This thesis work is strongly connected to the setup of the conservation strategy of a series of bronze artworks of the Cimitero Monumentale of Milan. Six bronze statues or bas-reliefs were selected being representative of the different conservation conditions observed on a wider group of 48 statues still to be restored. A multi-analytical in situ approach was associated to the analysis of few micro-samples to characterize the materials and their state of conservation, and to evaluate different cleaning procedures. Cleaning is indeed a very critical stage of the conservation of an artefact because it is intrinsically irreversible. Moreover, in case of bronze substrates it may lead to the re-activation of the surface and, consequently, to a temporary acceleration of the degradation rate. Standard guidelines for cleaning of metal artefacts are still not available; therefore, the most appropriate procedure should be evaluated case by case, possibly by using portable non-invasive techniques. Such techniques play a very important role in the monitoring and diagnostic of cultural heritage. In the case of metallic artefacts, it is possible to measure in a non-invasive way the real-time corrosion rate by means of electrochemical techniques. However, there is still the need for further systematic studies to better define the influence of the different parameters of the experimental setup on the recorded data. In such way, the potentiality of electrochemical measurements for Cultural Heritage conservation could be fully exploited. Corrosion inhibitor associated with protective coatings are frequently used after cleaning. The research towards innovative and highly effective solutions is very active; it is aimed at fulfilling requirements for the application to Cultural Heritage and to satisfy the environmental/health regulation. The research of green and non-harmful substances as alternative to Benzotriazole, the traditionally employed corrosion inhibitor, represents a particularly promising field. The purposes of this thesis are: i) to evaluate the influence of selected parameters and to set-up an optimized and sustainable methodology for Electrochemical Monitoring techniques; ii) to evaluate and monitor short and long-time variation of the surfaces’ properties after four different cleaning procedures; iii) to evaluate the performances of innovative protective treatments. Materials characterization was performed through a multi-analytical and micro-invasive approach. On site diagnostic and monitoring was performed by means of: digital portable microscopy to evaluate the morphology of the surfaces; portable VIS-light spectrophotometry in the CIEL*a*b* colour space to characterize the colour of the surface; Linear Polarization Resistance (LPR) and Electrochemical Impedance Spectroscopy (EIS) to evaluate Polarization Resistance by using a portable potenziostatic device. The laboratory diagnostic protocol on micro-samples included: Fourier Transformed Infrared Spectroscopy (FTIR); X-ray powder diffraction (XRD); scanning electron microscopy (SEM) analysis coupled with EDX analysis. Significant results were obtained after mechanical cleaning (by scalpel) of artefacts that presented an external low-adherent or porous patina with low organic residues: such technique allowed to reach a satisfactory cleaning level and to obtain a compact and uniform patina. In addition, the electrochemical monitoring over time showed a progressive increase in the polarization resistance. After six months from cleaning, the polarization resistance was higher than the one before cleaning, thus indicating a protective behaviour of the naturally evolved patina. This result may be useful with respect to the selection of the most suitable cleaning procedure and to the scheduling of the application of protective coatings after cleaning, in order to effectively exploit the natural protection provided by the patina.
Questo lavoro di tesi è fortemente connesso alla messa a punto della strategia di conservazione di una serie di opere di bronzo del Cimitero Monumentale di Milano. Sei statue bronzee o bassorilievi sono stati selezionati rappresentando le diverse condizioni di conservazione osservate su un più ampio gruppo di 48 statue ancora da restaurare. Un approccio multi-analitico in situ è stato associato all'analisi di pochi micro-campioni per caratterizzare i materiali e il loro stato di conservazione e per valutare le diverse procedure di pulizia. La pulizia è infatti una fase molto critica della conservazione di un artefatto perché è intrinsecamente irreversibile. Inoltre, in caso di substrati di bronzo può condurre alla riattivazione della superficie e, di conseguenza, ad un'accelerazione temporanea della velocità di degradazione. Le linee guida standard per la pulizia di manufatti metallici non sono ancora disponibili; Pertanto, la procedura più appropriata dovrebbe essere valutata caso per caso, possibilmente utilizzando tecniche portatili non invasive. Tali tecniche svolgono un ruolo molto importante nel monitoraggio e nella diagnosi del patrimonio culturale. Nel caso di manufatti metallici, è possibile misurare in modo non invasivo la velocità di corrosione in tempo reale mediante tecniche elettrochimiche. Tuttavia, c'è ancora la necessità di ulteriori studi sistematici per meglio definire l'influenza dei diversi parametri della configurazione sperimentale sui dati registrati. In tal modo potenzialmente potessero sfruttare la potenzialità delle misure elettrochimiche per la conservazione del patrimonio culturale. L'inibitore della corrosione associato a rivestimenti protettivi viene spesso utilizzato dopo la pulizia. La ricerca su soluzioni innovative e altamente efficaci è molto attiva; Ha lo scopo di soddisfare i requisiti per l'applicazione del patrimonio culturale e di soddisfare la normativa ambientale / sanitaria. La ricerca di sostanze verdi e non dannose in alternativa a Benzotriazole, l'inibitore della corrosione tradizionalmente utilizzato, rappresenta un campo particolarmente promettente. Gli scopi di questa tesi sono: i) valutare l'influenza dei parametri