3.1 The stones
Five stones historically used in the Venetian architecture, i.e. two crystalline marble, (white Carrara and Proconnesian marble), and three limestones (Istrian stone, Ammonitico Rosso from Verona, and Venetian Scaglia Rossa) were identified throughout a correlation of macroscopic and petrographic investigations. Further three well known lithotypes, i.e. one crystalline marble (Pavonazzetto Toscano), one vulcanite (Porfido Rosso Antico or Lapis Porphyrites,), and a cataclastic limestone (Fior di Pesco) were identified on the basis of their typical macroscopic characteristics observed in-situ.
3.1.1 Petrografic study of the thin-sectioned stones
White crystalline marble (Carrara marble)
This stone (samples 2E, 1S) correspond to a pure calcite (partially dolomitic) fine-grained marble with isotropic fabric and Grain Boundary Shapes (GBS) mainly straight, in a homeoblastic and polygonal mosaic microstructure made of calcite crystals often forming triple points (120°). The Maximum Grain Size (MGS) is 0.36 mm. The few accessory minerals observed are pyrite, graphite and quartz (Figure 2a-c), the latter detected also by XRPD analysis as well as a limited amount of dolomite (Table 3). These petrographic features identify this marble as the Carrara one, from the Italian Apuan Alps [21].
Grey crystalline marble (Proconnesian marble)
Another calcite marbles are present (sample 9W), showing heteroblastic fabric, mortar microstructure and GBS mainly sutured and, secondarily embayed. It is a coarse- to medium-size marble with MGS around 4.50 mm. The mineralogical composition is made by calcite with graphite as accessory mineral (Figure 2d-f). XRPD analysis shows predominant calcite and rare quartz (Table 4). The samples are so recognizable as Proconnesian marble fragments from the micro Asiatic island of Marmara [21].
Istrian stone
A compact sedimentary rocks corresponding to the Istrian stone, a marine micritic limestone [22] to be classified as a carbonate mudstone [23,24] were also identified (samples 3E, 5E, 5W, 8W). Very rare fragments of unidentifiable shells define the allochem constituents. Stylolites, as well as sedimentary joints, are frequent, with small deposits of clay minerals and iron oxides and hydroxides (hematite-limonite) often occurring along them (Figure 2g-i). XRPD investigation confirms the predominant calcite composition (only sample 5E shows scarce dolomite) coupled with traces of quartz (Table 3).
Ammonitico Rosso from Verona (Rosso di Verona)
This is a nodular limestone with abundant bioclasts within a micritic mud and rare spathic cement in which hematite is finely dispersed (Figure 2j-l) recognised in samples 2N, and 4N. The allochem constituents are ammonites (macroscopic), fragments of bivalves (pelagic lamellibranchs), planktonic micro-foraminifera and echinoderms. The stone can be classified as biomicrite [22] or wackestone [23,24]. XRPD analysis shows predominant calcite combined with scarce quartz (Table 3). All these characteristics are peculiar of Ammonitico Rosso from Verona [25,26].
Ventian Scaglia Rossa (or Pietra di Prun)
Finally a marine micritic limestone with sparite limited as filler of fossil footprints is also present (samples 1E, 1N, 3N, 1W). Allochem constituents are plankton microforaminifera, globotruncane, globigerinides. XRPD analysis detected only abundant calcite together with scarce presence of quartz (Table 3). The rock can be classified as biomicrite [22] or wackestone [17]) and belongs to the white variety of the Venetian Scaglia Rossa Formation, locally corresponding to the so called “Pietra di Prun” or “Pietra di Lessina” or also “lastame” (Figure 2m-o) [25,26].
3.1.2 Macroscopically-defined stones
Porfido Rosso Antico (lapis porphyrites).
The rock of the two rotae (round plates) surrounding the central archway to the Mercerie was easily identified, by naked eye, as the famous Porfido Rosso Antico or lapis porphyrites [27]; it is an effusive igneous rock (of the Paleozoic period) showing porphyritic texture made of whitish to pink saussuritized plagioclase and rarer blackish amphibole phenocrysts embedded in a purple glassy groundmass. The red-purple background colour is due to the presence of a combination of hematite and piemontite (the manganese-rich member of the epidote group) in the groundmass. Porfido Rosso Antico is typically classified as andesite-trachinadesite/dacite [27–29] and it originates from the Mons Porphyrites, a mountain massif today called Gebel Dokhan, located west of Hurghada in the Egyptian Eastern Desert.
Pavonazzetto Toscano Marble
The slabs, located as frame of the clock and the central archway, are attribuitale to Pavonazzetto Toscano. This is a fine-grained, calcite marble with dark purple magnetite-bearing layers, dated to the Hettangian age and, like Carrara marble, originated from the Apuan Alps [12,30].
Fior di Pesco (marmor chalcidicum)
To Fior di Pesco, a deformed light-pink (the colour of "peach flower") to purple-red cataclastic hematitic limestone, macroscopically correspond the lythotypes of the rotae located at the top of the bifore of the third floor. This colour of the rock is due to the dispersion of, sometimes manganesiferous, hematite. The ancient quarries, nowadays largely destroyed by persistent cultivation, are located just north-west of the city of Eretria on the island of Euboea (Greece).
