Leseprobe

23 Cagnini | Galeotti | Porcinai | Santagostino Barbone | Mercante Methods of investigation Different analytical techniques, both non-invasive and invasive, were used to identify the materials used for the production of the artefacts. For the creation of 3D models, an Ametek ® Creaform ® Go!Scan20 ® scanner was used with an accuracy up to 0.1 mm, a resolution of 0.2 mm, and a measurement rate of 1.500.000 measures/sec. The scanner uses the Structured Light technique and emits in visible light through a projector. It features a multiple acquisition system in the visible range with three digital cameras with CMOS sensors: two cameras for triangulation of the acquired points and one camera dedicated to the texture information (color from photo recording). The projected series of patterns are acquired by cameras and the dedicated software calculates the deformation of these patterns projected on real surfaces. From de­ formation, the software generates the 3D cloud of XYZ points. Real time calculation of 3D cloud generates a triangle mesh, a virtual representation of the object surface. X-ray fluorescence (XRF) measurements for the non-invasive identification of the alloys and of the polychromy were performed by an XGLab Elio portable spectrometer (incidence angle, 90°; spot size 2.5mm), equippedwith a Silicon-Drift Detector (active depth= 500 μm, Take-off angle = 63.5°, Sample-detector distance = 14 mm) and a Rh anode. Eddy Current techniques were used both for sorting metals and alloys and for measur- ing coating thickness. For the first, a Sigmascope SMP 10 (Fisher) with ES40 probe at 60 kHz frequency was used. Calibration of the device was verified using a copper certificate standard 101 % IACS. Coating thickness was measured with a Leptoskop 2042 (Karl Deutsch, Germany) equipped with a 1 mm diameter probe (diameter leaning point 12 mm). Eddy Current techniques allow the sorting of alloys and the detection of defects in metals, but also the measurement of the thickness of non-conductive coatings on conductive sub- strates. 5 In this study, this technique was very helpful for both aspects, and in particular, it helped identify areas on polychromy with higher thickness. This information allows addressing the sampling of small flakes where it is more likely to find the complete super- position of painting layers. The measurement is quick, easy to interpret, and fully non-invasive; however, limitations are connected to the surface characteristics, since the area being probed needs to be flat, and the measurement will be affected by surface roughness and defects. Samples including metal and polychromy were taken and analysed as flakes, or embed- ded in polystyrene resin, and examined as cross-sections. The latter were polished with abrasive paper down to P1200 with an average particle diameter of 15.3 µm and observed using a Zeiss Axioplan microscope, with UV and visible light. An EVO ® MA 25 Zeiss scan- ning electron microscope equipped with an Oxford EDS X-MAX 80 mm 2 microprobe and AZTEC ® system with a 20 keV voltage was used (SEM-EDS). Cross-sections were coated with a carbon coating prior to analysing them. FTIR analyses were performed using a Continuum Infrared Microscope linked to a Nicolet Nexus spectrometer, with a spectral resolution of 4 cm -1 (128 scans) in transmittance mode.