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.