Caracterização das litologias e patologias da Sé de Lamego: um estudo sobre alteração em monumentos graníticos
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2013-11-14
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A Sé de Lamego é um dos monumentos mais emblemáticos da cidade. O edifício, cuja génese se reporta ao século XII, sofreu várias intervenções ao longo dos diferentes bispados, o que explica a variedade de estilos presentes atualmente. Na sua construção foram utilizadas, pelo menos, cinco rochas graníticas que afloram na região: granito de Lamego, granito de Várzea de Abrunhais, granito de Valdigem, granito das Meadas e um aplito. O estudo petrográfico revelou que todas as litologias em estudo apresentam alguns aspetos de alteração hidrotermal e meteórica, com a presença de minerais secundários, de deformações intercristalinas e de fissuras intra e intergranulares. A caracterização petrofísica e dinâmica confirmou o estado de meteorização de todas as rochas e possibilitou o estudo do seu meio poroso. Essa caracterização foi feita através da determinação de várias propriedades, nomeadamente a densidade, a porosidade, a absorção de água por imersão à pressão atmosférica, a capilaridade e a velocidade de propagação das ondas P. No sentido de aferir a alterabilidade destes materiais pétreos foram ainda levados a cabo ensaios de envelhecimento acelerado: ensaio de resistência à cristalização de sais, ensaio de resistência ao envelhecimento por choque térmico e ensaio de alteração da cor. A identificação e caracterização química dos minerais de sais solúveis do extrato solúvel de 25 amostras recolhidas na superfície das pedras de alvenarias do edifício da Sé de Lamego revelou a predominância cloretos e nitratos, de cálcio e sódio, surgindo ainda raros sulfatos. O cloreto de sódio cristaliza sob a forma de halite e o sulfato de cálcio sob a forma de gesso. A origem destes sais deve-se, sobretudo, à ascensão de soluções salinas. No monumento em estudo é possível observar várias patologias, que são responsáveis, quer pelos danos gravosos nalgumas alvenarias, quer pelo denegrir da sua beleza estética e arquitetónica. A partir do mapeamento cartográfico concluiu-se que a colonização biológica é a forma de alteração dominante, seguida de alterações cromáticas, pátinas, crostas negras, placas, desagregação granular, filmes negros e fissuras. No estudo mineralógico destas patologias, com recurso à microscopia eletrónica de varrimento, foi atestada a presença dos minerais de sais solúveis, como o gesso, a halite e a calcite, sendo possível ainda correlacionar, devido à observação de cinzas volantes, a origem das crostas negras e das pátinas com a deposição de poluentes.
A colonização biológica, dominada por líquenes, constituiu um aspeto relevante no empobrecer da imagem deste imóvel. Através de análises moleculares, e tendo por base a amostragem de 27 espécies liquénicas, foi possível identificar 15 espécies de fungos colonizadores do monumento. Os géneros Phoma, Epicoccum e Alternaria foram os mais comuns. Estes microrganismos, sendo endolíticos, são responsáveis por processos de meteorização física por penetração das suas hifas, e por uma ação bio-corrosiva, devido à secreção de ácidos orgânicos. Na salvaguarda dos marcos históricos, culturais e religiosos de cada território é preponderante o papel cívico dos jovens. A preocupação destes pela perda insanável do património poderá ser fomentada a partir do interesse despertado nas escolas pelas Geociências. Cabe aos professores atualizar conhecimentos e práticas – através da formação contínua - por forma a melhor traduzir a informação científica e assim viabilizar a concretização deste objetivo.
