B. Silva

Mineralogical transformation and neoformation in granite caused by the lichens Tephromela atra and Ochrolechia parella

Abstract Tephromela atra and Ochrolechia parella are among the most abundant lichens colonizing granitic monuments in the region of Galicia (northwest Spain). In this work, their interaction with a two-mica granite used in the construction of the Toxosoutos Monastery (Noia, Galicia) was studied, using optical microscopy, scanning electron microscopy (with back-scattered-electron and energy-dispersive X-ray detection), X-ray diffractometry and atomic absorption spectroscopy to evaluate their physical, mineralogical and chemical effects. Both lichens contributed to physical weathering by penetrating intermineral voids and mineral cleavage planes, disaggregating the rock and entrapping the loosened mineral grains in their thalli. Significant chemical and mineralogical weathering also occurred, including depletion of potassium from biotite, transformation of this mica into hydroxyaluminium-vermiculite, and neoformation of whewellite and calcite in the lichen thalli. Neoformation of these calcium minerals on a calcium-poor rock such as granite is noteworthy, and this is the first time calcium carbonate has been detected within a lichen colonizing a granitic rock. Precipitation of the calcium carbonate was attributed to the local pH in the thalli having been raised due to release of sodium from nearby plagioclase during weathering.

Application of mercury porosimetry to the study of xerogels used as stone consolidants

Abstract Alkoxysilanes, low-viscosity monomers that polymerize into the porous network of stone by a sol–gel process, are widely used in the restoration of stone buildings. We have used the mercury porosimetry technique to characterize changes in microstructure of three granites following their consolidation with two popular commercial products (Wacker OH and Tegovakon V). The suitability of this technique is questioned because a surprising increase of stone porosity is observed. In order to investigate the feasibility of porosimetry, we analyze the behavior of xerogels prepared from the two commercial products, under mercury pressure. Gels are basically compacted and not intruded by mercury. Thus, the increase of stone porosity after consolidation can actually be associated with gel shrinkage. Mercury porosimetry, therefore, has been found unsuitable for characterizing the microstructure of consolidated rocks. However, it can be employed usefully to evaluate shrinkage of gels under mercury pressure, which permits the behavior of a consolidant during the process of drying in stone to be predicted. It is a key factor because many problems of consolidants are related to their drying process within the stone. Gels under study exhibit a high rigidity and an elastic behavior, as consequence of their microporous structure. Finally, the reduction in the porous volume of gels after the porosimetry test demonstrates that the shrinkage mechanism is based on pore collapse.

Rapid biological colonization of a granitic building by lichens

Abstract The Galician Centre of Contemporary Art (Santiago de Compostela, Spain) is a large, modern building finished with a granitic cladding. Completed in September 1993, it is already showing signs of deterioration, including biological colonization and intense blackening of exterior surfaces, and the breaking off of cladding. In this work we investigated the causes of this premature deterioration. Biological colonization was most intense on the lower areas of shaded walls, the most abundant organisms being the lichens Trapelia coarctata and T. involuta . The blackening was chiefly due to the build up of microorganisms or their remains (no elements indicative of atmospheric pollution were detected). Both the cladding and granite from the source quarry showed signs of severe weathering (fissuration, high porosity, the presence of kaolinite) and had high capillarity, rapidly absorbing large amounts of water and releasing it only slowly. Because of this and the wet climate in Santiago the cladding was almost permanently damp, which, together with its weathered condition, made it highly bioreceptive and, at the same time, reduced its mechanical resistance, making it susceptible to breakage.

Colonization by lichens of granite dolmens in Galicia (NW Spain)

Abstract We studied colonization by linchens of five granite dolmens in Galicia (NW Spain). A total of 60 species, 3 subspecies and 2 varities (some cited for the first time in Galicia) were identified, and the positions in which they occurred on the dolmens were mapped. The factors most strongly influencing colony initiation and position were degree of exposure to rain, insolation, substrate verticality/horizontality, humidity and location (on the inside or outside of the chamber). We also evaluated microscopic-level effects of colonization by five species of lichen Xanthoria parietina, Aspicilia cinera, Diploschistes scruposus, Pertusaria coccodes and Ochrolechia parella ) in samples taken from nearby outcrops of the same granite as the dolmens. Hyphae were observed to penetrate almost exclusively through intermineral voids, except in the case of micas which were penetrated between layers. The only minerological effect observed was degradation of micas to mica-aluminium hydroxy vermicultie intergrades.

