## 1 Introduction  
Historical architectural heritage requires specific maintenance and restoration interventions, especially in the presence of prolonged deterioration over time. Restoration works may control deteriorations, which can affect monuments, without secondary effects on treated surfaces of the historical elements [1; 2; 3; 4; 5]. Similarly, diagnostics, which is a fundamental phase of knowledge in order to deepen the critical aspects of structure assessments and degradation causes, must operate with technologies that can be used in a non-destructive and non-invasive way for the historical buildings [6; 7; 8; 9; 10; 11; 12; 13]. The main purposes related to the general framework of the material maintenance conditions are those to refine methodological paths for such contexts, to have well-founded analytical support and to reduce the error range in the qualitative and quantitative evaluation of the damage process. Ultrasonic tests are one of the non-destructive techniques most often used in the field of diagnostics on historical buildings [14; 15; 16]. They measure the wave speed through the material thickness, which is in relation to the dynamic elastic modulus. The results obtained by tests on architectural heritage are representative, if compared with known physical characteristics of similar materials, of the degradation inside the structural section of historical elements. In addition, the impact tests, performed with an instrumented hammer in the same points of the ultrasonic test, enable the evaluation of the surface degradation level [17]. This research concerns the mechanical characterization, through non-destructive techniques (NDT), of the 13 stone columns (granitoid rock) that support the Via Crucis portico inside the monumental complex of the Sacro Monte in Ghiffa. The Sacri Monti of northern Italy are a UNESCO heritage site that includes nine complexes that constitute groups of chapels and other buildings with many artistic masterpieces. All buildings are part of a high-quality natural environment. The first complex was founded in the late 15 th century in Varallo Sesia, and the others were constructed up to the end of 17 th century and dedicated to different aspects of the Christian faith [18]. In addition to their symbolic spiritual meaning, they are of great beauty by virtue of the skill with which they have been integrated into the surrounding natural landscape of hills, forests and lakes. They also house much important artistic material in the form of wall paintings and statuary. Many buildings within the Sacri Monti monumental complexes suffer from different conditions of degradation, linked in particular to the climatic conditions of moisture, rain, damp action from capillary rising, freeze-thaw cycles that cause the detachment of plaster and the degradation of the stone elements [19]. A research group of the Politecnico di Torino has been interested for years in the study of non-destructive diagnostic techniques for the evaluation of degradation levels of the architectural and structural elements of historical buildings in the Sacri Monti complexes [1; 20; 21; 22; 23]. In the specific case of the Sacro Monte at Ghiffa, the Via Crucis portico (Figure 1) is supported by 13 stone columns with evident intense superficial material degradation. The portico was built between 1752 and 1761. It is marked by 14 spans and has columns and capitals made by stones derived both from local \"granitic stones\". It is possible that the origin of the stone of columns is not from historical quarries but from erratic boulders found near the construction site (aplite is the current interpretation, but other investigations are in progress), referred to in slang as \"trovanti\", while for the capitals there is a strong similarity with a serizzo quarried near Locarno (Valle Verzasca, Switzerland) (Figure 2).