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The enemy within: deszincificação e corrosão cíclica numa escultura equestre em latão patinado
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A presente dissertação tem como objetivo a caracterização, avaliação do estado e a
elaboração de uma estratégia para a conservação de uma escultura equestre em latão, que
apresenta um mau estado de preservação, devido à presença de corrosão cíclica localizada.
Considerando a falta de informação relativamente à proveniência, data de produção deste
objeto e à sua história procedeu-se à elaboração de uma contextualização com base estilística
e à revisão de literatura sobre o processo de deszincificação em meios com cloro.
Para a elaboração de uma proposta adequada às necessidades de conservação do objeto
recorreu-se a métodos de análise física e química, que forneceram informações essenciais
relativamente à sua constituição e composição, aos problemas de corrosão e dano, e
respetivas causas. O exame de superfície foi realizado por observação por microscopia ótica
com lupa estereoscópica; as análises estrutural e microestrutural foram realizadas,
respetivamente, por radiografia digital e por metalografia; os estudos composicionais foram
realizados por EDXRF, SEM-EDS e DRX.
A escultura, em latão Cu62.2 - Zn36.5 - Sn1.3, foi fundida pela técnica de fundição em areia
em partes distintas e posteriormente unido por brasagem e patinado de castanho. Com base
no estudo do material, foi possível concluir que a presença de cloretos provenientes do
interior do objeto é a principal responsável pela reincidência do processo de deszincificação e
corrosão autocatalítica, desenvolvendo-se um processo de acentuada corrosão ativa. A
progressão deste mecanismo poderá levar à sua instabilidade estrutural, e poderá pôr em
causa o manuseamento e apreciação do objeto.
Foi necessário otimizar uma metodologia de limpeza — mecânica e química —
investigativa da superfície e pré-estabilização, que obedeceu aos critérios de intervenção
mínima, por forma a não comprometer a decoração da superfície, e a promover a
estabilização das camadas de corrosão com um inibidor de corrosão. Elaborou-se ainda uma
proposta metodológica de dessalinização interna controlada, de modo a se atingir uma
estabilidade química segura e que não afetasse a decoração. Foi ainda assegurado o bom
acondicionamento do objeto.
This dissertation aims to characterize, assess the condition and develop a strategy for the conservation of an equestrian sculpture in brass, which is in a poor preservation state, due to the presence of localized cyclic corrosion. Considering the lack of information regarding the origin, date of production of this object and its history, a stylistic-based contextualization was carried out, as well as a literature review on the dezincification process in chlorine-containing media. To prepare a proposal suited to the conservation needs of the object, methods of physical and chemical analysis were used, which provided essential information regarding its constitution and composition, corrosion and damage problems, and their respective causes. The surface examination was performed by optical microscopy observation with a stereoscopic magnifying glass; structural and microstructural analyzes were carried out, respectively, by digital radiography and metallography; compositional studies were performed by EDXRF, SEM-EDS and DRX. The sculpture, in brass Cu62.2 - Zn36.5 - Sn1.3, was cast using the sand-casting technique in different parts and later joined by brazing and patinated in brown. Based on the study of the material, it can be concluded that the presence of chloride inside the object is the main responsible for the development of a dezincification and autocatalytic corrosion process, thus being under aggravated active corrosion process. The progression of this mechanism could lead to structural instability and could jeopardize the handling and appreciation of the object. It was necessary to optimize a methodology of investigative cleaning — mechanical and chemical — of the surface and pre-stabilization protocol, which followed the criteria of minimum intervention, so as not to compromise the surface decoration, and to promote the stabilization of the corrosion layers with a corrosion inhibitor. A methodological proposal for controlled internal desalination was also elaborated to achieve chemical stability without affecting the decoration. The good packaging of the object was also ensured.
This dissertation aims to characterize, assess the condition and develop a strategy for the conservation of an equestrian sculpture in brass, which is in a poor preservation state, due to the presence of localized cyclic corrosion. Considering the lack of information regarding the origin, date of production of this object and its history, a stylistic-based contextualization was carried out, as well as a literature review on the dezincification process in chlorine-containing media. To prepare a proposal suited to the conservation needs of the object, methods of physical and chemical analysis were used, which provided essential information regarding its constitution and composition, corrosion and damage problems, and their respective causes. The surface examination was performed by optical microscopy observation with a stereoscopic magnifying glass; structural and microstructural analyzes were carried out, respectively, by digital radiography and metallography; compositional studies were performed by EDXRF, SEM-EDS and DRX. The sculpture, in brass Cu62.2 - Zn36.5 - Sn1.3, was cast using the sand-casting technique in different parts and later joined by brazing and patinated in brown. Based on the study of the material, it can be concluded that the presence of chloride inside the object is the main responsible for the development of a dezincification and autocatalytic corrosion process, thus being under aggravated active corrosion process. The progression of this mechanism could lead to structural instability and could jeopardize the handling and appreciation of the object. It was necessary to optimize a methodology of investigative cleaning — mechanical and chemical — of the surface and pre-stabilization protocol, which followed the criteria of minimum intervention, so as not to compromise the surface decoration, and to promote the stabilization of the corrosion layers with a corrosion inhibitor. A methodological proposal for controlled internal desalination was also elaborated to achieve chemical stability without affecting the decoration. The good packaging of the object was also ensured.
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Palavras-chave
Conservação Deszincificação Contaminação EDXR Metalografia SEM- EDS/BSE