NON DESTRUCTIVE TECHNIQUES IN THE POST-INTERVENTION: DIAGNOSIS AND MONITORING OF ARCHITECTURAL SURFACES IN BUILT CULTURAL HERITAGE .

The present contribution deals with the use of non destructive techniques (NDTs) in the assessment of the conservation state, evaluation of the intervention effectiveness and monitoring of restored historical architectural surfaces [1]. A brief review of ND methods tailored on the authors experience will be given, together with the description of some NDT applications on real case studies, in relation to selected types of decay commonly found. Furthermore, an open discussion on i) the role and the good practices for ND monitoring; ii) the problems related to the individuation of indicators and corresponding threshold values for evaluating the materials conservation state; iii) the efficacy/durability of conservative interventions are discussed. 1. Introduction In the last 30 years, conservators have become more conscious of the restoration risks linked to the incomplete assessment of the intervention effectiveness and to the use of innovative restoration materials and techniques, whose behavior and compatibility over time is not entirely known, yet. The post-intervention diagnosis and the monitoring over time have been recognized as an essential action in the conservation of the Cultural Heritage (CH). n this frame, NDTs could give a significant contribution not only in the definition of the conservation state of architectural surfaces, but also during and after maintenance and restoration interventions. The NDTs can address and reduce invasive sampling and allow an extensive evaluation of the variation of some characteristics (colour, water absorption, surface texture, cohesion, detachment, etc.) [2]. The importance of NDTs is enhanced for continuous or repeated monitoring campaigns in order to assess the restoration functionality accordingly to specific conservation questions. Throughout these campaigns, the use of invasive techniques might be however necessary but they should not be a priori choice if not supported and planned on the results obtained by NDTs. 2. Consideration about NDT and common decay in historical surfaces In this contribution different case studies will be presented in relation to decay forms and material properties, significant of the post intervention state of architectural surfaces. The decay forms considered, chosen because of their frequency and impact on historical surfaces, are: detachments, disaggregation/loss of material coherence, discolouration, deposits and moisture. The mural paintings of Léon Tarasewicz in the Perusini’s Tower of Corno di Rosazzo, Udine (Italy), has been analyzed by IR-thermography, working in the middle IR range 3-5 µm. This NDT allowed the detection of subsurface defects, such as: detachments of the painting layers not visible by naked eye observation, cracks and air voids[3,4]. The investigation of the material coherence might be evidenced by 2D and 3D ultrasonic tomography. In the cases of 'Loggia dei Viretti’ at the Sacra di San Michele near Turin and of the Ghirlandina Tower in Modena the US measurements evidenced the increased coherence after the consolidation intervention and homogenously treated area [5-7]. The variation of the surface behaviour after the application of a protective product can be investigated by colorimetry of treated surfaces [9] or, in relation to liquid water, with in-situ contact SOCIETÀ CHIMICA ITALIANA Divisione di Chimica dell’Ambiente e dei Beni Culturali 78 angle measurement and water absorption at low pressure with the pipette method [10]. These three methods were useful in the case of Ghirlandina Tower in Modena where they evidenced the decrease of the water repellent effect after 5 years. The analyses of the surface composition can show the difference among original, decay deposits and superimposed materials. For this purpose, Raman spectroscopy with hand-held instrument was used in the didactic workshop held in Cibiana di Cadore (Belluno) [8]. 3. Conclusion and perspectives Performing an analysis implies: economic costs, availability of analytic techniques and of skilled professionals; furthermore in the field of cultural heritage it implies also a “cultural-social” cost (e.g. the collection of samples might be a in itself a cost since it affects the artefact integrity). Therefore, the balance cost-benefit should be taken into account in CH diagnosis and monitoring, by involving non or micro invasive, easy to do, effective analyses before more expensive and invasive investigation [2, 8]. Furthermore, an increased contribution of NDTs in the post intervention monitoring and safeguarding of historical surfaces is of particular importance, since it allow a farsighted and well-planned monitoring that permits timely and effective conservation actions. Specific protocols and best practices should be developed in order to reduce the subjectivity and to obtain complementary information from several ND methods.