Читать книгу Monument Future - Siegfried Siegesmund - Страница 223

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 cross sections has allowed us to conclude that the columns were originally most probably painted. A white-yellowish layer made of lead-white was mixed with a small amount of calcium carbonate, orpiment, minium, vermilion and yellow ochre (in decreasing order of abundance) identified by Raman spectroscopy and SEM-EDX (Fig. 7, 8). It is approximately 0.15 mm thick, and may be considered as a sort of imprimitura (priming) applied directly over the marble substratum, very much likely the one often found in variously dated panel paintings (Gettens et al. 1967: 123). Over this layer, was a much thinner one (around 0.06 mm) composed of white-lead mixed with a proteinaceous medium, likely an animal glue, as in preparation for gold leaf. This original gilding is covered by dirt, carbonaceous matter likely from candle-burning, and by two later gildings; the first again applied on a thin lead-white layer mixed with a proteinaceous binder. The second is not preserved, however from the presence of a thick red layer of minium (0.2 mm) it can be assumed this was another preparation for gold leaf, as commonly used in the Venetian Renaissance. A natural oil-resin, most probably dammar impregnating all layers (Fig. 9), was applied in order to fix and consolidate the gildings: its discolouration into a brownish matter is probably responsible for the overall brown aspect currently assumed by the columns.

Figure 7: Photomicrographs of the polished cross section of sample A3 (column A), a) in reflected light mag; b) stratigraphic scheme; c) SEM in backscattered electrons; d) same as c), but detail of the strata. The strata represented in b) and d) are: 1) painted preparation layer, 2) preparation for the gold leaf, 3) the original gilding, 4) dirt layer, 5) preparation for the gold leaf, 6) the second gilding, 7) dirt laye, 8) red layer of minium.

Figure 8: Detail of sample A3, a) SEM in backscattered electrons.; b) EDS mapping of the distribution of Pb; c) same of Ca; d) same of Au.