## 4 Multi-Analytical Methods  
To pursue the described diagnostic and monitoring purposes, and to obtain a complete and exhaustive assessment of the conservation status of the facades, the Pietraforte elements of the Palazzo Medici Riccardi were investigated following the multi-analytical approach described in Figure 1.  
The main instrument of documentation, general and detail, was the architectural survey carried out with 3D laser scanner and digital photogrammetry. From this survey, perspective drawings (CAD file), orthophotos, and point clouds of the facades were obtained. Two instruments were used: a Faro Focus X330 laser scanner and Leica TS02 total station. The software used to combine the outputs of the laser scanner and obtain a unique point cloud was Autodesk Recap Pro. The photogrammetric processing was performed with the Agisoft Metashape software. Based on the architectural survey, diagnostic investigations were carried out. First, stone blocks were investigated through a critical optometric survey of the characteristics that may condition their stability, such as overhang, convolute laminations, detachments, and discontinuities (e.g., calcite veins). NDT, including an ultrasonic test and the Schmidt hammer test, were carried out with the aim of assessing the on-site mechanical properties of investigated materials without the need for sampling. These NDT methods are widely applied to investigate the mechanical properties and the state of the conservation of natural and artificial stone and concrete [7; 8; 30; 31; 32; 33; 34; 35; 36; 37]. NDT methods represent a reliable contribution to the monitoring and assessing of stone decay if traditional destructive tests cannot be employed. The ultrasonic pulse velocity test allows for the measurement of the propagation velocity (Vp, m/s) of high-frequency (elastic) mechanical waves (20-1000 Mhz) between two probes within materials; this technique is used for the qualitative evaluation of the compactness and homogeneity of the stone and the detection of cracks, discontinuities, and cavities inside the material. The ultrasonic test was employed through an IMG 5200 CSD, with a resonance frequency of 50 kHz. The Schmidt hammer test, applied for the sclerometric measurements, gives an indication about the strength of the surface portion of the test sample. The instrument used, known as a Schmidt hammer, is a steel hammer operated by a spring. The test measures the rebound of the impact mass on a piston resting directly on the stone surface (Rebound index, R). Various researchers have correlated the Schmidt hammer rebound index with compressive strength, relating rock density, tangent modulus, and Young's modulus. A thorough list of such correlations was presented by [PERSON] and [PERSON] [38]. Estimation of compressive strength is allowed, providing a detailed correlation is developed for the specific material investigated. This work, which wants to propose an effective, quick, easily run, and economic method of investigation, was limited to the use of the rebound index to quickly assess the uniformity in-place of Pietraforte, finding regions of poor quality or deteriorated. The Schmidt hammer used was the Original Schmidt Live L by Proceq, with an impact energy level of 0.735 Nm. This L-type hammer was chosen because of its greater sensitivity in testing weathered rocks [38; 39]; moreover, ISRM endorsed the use of the L hammer for testing rocks [38; 40]. The combination of the two techniques, performed according to a scheme tailored to the object of study, allows us to examine in depth the stone's properties: an ultrasonic pulse velocity test makes it possible to detect cracks, discontinuities, or other damage of the stone matrix in depth, while the Schmidt hammer allows us to evaluate the quality of the surfaces of stone elements. Although the two techniques lead to an estimate of the mechanical characteristics, the integrated and combined use of these in the same points of the stone allows a satisfying characterization.