Deterioration of graffiti spray paints applied on granite after a decade of natural environment.

Graffiti spray paints are commonly used in contemporaneous mural paintings in public spaces, contributing to the transformation of sites and urban life. These outdoor artworks are now beginning to show different deterioration forms, such as physical-mechanical alteration (loss of material and cohesion, etc.) and chromatic changes. However, the deterioration has not been formally characterized, and the influence of the paint composition and underlying substrate are not known. In this study, three non-metallic (red, blue and black) alkyd graffiti spray paints and one metallic (silver) polyethylene graffiti spray paint were applied to two granite stones with different mineralogy and texture and exposed to a natural urban-marine environment near Vigo (NW Spain) for one decade (2010-2020). Physical changes were evaluated by stereomicroscopy, colour spectrophotometry, measurements of gloss, surface roughness and static contact angle, and peeling test. Mineralogical changes were determined by x-ray diffraction and molecular changes by Fourier transform infrared spectroscopy. Moreover, micromorphological and chemical characterization of the surfaces was conducted by scanning electron microscopy. Physical-mechanical changes, such as craquelure and paint loss, depended on the texture of the granite. More specifically, paint on the granite with the finest grain size showed most intense cracking and loss of material. Chemical changes, which were not related to the granite substrate, were most intense in the red and silver paint coatings. In the red paint, loss of binder was accompanied by an intense fading of the colour (due to titanium dioxide relative enrichment), while in the silver paint coating, chemical changes occurred in both the organic binder and aluminium particles, thus darkening the colour. Fewer chemical changes were observed in the blue and black paints. Physical and chemical changes detected in these paints were not correlated.

Cracks in consolidants containing TiO2 as a habitat for biological colonization: A case of quaternary bioreceptivity.

The recently proposed concept of quaternary bioreceptivity applies to substrates treated with coating materials and it is considered in the present study with the alga Bracteacoccus minor and the cyanobacterium Nostoc sp. onto granite specimens treated with ethyl silicate and nano-sized silica doped with different amounts of TiO2 (0, 0.5, 1 and 3 wt%). The findings showed a lack of correlation between the amount of TiO2 and the level of colonization (main bioreceptivity estimator) to the presence of cracks on the surface, which annul the biocidal power of TiO2. Crack formation, which depends on the mechanical properties, greatly influences the bioreceptivity of the material. Thus, the cracks provided anchor points where water is retained, in turn strongly influencing the early stages of colonization kinetics, to a greater extent than the biocidal power of TiO2, which will probably increase as the biofilm develops over the entire surface. In addition, although the cracks were more abundant and wider in the ethyl silicate-based consolidant, the nano-sized silica provided better anchoring points, making the material treated with the corresponding consolidant more bioreceptive.

Impact of wildfire on granite outcrops in archaeological sites surrounded by different types of vegetation.

The lack of scientific information about the effects of wildfire on prehistoric structures and rock art, such as dolmens and petroglyphs, impedes the development of conservation guidelines. In this study, the impact of a recent wildfire (late 2017) on granite outcrops in the San Salvador de Coruxo archaeological site (Vigo, SW Galicia) was evaluated. Samples of the same type of granite were obtained from three sites characterised by different types of vegetation (natural scrub, native deciduous oak and non-native pine-eucalypt forest) in order to determine how the vegetation influences the fire-caused damage to the rock. Three subsamples were taken from each of the granite samples at depths of 1 cm-3 cm to study how fire affects the rock at depth. In all sites, the temperature reached at the granite surface was below 380 °C. No mineralogical changes due to fire exposure were detected, and no physical changes that could be attributed to the effect of the fire on the fissure system of the granite were identified. However, aesthetic colour changes due to the deposition of organic and charred matter, which even penetrated the fissures, were detected. The existence of lignin-derived compounds, lipids and carbohydrates in the samples from the oakwood site indicates greater resistance to fire effects in this type of vegetation than in the other two types. Although preliminary, these findings suggest that oakwoods could act as protective belts around archaeological sites by reducing the wildfire severity, because of their greater resistance to being burnt, and that they could buffer the damaging effects of fire in natural areas where parietal art is found.

Influence of the weathering rate on the response of granite to nanosecond UV laser irradiation.

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.