Relationship between color and pigment production in two stone biofilm-forming cyanobacteria (Nostoc sp. PCC 9104 and Nostoc sp. PCC 9025)

Previous studies have provided evidence that color measurements enable on site quantification of superficial biofilms, thereby avoiding the need for sampling. In the present study, the efficiency of color measurements to evaluate to what extent pigment production is affected by environmental parameters such as light intensity, combined nitrogen and nutrient availability, was tested with two cyanobacteria, Nostoc sp. strains PCC 9104 and PCC 9025, which form biofilms on stone. Both strains were acclimated, in aerated batch cultures for 2 weeks, to three different culture media: BG-11, BG-110, and BG-110/10 at either high or low light intensity. The content of chlorophyll a, carotenoids, and phycocyanins was measured throughout the experiment, together with variations in the color of the cyanobacteria, which were represented in the CIELAB color space. The results confirmed that the CIELAB color parameters are correlated with pigment content in such a way that variations in the latter are reflected as variations in color.

New Nanomaterials for Protecting and Consolidating Stone

The sol-gel process has been found to be successful in applications for the conservation and restoration of stone. However, a well-known drawback of the materials obtained by this process is their tendency to crack during drying inside the pores of the treated stone. In this article, we present an overview of our current research centred on producing crack-free sol-gel materials for consolidating and protecting building stone. A novel synthesis, in which a surfactant acts as a template to make the pore size of the gel network coarser and more uniform, is shown to provide an effective alternative for preventing the cracking of consolidants. We also highlight an alternative pathway, in which we add an organic component to the silica precursor in the presence of the surfactant. The hybrid organic-inorganic gel prepared in our laboratory provides excellent waterproofing to the stones under study.

Effects of lichens on the geochemical weathering of granitic rocks

Abstract This paper examined the role of lichens in geochemical weathering of granite for several lichens representative of the species colonizing granitic monuments in Galicia (N.W. Spain). The salient results were that although the lichens affected the mobility of some elements, overall geochemical weathering was similar for colonized and uncolonized rock. Where differences were detected the various lichen species generally had similar effects on the weathering of the rock. Lichens seemed to protect the exposed surface of the rock from atmospheric weathering, thereby reducing losses of the most mobile elements. However, the overriding factor determining the extent to which an element was accumulated at, or mobilized from, the surface of colonized rock was the susceptibility to weathering of the source mineral. A new term, “interphase”, was introduced to describe rock engulfed in lichen thalli.

Spectrophotometric color measurement for early detection and monitoring of greening on granite buildings

This paper addresses the detection and monitoring of the development of epilithic phototrophic biofilms on the granite façade of an institutional building in Santiago de Compostela (NW Spain), and reports a case study of preventive conservation. The results provide a basis for establishing criteria for the early detection of phototrophic colonization (greening) and for monitoring its development on granite buildings by the use of color changes recorded with a portable spectrophotometer and represented in the CIELAB color space. The results show that parameter b* (associated with changes of yellowness-blueness) provides the earliest indication of colonization and varies most over time, so that it is most important in determining the total color change. The limit of perception of the greening on a granite surface was also established in a psycho-physical experiment, as Δb*: +0.59 CIELAB units that correspond, in the present study, to 6.3 μg of biomass dry weight cm−2 and (8.43 ± 0.24) × 10−3 μg of extracted chlorophyll a cm−2.

Effect of Surface Finish on Roughness, Color, and Gloss of Ornamental Granites

The effects of four of the most common types of surface finish on the appearance of five varieties of ornamental granite, all widely used in building construction and selected for their different colors, were analyzed by means of roughness, color, and gloss measurements. The results demonstrated that different surface finishes produce differences in color, especially in the lightness parameter (L* ), and that the magnitude of these differences depends on the color of the ornamental granite and is greatest in dark colored rocks. However, the variation in the color parameters with the different surface finishes did not depend on roughness, and no general conclusions could be drawn regarding the influence of the roughness on the color of ornamental granite. Gloss values were affected by the color of the ornamental granite, but in a different way for smooth and rough surfaces. Variation in gloss also depended on the mineral composition of the rock. Gloss and roughness were inversely related, but only within t...

Color of cyanobacteria: some methodological aspects

Although the color of cyanobacteria is a very informative characteristic, no standardized protocol has, so far, been established for defining the color in an objective way, and, therefore, direct comparison of experimental results obtained by different research groups is not possible. In the present study, we used colorimetric measurements and conventional statistical tools to determine the effects on the measurement of the color of cyanobacteria, of the concentration of the microorganisms and their moisture content, as well as of the size of the target area and the minimum number of measurements. It was concluded that the color measurement is affected by every factor studied, but that this can be controlled for by making at least 10 consecutive measurements/9.62 cm(2) at different randomly selected points on the surface of filters completely covered by films of cyanobacteria in which the moisture contents are higher than 50%.