selezionati e impostare una metodologia ottimizzata e sostenibile per le tecniche di monitoraggio elettrochimico; Ii) valutare e monitorare la variazione breve e lunga delle proprietà delle superfici dopo quattro diverse procedure di pulizia; Iii) valutare le prestazioni dei trattamenti protettivi innovativi. La caratterizzazione dei materiali è stata eseguita attraverso un approccio multi-analitico e micro-invasivo. La diagnostica e il monitoraggio in loco sono state effettuate mediante: microscopia portatile digitale per valutare la morfologia delle superfici; Spettrofotometria portatile VIS-light nello spazio CIEL * a * b * per caratterizzare il colore della superficie; La resistenza di polarizzazione lineare (LPR) e la spettroscopia di impedenza elettrochimica (EIS) per valutare la resistenza di polarizzazione utilizzando un dispositivo potenziostatico portatile. Il protocollo diagnostico di laboratorio sui micro-campioni comprendeva: Spettroscopia ad Infrared Trasformata (FTIR) di Fourier; Diffrazione della polveri a raggi X (XRD); Scansione microscopia elettronica (SEM) accoppiata con analisi EDX. Risultati significativi sono stati ottenuti dopo la pulizia meccanica (mediante bisturi) di manufatti che presentavano una patina esterna bassa o aderente con bassi residui organici: tale tecnica consentiva di raggiungere un livello di pulizia soddisfacente e di ottenere una patina compatta e uniforme. Inoltre, il monitoraggio elettrochimico nel tempo ha mostrato un aumento progressivo della resistenza di polarizzazione. Dopo sei mesi dalla pulizia, la resistenza alla polarizzazione è stata superiore a quella prima della pulizia, indicando così un comportamento protettivo della patina evoluta naturalmente. Questo risultato può essere utile per la scelta della procedura di pulizia più idonea e per la pianificazione dell'applicazione di rivestimenti protettivi dopo la pulizia, al fine di sfruttare efficacemente la protezione naturale fornita dalla patina
In situ non-destructive techniques for the study of cleaning and protection methodologies for the conservation of bronze artefacts
ZAPPA, MARIA AGNESE
2015/2016
Abstract
This thesis work is strongly connected to the setup of the conservation strategy of a series of bronze artworks of the Cimitero Monumentale of Milan. Six bronze statues or bas-reliefs were selected being representative of the different conservation conditions observed on a wider group of 48 statues still to be restored. A multi-analytical in situ approach was associated to the analysis of few micro-samples to characterize the materials and their state of conservation, and to evaluate different cleaning procedures. Cleaning is indeed a very critical stage of the conservation of an artefact because it is intrinsically irreversible. Moreover, in case of bronze substrates it may lead to the re-activation of the surface and, consequently, to a temporary acceleration of the degradation rate. Standard guidelines for cleaning of metal artefacts are still not available; therefore, the most appropriate procedure should be evaluated case by case, possibly by using portable non-invasive techniques. Such techniques play a very important role in the monitoring and diagnostic of cultural heritage. In the case of metallic artefacts, it is possible to measure in a non-invasive way the real-time corrosion rate by means of electrochemical techniques. However, there is still the need for further systematic studies to better define the influence of the different parameters of the experimental setup on the recorded data. In such way, the potentiality of electrochemical measurements for Cultural Heritage conservation could be fully exploited. Corrosion inhibitor associated with protective coatings are frequently used after cleaning. The research towards innovative and highly effective solutions is very active; it is aimed at fulfilling requirements for the application to Cultural Heritage and to satisfy the environmental/health regulation. The research of green and non-harmful substances as alternative to Benzotriazole, the traditionally employed corrosion inhibitor, represents a particularly promising field. The purposes of this thesis are: i) to evaluate the influence of selected parameters and to set-up an optimized and sustainable methodology for Electrochemical Monitoring techniques; ii) to evaluate and monitor short and long-time variation of the surfaces’ properties after four different cleaning procedures; iii) to evaluate the performances of innovative protective treatments. Materials characterization was performed through a multi-analytical and micro-invasive approach. On site diagnostic and monitoring was performed by means of: digital portable microscopy to evaluate the morphology of the surfaces; portable VIS-light spectrophotometry in the CIEL*a*b* colour space to characterize the colour of the surface; Linear Polarization Resistance (LPR) and Electrochemical Impedance Spectroscopy (EIS) to evaluate Polarization Resistance by using a portable potenziostatic device. The laboratory diagnostic protocol on micro-samples included: Fourier Transformed Infrared Spectroscopy (FTIR); X-ray powder diffraction (XRD); scanning electron microscopy (SEM) analysis coupled with EDX analysis. Significant results were obtained after mechanical cleaning (by scalpel) of artefacts that presented an external low-adherent or porous patina with low organic residues: such technique allowed to reach a satisfactory cleaning level and to obtain a compact and uniform patina. In addition, the electrochemical monitoring over time showed a progressive increase in the polarization resistance. After six months from cleaning, the polarization resistance was higher than the one before cleaning, thus indicating a protective behaviour of the naturally evolved patina. This result may be useful with respect to the selection of the most suitable cleaning procedure and to the scheduling of the application of protective coatings after cleaning, in order to effectively exploit the natural protection provided by the patina.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/133336