IR irradiation to remove a sub-aerial biofilm from granitic stones using two different laser systems: An Nd: YAG (1064 nm) and an Er:YAG (2940 nm).

A sub-aerial biofilm (SAB) developed on a granite commonly found in the built cultural heritage of the NW Iberian Peninsula was extracted with 2 different IR irradiations using an Nd:YAG laser at 1064 nm and an Er:YAG laser at 2940 nm. The methodology was based on the application of only one scan in order to evaluate the effect of the laser cleaning operated by applying different consecutive laser scanning and the suitability of these lasers as quick tools. The aim of this comparative study was twofold. The first goal was to find the most satisfactory level of extraction by comparing the results obtained by the different laser sources (IR wavelengths). The other aim was to investigate the by-effects induced by both lasers on each granite-forming mineral. Evaluations were made using stereomicroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy and scanning electron microscopy with energy-dispersive x-ray spectroscopy. The results were interpreted in terms of SAB extraction and damage induced on the granite. The results showed that the Nd:YAG laser achieved the most successful level of cleaning, because it extracted the most SAB, while causing the least amount of damage to the surfaces. Regardless of the fluence applied, the Er:YAG laser did not completely extract the SAB in only one scan; in addition, a more intense melting of biotite grains was found, producing amorphous fusion crusts and losing the distinction of the cleavage planes.

Marking petroglyphs with calcite and gypsum-based chalks: Interaction with granite under different simulated conditions and the effectiveness and harmfulness of cleaning methods.

Marking petroglyphs with chalk is a common practice to enhance them for documentation and reproduction. Although this procedure has started to be less frequently used, there is no knowledge about the interaction between the rock engravings nor about the effectiveness achieved by the common cleaning procedures of such markers considering the chalk extraction and the induced damage to the rock. This study evaluates the interaction between two chalks of different composition (calcite and gypsum) and a granite on which the majority of NW Iberian Peninsula-petroglyphs are carved. Granitic samples marked with these chalks were subjected to artificial rain events and high temperatures (700°C) related to fires. After each aging test, chemical and physical modifications on the rock were analysed by means of stereomicroscopy, x-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy and colour spectrophotometry. Moreover, the evaluation of the effectiveness and harmfulness of several mechanical and chemical cleaning procedures commonly used in the field of cultural heritage conservation was carried out. Both chalks remained at different extent on the surface after the artificial rain events. Water would promote a different penetration-depth of the chalks into the stone, depending on their solubility. High temperatures led to mineral phase transformations of the chalks influencing the interaction with the rock. Regarding cleaning effectiveness, despite a few chalk remains were found in all the cleanings, chemical methods showed higher effectiveness than mechanical procedures even though some of them leave chemical contamination. Benzalkonium chloride can be considered as the cleaner with the best results to extract both types of chalk on granite.

Microscopic characterisation of black crusts on different substrates.

Black crusts on different substrates (carbonate and silicate stones, cement-based mortar and carbonate detrital deposits) in urban environments were characterized microscopically by analysing their morphologies and compositions. The objective of this article is to study the interaction between the substrate and the crust and the influence of the environmental conditions on the crust development. On the one hand, the internal structure and morphology of each sample were evaluated with stereo and scanning electron microscopies. On the other hand, the black crust composition was accessed by X-ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscopy in backscattered electron mode combined with energy dispersive x-ray spectroscopy. The results of these analytical techniques provided interesting information about the composition and the mineralogical phases present in the collected black crusts. In most of the cases, gypsum was detected as the main component exhibiting different habits. Calcite was also detected in all the evaluated gypsum black crusts; its presence was attributed to different origins. The substrate-crust interaction was also evaluated, contributing to distinguish different crust development processes in relation to the substrate. In carbonate substrates (limestones, mortar and carbonate detrital deposits), it was detected a continuous diffuse boundary related to the replacement of Ca-carbonate by Ca-sulphate, while this boundary was significantly more defined for the granitic stone. This study shows that the substrate, the presence of different construction materials, (e.g. mortars), the motor exhaust particulate substances and the concentrations of atmospheric pollutants, the marine influence as well as biological or other anthropogenic compounds are decisive factors in the development of the black crust. Some ideas about the establishment of conservation strategies are also shown.

Effectiveness of granite cleaning procedures in cultural heritage: A review.

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.