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      Figure 2: Details of the four carved marble column: a) column A at the back and B at the front; b) column C at the back and D at the front.

       Sampling and experimental

      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).

       Results and their discussion

      The results of the minero-petrographic and isotopic analyses of the columns’ marbles are summarised in Table 1. There was considerable homogeneity of the results for the fabric parameters, hetero/homeoblastic mosaic type, embayed crystal boundaries and maximum grain size around 0.6–0.8 mm (Fig. 4), and mineral composition: rather pure calcite, with small amounts of accessories, mainly carbonaceous matter/graphite and traces of quartz and k-mica. The results of the isotopic analyses indicate that the columns clearly forms two groups, one with δ values around –0.8, and one with δ +1.6/–1.7, while for 3 columns gave similar δ values (around –9.4/–9.7), one totally different (–5.4). From the plot of such results in the reference diagram (Fig. 5) it may be deduced that columns A and D were cut from the same quarry locus, possibly the same block of marble, while columns 154B and C came from different quarries. Combining the results of the minero-petrographic and isotopic analyses, with reference to the most updated databases (Antonelli, Lazzarini 2015) and with direct comparison of thin sections of ancient quarry samples, it may be concluded that the marble of three columns is most probably from ancient Docimium (corresponding to the present-day village of Iscehisar, province of Afyon, Turkey), namely from two different loci, one for columns A and D, and one for column C. The isotopic ratio of the fourth column is outside all the known reference fields of the most important fine-grained marbles used in antiquity and therefore, cannot be assigned isotopically. Considering that its petrographic features are very close to those of the other three shafts, however, one may hypothesize that column B is also of the same marble, but of an unknown ancient quarry.

      Figure 4: Photomicrograph of the thin sections of the marbles of columns A, C, B, D (left to right, top to button), N+, long side = 3.8 mm: all showing similar mosaic fabric formed by calcite crystals with curved-to-embayed boundaries.

      In a thin cross section of column D, a superficial brown film of Ca-oxalates (Fig. 6a) was observed by optical microscopy, formed from the mineralization of an unknown organic treatment material. The presence of abundant phosphorus (P) (Fig. 6b), covered by a deposit of airborne quartz particles and gypsum (Fig. 6c and d) suggests the organic matter to be casein, corresponding to an ancient conservation treatment, with a weak sulphation process affecting the marble of the column.

      Figure 5: Plot on the reference isotopic diagram of the most important fine-grained marbles (Antonelli, Lazzarini 2015) of the resulting ratios of the 4 columns.

Table 1: Summary of the minero-petrographic and isotopic analyses (HE, heteroblastic; HO, homeoblastic; M. G. S., maximum grain size; +++, very abundant; ++, abundant; +, present; ±, traces; –, absent).

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      Figure 6: Photomicrograph of the thin section of column D: a) showing the old brown treatment layer on top of the marble substratum, covered by a thick gypsum layer, N+, long side = 0.96 mm; b) SEM-EDS mapping of the phosphorous distribution; c) same for Si; d) same for S.

      The microscopic study (OM and SEM) of several unmounted samples and of four

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