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paINTed mEtal aRteFActs ConsErvation

Periodic Reporting for period 1 - INTERFACE (paINTed mEtal aRteFActs ConsErvation)

Reporting period: 2018-09-03 to 2020-09-02

The use of science for the conservation of cultural heritage is nowadays widespread. Many studies have been conducted on artworks made of single materials such as paintings, stones and metals. However, cultural heritage conservation scientists are beginning to be interested in the characterisation and conservation of composite artefacts, that have multiple conservation issues to be addressed.
This project focused on composite artworks made of painted metal. Indeed, the particular use of metals as “canvas” has never been investigated even though many masterpieces were created using this technique. The degradation mechanisms occurring to metal artefacts as well as to paints as single materials are known. However, rare studies about painted metals and paint-metal interactions have been undertaken so far. Indeed, there is an extended lack of knowledge about the degradation processes that occur on such artefacts and about the conservation methodology to adopt. The project INTERFACE (paINTed mEtal aRteFActs ConsErvation) aimed to fill this lack of scientific information, having two main objectives:

1. The characterisation of the degradation mechanisms, with particular attention to the processes occurring at the paint-metal interface;
2. The development of a conservation methodology to preserve both paint film and metal substrate.

In particular, the decay mechanisms and the conservation approaches of copper and iron as substrates decorated with linseed oil paints were investigated.
In order to understand the degradation processes that occur on paintings on copper and iron plates, artificial samples painted with ten different pigments in linseed oil were produced. Particular attention was paid to the composition of the metal sheet and to the pigments used, in order to create artificial samples as similar as possible to the ancient materials (WP1). The surface and cross section of the samples were characterized throughout a variety of analytical techniques such as optical and metallographic microscopy (OM), Scanning Electron Microscopy coupled with Energy Dispersive X-Ray Spectroscopy (FE-SEM-EDS), infrared spectroscopy (FTIR) and colorimetry. After the first characterization, samples were artificially aged by cycles of temperature and humidity and by continuous irradiation of UV light. Samples were aged for a total of four weeks (28 ageing cicles). In order to early identify and characterize the degradation processes occurring, after each week of ageing samples were analysed with the same analytical techniques previously described (WP2). During this first part of the project, it was possible to identify the oil and pigments degradation as well as the corrosion processes occurring to the metal plates. Furthermore, for the first time, it was possible to monitor and characterize the formation of new degradation products at the interface of the paint layer and the metal plates, characterizing their morphology, chemical composition, and formation kinetic. At the end of this first part of the project, the first objective was fully achieved.

After the characterization of the degradation processes, the project focused its attention to the study of potential conservation treatments to be applied for the preservation of paintings on copper and iron plates. Two different corrosion inhibitors for the protection of iron and one corrosion inhibitor and two varnishes for the protection of copper were selected and tested (WP3 and partially WP4). The main characteristics tested were the compatibility with both materials, efficiency on paint and metal degradations, aestheticism and minimal colour variation after treatment and safeness for human and environment. Right after the application of the products on bare samples, electrochemical impedance spectroscopy (EIS) and infrared spectroscopy (FTIR) were performed to assess the efficiency of each product, as well as optical microscopy (OM) and colorimetry were performed to evaluate the colour variation. For each metal substrate one product or a mixture of two were selected as the most suitable to be used for the protection of these artefacts.
Furthermore, an extended in-situ measurements campaign was performed at the Gallerie Nazionali Braberini Corsini on real artworks. Several paintings on copper were analysed by non-invasive analytical techniques. Particular attention was paid to the characterization of degradation processes occurring on the artworks.

So far, the project resulted in one peer reviewed paper and two MSc thesis:
1) M. Albini, S. Ridolfi, C. Giuliani, M. Pascucci, M. P. Staccioli, C. Riccucci. Multi-Spectroscopic Approach for the Non-invasive Characterization of Paintings on Metal Surfaces. Frontiers in Chemistry, 2020 (doi: 10.3389/fchem.2020.00289)
2) Loredana Modanesi, Msc Thesis: Studio e caratterizzazione dei processi di degrado in dipinti a olio su supporti di rame. 2020, Sapienza Università di Roma
3) Flavia bravetti, Msc Thesis: Study of conservation treatments for the preservation of oil paintings on metal plates. 2020, Sapienza Università di Roma

The project and its results were also presented during two international conferences and one national workshop.
This worked allowed for the first time to monitor and evaluate the degradation mechanisms occurring on painting on copper and iron plates, with particular attention to the interface layer. Indeed, due to the artistic value of these artefacts, cross-section destructive analyses are precluded on real artworks. The possibility of analysing artificial samples allowed a better understanding of the degradation processes to a greater extent than ever before. Furthermore, the study of possible conservation procedures dedicated to such artworks will allow museums and conservation institutions to preserve these artefacts for the future generations.
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