Table 4
XRPD semi-quantitative and qualitative mineral composition of the decay products micro-samples.
Minerals
|
Cal
|
MgCal
|
Dol
|
Gp
|
Qz
|
We
|
WM
|
|
Rocks
|
|
1E
|
xxx
|
|
|
|
**
|
|
|
|
1N
|
xxx
|
|
|
|
*
|
|
|
|
1S
|
xxx
|
x
|
|
|
**
|
|
|
|
1W
|
xxx
|
|
|
|
**
|
|
*
|
|
2N
|
xxx
|
|
|
|
*
|
|
|
|
3E
|
xxx
|
|
|
|
|
|
|
|
3N
|
xxx
|
|
|
|
**
|
|
|
|
4N
|
xxx
|
|
|
|
**
|
|
|
|
5E
|
xxx
|
|
**
|
|
**
|
|
|
|
5S
|
xxx
|
|
|
x
|
**
|
|
|
|
5W
|
xxx
|
|
|
|
**
|
|
|
|
6E
|
xxx
|
|
|
|
|
|
|
|
8W
|
xxx
|
|
|
|
**
|
|
|
|
9W
|
xxx
|
|
|
|
*
|
|
|
|
Black crusts
|
2S
|
xxx
|
|
|
x
|
|
|
|
|
4S_b
|
|
|
|
xxx
|
x
|
x
|
|
|
5S-b
|
|
|
|
xxx
|
x
|
x
|
x
|
|
Patinas
|
4W
|
xxx
|
|
|
|
**
|
|
|
|
7E
|
xxx
|
|
|
|
*
|
|
|
|
7W
|
xxx
|
|
|
|
*
|
|
|
|
8E
|
xxx
|
|
|
|
**
|
|
|
|
Mineral abbreviations after Whitney & Evans (2010) [31]: Cal = calcite; Dol = dolomite; Gp = Gypsum; Qz = quartz; We = weddellite; WM = white mica. Other abbreviations: MgCal = Mg-rich calcite. Relative quantity: xxx = very abundant; xx = abundant; x = present; ** = scarce; * = rare.
3.2 Deterioration morphologies and their distribution
The most widely recognised deterioration morphologies divided into macro groups as indicated by ICOMOS [16], are as follows below.
Cracks and Deformations
In this group the most abundant morphologies are due to cracks, more specifically fractures (Figure 3a,b), with deterioration level (DL) value between 1 and 2. The façade more afflicted by this phenomenon is the southern one (Figure 4a). Following, deformations, mainly as bowing phenomenon (i.e. a convex deformation essentially due to thermal shocks) involve especially the median section of the marble slabs and show DL 1 to 2 (Figure 3c). The latter deterioration pattern is more evident in the eastern façade. A more intense deformation (DL 3) involves the extreme NE lesene, where one stone block appears to be slightly tilted (Figure 3d).
Detachment
Only in the southern façade, where salts crystallization and black crusts are more intense, small areas show blistering phenomena (Figure 3e) with DL between 1 and 2. Moreover, both limestone and crystalline marbles panels exhibit disintegration features (i.e. intracrystalline decohesion and sugaring phenomena for marbles) (Figure 3f); these morphologies are more common on the side façades (Figure 4b) having DL of 1 or 2.
Features induced by material loss
The mapping revealed that the main morphologies belonging to this group are differential erosion with a DL between 1 and 2, in particular as a consequence of a major recession rate (due to dissolution effect) of the fine-grained calcite portions of limestone (Figure 5a,b) with respect to those with average-to-coarse grained carbonate. This decay morphology is more common in the eastern and western façades (Figure 6a). Microkarst of slight intensity (DL 2) is also present, affecting all the small columns of the balustrades in Istrian Stone (Figure 5c). Finally, missing parts with the intensity varying between very slight and very severe (DL 1 to 5) were identified; in particular, some rotae of Fior di Pesco are partially or completely missing.
Discoloration and deposits
For this group quite frequent are the black crusts mainly in the western side of the ground floor of the southern façade (Figure 6b), heavily afflicting the Corinthian capitals (Figure 5d). XRPD, performed on sample 4S and 5S (Table 3), is consistent to the classic gypsum-based composition of the black crust incorporating atmospheric particulate (especially carbon particles, pollens and wind quartz), showing also traces of weddellite oxalate [12,32–34]. The intensity of this alteration morphology covers all the DL range from 1 to 5. Some samples reviled also the presence of subflorescence testified by the presence of a layer between the stone and the black crust itself. SEM-EDS investigation proved high values of Cl and Na, indicating a recrystallization of halite (NaCl) (Figure 7a-d).
