform infrared (FTIR) spectroscopy and scanning electron microscope (SEM); paint absorbance in the ultraviolet–visible-infrared range (200–2000nm) was also assessed. The studied granites had different mineralogy, texture and porosity properties. Cleaning efficiency was evaluated by polarized microscopy, SEM, FTIR spectroscopy and spectrophotometer colour measurements. The results indicate differences in the effectiveness of surface cleaning for the blue, red and black paints as opposed to the silver paint, mainly attributed to chemical composition. No evidence was found that the granite properties had a bearing on laser effectiveness, although the degree, type and spatial distribution of transgranular fissures in the stone affected the overall assessment of cleaning effectiveness. Polarized light microscopy observations and colour measurements showed that the intensity and distribution of fissures affect the depth of paint penetration, ultimately affecting the cleaning efficiency for both granites.
•Lichens were cleaned with a Nd:YVO4 laser at 355 nm.•Covering level of the thallus and colour influence on the laser cleaning effectiveness.•Laser caused mineral damages, mainly on biotite.•Laser ...achieved better cleaning of lichens than scalpel.•Scalpel followed by laser offered better results than these procedures alone.•Colour spectrophotometry cannot be used as a reliable tool to assess the cleaning effectiveness.
This paper addresses the evaluation of the cleaning of crustose lichens developing on granite. The evaluation was performed considering the effectiveness of the cleanings and harmfulness exerted on the granite. The laser cleaning of lichen was compared with the most conventional procedure, scalpel. The combination of both procedures was also tested. The study, which was carried out with two different species of crustose lichen, was also focused on the influence of the intrinsic characteristics of the lichen on the effectiveness.
The cleanings were evaluated by optic and electronic microscopies, FTIR and colour spectrophotometry. A previous characterization of the lichen and its interaction with the granite using those analytical techniques were also performed. The laser cleaning effectiveness depends on the coverage and the colour of the lichen; also, the prior mechanical weakening of the lichen by scalpel seemed to improve the laser cleaning. The darkest lichen was satisfactorily removed by laser and with the combined cleaning. Conversely, the lightest lichen was more difficult to extract with laser than the darkest lichen, being necessary to apply both methods sequentially. Despite laser and the combination of methods cleaned satisfactorily the surface, they were unable to eliminate the thalli into fissures.
Three cleaning methods, namely i) a commercial biocide applied by brush with the subsequent extraction of organic remains by a rod, ii) a scalpel or iii) a Nd:YVO4 laser at 355 nm, single application ...as well as paired combinations, were tested for removing from a granite two different crustose lichens: Diploschistes scruposus (lighter with a clearly distinguished thallus) and Polysporina simplex (darker with a poorly developed thallus). In addition to the removal rate, side effects such as melting of biotite and feldspar grains and chromatic changes were assessed. The multianalytical approach was based on stereomicroscopy, light microscopy, Fourier Transform Infrared spectroscopy, Scanning Electron Microscopy and color spectrophotometry. Combining cleaning methods enhanced the removal efficacy. Biocide followed by laser was the most effective method for removing both lichens colonizing this granite. Mineral melting was more intense for the samples covered by Polysporina simplex due to its lower coverage degree and darker color.
•Removal of Diploschistes scruposus and Polysporina simplex with biofilm on granite was tested.•Cleaning efficacy of scalpel, biocide and laser were evaluated singly and dually.•Dual combination is more effective to remove lichen thallus than the single methods.•The best combination for removing lichen thalli (biocide + laser) induced a slight biotite fusion.
•Hyperspectral imaging techniques for determining the degree of crust removal on granites used in Cultural Heritage.•Hyperspectral imaging techniques allow to in situ evaluate of the effectiveness of ...the laser cleaning.•Hyperspectral imaging data are consistent with the information obtained by conventional techniques about the cleaning effectiveness.
In this paper, we present a study of the application of the hyperspectral imaging technique in order to non-destructively evaluate the laser cleaning of the biogenic patina and the sulphated black crust developed on a fine-grained granite used in the construction of Cultural Heritage in NW Spain. The grained polymineral texture of the granite hinders the adjustment of laser irradiation parameters during the cleaning, and therefore the in situ process control. The cleaning was performed with a nanosecond pulsed Nd:YVO4 laser at 355nm. A hyperspectral camera was used to in situ assess the effectiveness of cleaning by recording images of the rock surfaces before and during the laser treatment. Different analytical techniques were used to test the ability of the hyperspectral imaging technique to evaluate the cleaning process of the granite samples: optical microscopy, scanning electron microscopy with energy dispersive X-ray spectrometry (SEM - EDX), Fourier transform infrared spectroscopy (FTIR) and spectrophotometer colour measurements. The results indicated that hyperspectral imaging technique is a reliable and more affordable technique to in situ evaluate the process of laser cleaning of the biogenic patina and the sulphated black crust in fine-grained granites.
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