Читать книгу Monument Future - Siegfried Siegesmund - Страница 222

The sampling was made by removing with a small, very sharp chisel micro fragments of the marble of the four columns (Fig. 3) from now on indicated as follows: column A = back, left (looking from the front of the ciborium); column B = front, left; column C = back, right; column D = front, right.

Also sampled were some of the gildings (including the relative red bole-preparation) and of the patinas. Polished cross-sections of the latter two components were prepared and later examined in polarized reflected light and in ultraviolet light under a LEITZ DM RXP microscope. The same sections were then studied under a SEM coupled with an EDS microanalysis (EVO+BRUKER) for the topographical and chemical analyses. Raman spectroscopy supported the identification of the pigments used for the preparation of gilding layers. The chemical nature of the gilding preparation and of eventual past treatments was determined by FTIR and µFTIR analysis using ThermoScientific instruments (iZ10 and iN10 Infrared Microscope). The µFTIR investigations of allowed the mapping of the distribution and penetration depth of the organic components, whereas FTIR analyses of microflakes on standard KBr pellets allowed better identification of the mixture-components.

To identify the marble provenance, a small portion of each marble sample was finely ground and the powder subjected to X-Ray diffraction (PANalytical Empyrean X-ray diffractometer, Cu-Kα radiation at 40 kV and 20 mA) to evaluate the possible presence and relative abundance of dolomite. O and C stable isotopes ratio analyses were performed on the same powders through a Gasbench II preparation line connected on-line to a Thermo Finnigan Five Plus mass spectrometer in a continuous flow mode. All δ13C and δ18O values were measured against a PDB standard: the results were then plotted in reference isotopic diagrams obtained from the most updated database (Antonelli and Lazzarini, 2015). The remaining larger fragment of each marble sample was used for the preparation of a thin section studied under a LEITZ DM RXP polarising microscope. The main petrographic features of marbles (fabric, boundary shapes of the carbonate crystals, maximum grain size (MGS) of the largest crystal of calcite expressed in mm, presence and relative quantity of accessory minerals) were compared with both the most recent published data and with reference samples taken from ancient quarries (the extensive thin section collection present in the LAMA, University Iuav of Venice).

Figure 3: Detail of columns A (left) and D (right).