Indagini diagnostiche e procedure chimico-fisiche per i Beni Culturali

Nowadays, the concept of Cultural Heritage is deeply heart-felt; each country tries to promote and protect the artistic works standing on its territory, as they are a manifestation of culture and, at the same time, they can become an instrument of wealth for the country itself. Talking about protection and enhancement of the Cultural Heritage, it results mandatory to protect these artistic assets from degradation phenomena as they are often subjected to: these kind of phenomena are becoming even more pressing. Research in this field is putting a great deal of efforts, as the protection of artistic works is now widespread and relies on scientific techniques to prevent, treat and improve the status of the works. The doctorate project focused both on the research of new methods for the conservation and restoration of artistic works (in particular the made of inorganic materials, such as artificial and natural stones) and on the increase of knowledge on Giulio Monteverde’s artistic techniques, a well-known 20th Century sculptor, by means of diagnostic tools. The cleaning method for porous materials affected by saline degradation developed during this research work is based on the international patent PCT / IB2015 / 055129, G. Torrielli, L. Gaggero, M. Ferretti owned by the University of Genoa. For brevity and clarity, during the elaborate, we will refer to this patent as "extraction method by suction" [1,2]. Currently, there are several techniques that face the salts problem: preventive solutions, such as polarity reversal devices, electromagnetic fields [3,4] and isolation of the foundations, which try to solve the problem at the origin in order to avoid that the salts enter in contact with and then penetrate into the materials. Differently, when the degradation is already underway, there are possible solutions where it is necessary to extract the present salts, such as extractive poultice applications, which are available today on the market [5,6]. These poultices are made of absorbent materials, such as clays, kaolin, talc, cellulose pulp, etc. The object of this thesis fits itself in the second group of methods for the soluble salts removal. The proposed method exploits a mechanical action in the salts extraction from the material phase: the process of removing the salts in solution towards the outside is speeded up through a micro-suction point. The first part of the work, which was carried out in the laboratory, was designed in order to define all the optimal operating parameters of the extractive method on different porous materials (such as plasters, frescoed plasters, bricks and stone materials). Once contaminated with known concentrations of saline solutions, the materials were cleaned both with the commercial technique and with the suction extractive method. The results showed that the proposed technique is faster and more efficient than traditional wraps. In the light of the obtained results, the extractive method was applied on two different real cases: a farmstead of the early ‘900 not subject to protection restrictions, and a protected property (Santa Giulia’s Church of Monastero Bormida, 18th century). In both structures, a monitoring was carried out over time to assess any saline re-growth. It emerged that the suction extractive method, as well as the wraps, allows a surface cleaning that may be however in vain, if the main deterioration source is not drained, like, for example, architectural interventions able to isolate the capillary rising of the water from the ground. The results obtained have shown that the suction extractive method can be used for the cleaning of porous materials, allowing a good removal of the salts, conveying even the ones situated in depth towards the surface. In collaboration with the Department of Earth Sciences of the University of Zaragoza, a preliminary method was developed to define saline distribution in stone materials inspired by a known technique in the geological field for land geophysical measurements [7]. The method has led to the development of a model that correlates the measured resistance of the material to the presence and distribution of salts; furthermore it could be applied even in small scale and providing non-invasive measurements for the material. In addition, another technique to be applied in the restoration field for the removal of organic coatings from surface of stone materials was investigated. This technique exploits titanium dioxide nanoparticles (TiO2). This material is well-known since many years for its photocatalytic properties, and, for this reason, it was employed in the medical, environmental and restoration field [8-10]. The photocatalytic mechanism exploits solar energy, which is able to activate the photocatalyst. When TiO2 is irradiated with a suitable wavelength, an electron-hole couple is formed and they can lead to the formation of radical species that are capable of mineralizing completely the organic substances. These features, specific of TiO2 were tested on natural and artificial stone material (brick) at the Chiostro Vecchio of Lodi, an architectural asset protected by the Superintendence of Lombardy. The structure showed a widespread aesthetic degradation due to biological coatings. In the literature the use of TiO2 for the removal of bio-degrading agents and the creation of self-cleaning films is already reported and consolidated [11-15]; therefore, in agreement with the Superintendent, it was decided to apply photocatalytic nanoparticles to test their restoring capability. The diagnostic investigations carried out on the site have shown the presence of a thin layer of natural wax on the bricks, probably due to a restoration of the nineteenth century. Before performing the tests in the real case, a laboratory study was carried out in order to identify the type of nanoparticles able to degrade the surface biological coating without affecting the underlying wax layer, which was decided to be preserved. Tests were carried out on natural wax samples treated with four different types of titanium dioxide nanoparticles: three of them were synthesized in the laboratory using the sol-gel technique (anatase TiO2, N-doped TiO2, S-doped TiO2) [16], the fourth is a commercial one (TiO2 P25 Sigma-Aldrich). The results showed that only the TiO2 P25 produced a degradation of the natural wax layer; for on-site application synthetic anatase TiO2 was chosen as it did not degrade the wax layer, but at the same time presented the best photocatalytic yield. The TiO2 application at the Chiostro Vecchio led to a good degree of cleaning of the biological coating. The last part of the work concerned the diagnostic investigation of samples taken from a plaster sculpture of Giulio Monteverde, a well-known artist of the twentieth century. The investigations allowed to widen the knowledge on a part of his unknown work, in particular on his executive technique for the realization of the plaster works and on the components of mixtures he used. The study was conducted on one of the most Monteverde famous gypsum works: "Ideality and Materialism" [17,18]. This sculpture is part of a series of gypsum artworks, a cheap material used by Monteverde for copies to be reproduced in more durable and precious materials, like marble or bronzes. With the collaboration of the Superintendence of Piedmont, it was possible to pick up samples from the surface of the “Materialism” figure and perform in-depth instrumental analyses (optical microscopy, SEM-EDS electron microscopy- microprobe, FTIR spectrometry, XRD of powder) that allowed us to identify the presence of a thin layer of finishing, the sculpture "skin", composed of a mixture made of gypsum, calcite and anhydrite, which is a different mixture with respect to the underlying plaster layer. These data provided important information on Giulio Monteverde’s sculptural technique, and proof of the artist’s choice to create a very particular blend for the final layer of his work, perhaps for aesthetic reasons and to improve the mechanical properties of the layer itself.