The Cathedral of Lamego is one of the most emblematic monuments of the town. The building, dating back to the twelfth century, underwent several interventions over the different dioceses, which explains the current variety of styles. At least five granitic rocks that outcrop in the region were used in its construction: granite from Lamego, granite from Várzea de Abrunhais, granite from Valdigem, granite from Meadas and an aplite. The petrographic study revealed that all these lithologies present some evidence of hydrothermal alteration and also meteoric action, such as the presence of secondary minerals, intercrystal deformations and intra and intergranular fractures. The petrophysics and dynamic characterization confirmed the degree of weathering of all the rocks and allowed the study of their porous network. That characterization was done through the determination of several properties, namely: density, porosity, free absorption, capilar transmission, and velocity of the P waves propagation. In order to assess the alterability of these stones, accelerated weathering tests were also carried out: resistance to crystallization of salts, resistance to ageing by thermal shock and a color change testing. The identification and chemical characterization of salt soluble minerals in the soluble extract of 25 samples taken at the surface of stones of the building revealed the predominance of chlorides and nitrates, of calcium and sodium, and rare sulfates. Sodium chlorite crystalizes in the form of halite and calcium sulfate in the form of gypsum. The origin of these salts is due mainly to the underground waters that ascend by capillarity through the walls. At the monument it is possible to observe several types of weathering forms which are responsible, either for some significant damage in the masonry, or for denigrating their aesthetic and architectural beauty. From the cartographic mapping it was possible to conclude that the biological colonization is the dominant weathering form, followed by color changes, patinas, black crusts, plates, granular disintegration, black films and fissures. In the mineralogical study of these weathering forms, using electronic microprobe, evidence of the presence of soluble salts such as gypsum, calcite and halite, was found, making it also possible to correlate the origin of black crusts and patinas with the deposition of pollutants due to the observation of fly ash. The biological colonization, dominated by lichens, is an important aspect in the impoverishing of this building image. Through molecular techniques, and based on sampling of 27 species of lichens, it was possible to identify fifteen fungal species colonizing the monument. Phoma, Epicoccum and Alternaria genera were the most common. These microorganisms being endolithic, are responsible for physical weathering truth the penetration of their hyphal, and for a bio-corrosive action due to the excretion of organic acids. Safeguarding some of the historic, cultural and religious milestones of each territory should be considered a preponderant civic role of youth. The concern for the irreparable loss of heritage could be enhanced through the interest motivated by schools for the Geosciences. Teachers will have to update their knowledge and practices - through training – so that they can translate scientific information better and thus enable the achievement of this goal.
The Cathedral of Lamego is one of the most emblematic monuments of the town. The building, dating back to the twelfth century, underwent several interventions over the different dioceses, which explains the current variety of styles. At least five granitic rocks that outcrop in the region were used in its construction: granite from Lamego, granite from Várzea de Abrunhais, granite from Valdigem, granite from Meadas and an aplite. The petrographic study revealed that all these lithologies present some evidence of hydrothermal alteration and also meteoric action, such as the presence of secondary minerals, intercrystal deformations and intra and intergranular fractures. The petrophysics and dynamic characterization confirmed the degree of weathering of all the rocks and allowed the study of their porous network. That characterization was done through the determination of several properties, namely: density, porosity, free absorption, capilar transmission, and velocity of the P waves propagation. In order to assess the alterability of these stones, accelerated weathering tests were also carried out: resistance to crystallization of salts, resistance to ageing by thermal shock and a color change testing. The identification and chemical characterization of salt soluble minerals in the soluble extract of 25 samples taken at the surface of stones of the building revealed the predominance of chlorides and nitrates, of calcium and sodium, and rare sulfates. Sodium chlorite crystalizes in the form of halite and calcium sulfate in the form of gypsum. The origin of these salts is due mainly to the underground waters that ascend by capillarity through the walls. At the monument it is possible to observe several types of weathering forms which are responsible, either for some significant damage in the masonry, or for denigrating their aesthetic and architectural beauty. From the cartographic mapping it was possible to conclude that the biological colonization is the dominant weathering form, followed by color changes, patinas, black crusts, plates, granular disintegration, black films and fissures. In the mineralogical study of these weathering forms, using electronic microprobe, evidence of the presence of soluble salts such as gypsum, calcite and halite, was found, making it also possible to correlate the origin of black crusts and patinas with the deposition of pollutants due to the observation of fly ash. The biological colonization, dominated by lichens, is an important aspect in the impoverishing of this building image. Through molecular techniques, and based on sampling of 27 species of lichens, it was possible to identify fifteen fungal species colonizing the monument. Phoma, Epicoccum and Alternaria genera were the most common. These microorganisms being endolithic, are responsible for physical weathering truth the penetration of their hyphal, and for a bio-corrosive action due to the excretion of organic acids. Safeguarding some of the historic, cultural and religious milestones of each territory should be considered a preponderant civic role of youth. The concern for the irreparable loss of heritage could be enhanced through the interest motivated by schools for the Geosciences. Teachers will have to update their knowledge and practices - through training – so that they can translate scientific information better and thus enable the achievement of this goal.
Descrição
Tese de Doutoramento em Ciências da Terra e da Vida.
Palavras-chave
Monumentos graníticos , Litologias , Patologias , Sé de Lamego