Bleaching and staining (Figure 6b) represent the main discoloration features. In the former case, pink and red limestones (i.e. Ammonitico Rosso from Verona and Pietra di Prun), due to weathering, leaching and iron mobilisation loss their hematitic pigments from the most superficial layers and change their colours to lighter ones [35]. This phenomenon is clearly reviled by BSE images of sample 4N where iron-bearing minerals are absent from the surface and appear in greater concentration at a depth of around 400-500 mm (Figure 7e). This pattern is more present in the two side façades, ranging in DL from 1 to 4.
On the other hand, staining creates greenish and reddish localized halos due respectively to the dispersion of copper, from the Moors sculptures (Figure 5g), and iron products (mainly hydroxides), deriving from the weathering and dispersion of metallic elements of inserts. The elemental mapping by SEM-EDS of samples with macroscopically green surface deposits proved the presence of Cu, Cl and Na (Figure 7g,h). The DL values of this decay product range from 1 to 3. Other deposits are made of efflorescences and mainly appear as whisker-like crystals (Figure 5f) of very slight and slight intensity (corresponding to DL 1 and 2, respectively) on the surface of the small columns of the balustrades (Figure 6b). In Table 5 are listed the anions detected by IC that identify the main salts constituting the efflorescence. The samples were tested for all the following ions: fluorides, chlorides, nitrites, bromides, nitrates, phosphates, sulphates, lithium, sodium, ammonium ion, potassium, magnesium, calcium. The main contents are of chlorides and sulphates for the anions, and sodium and calcium for the cations, composition consistent with halite (NaCl) and gypsum (CaSO4⋅H2O) crystallizations. In addition, sample 5E showed a significant amount of magnesium, which could be linked to the presence of dolomite (Ca,Mg(CO3)2), a Mg and Ca carbonate, in this sample, as also indicated by its XRPD (Table 3). Finally, all the samples show significant calcium contents, which can easily be linked to the carbonate nature of most of the samples and/or their stone substrates.
Table 5
Anions and cations obtained by IC and expressed as percentages by weight. In bold ions connected with the presence of Halite (NaCl); in italic ions connected with the presence of Gypsum (CaSO4∙H2O); in underlined ions connected with the presence of Mg-bearing carbonates.
Sample
|
1E
|
1N
|
1W
|
2N
|
2S
|
4N
|
5E
|
5S
|
5W
|
7E
|
8W
|
Anions
|
Fluorides
|
-
|
-
|
0.01
|
-
|
0,01
|
-
|
-
|
0,01
|
-
|
-
|
-
|
Chlorides
|
0.02
|
0.03
|
0.03
|
0.03
|
0,13
|
0.03
|
0.03
|
0,07
|
0,05
|
0,04
|
0.03
|
Nitrates
|
-
|
-
|
-
|
-
|
0,05
|
-
|
-
|
0,04
|
0,06
|
0,03
|
-
|
Sulphates
|
0.03
|
0.03
|
-
|
0.03
|
1,09
|
0.03
|
-
|
0,68
|
0,13
|
0,05
|
0.04
|
Cations
|
Sodium
|
0,04
|
0,03
|
0,02
|
0,04
|
0,26
|
0,03
|
0,04
|
0,11
|
0,02
|
0,04
|
0,04
|
Ammonium
|
0,08
|
0,10
|
0,05
|
0.09
|
-
|
0,06
|
0,07
|
0,11
|
0,10
|
0,09
|
0,09
|
Potassium
|
0,08
|
-
|
0,08
|
-
|
-
|
0,10
|
0,09
|
0,14
|
-
|
-
|
-
|
Magnesium
|
0,13
|
0,05
|
0,38
|
0,07
|
0,02
|
0,15
|
0,59
|
0,08
|
0,28
|
0,13
|
0,66
|
Calcium
|
1,63
|
1,66
|
1,44
|
1,66
|
5,06
|
1,82
|
1,58
|
3,03
|
2,36
|
1,75
|
2,19
|
Patinas, resulting from ageing of ancient treatments, appear as a yellowish colour alteration of crystalline marble slabs and columns/pilasters (Figure 5h). The DL of this decay product range from 1 to 3. The study performed by FTIR compared with EDS elemental mapping allowed to determine with great accuracy the nature of the past treatments applied on the marble columns. The abundance of fluorine diffused in the superficial micro-cracks (Figure 7i,l) combined with the recognition by FTIR of fluosilicates (3432 cm-1 due to ν Si-OH, 1260 cm-1 related to ν C-F3, 1161 cm-1 linked to ν C-F and 1115-1033-1007 cm-1 attributed to νs Si-O-Si), confirms the use of these products on the stone surfaces (samples 1S, 6W). Moreover, in the same samples, the peaks at 2959-2928-2873 cm-1 relate to ν C-H chemical bond, 1737 cm-1 linked to ν C=O, 1260 due to ν Si-(CH3) suggests the application also of an organic treatment based on acrylic-siloxane resin. The presence of peaks related to acrylic-silicone compounds were also detected in sample 6E from the eastern façade. The characteristic pecks of calcium oxalate dehydrate (weddellite) have been also detected (3400 cm-1 due to ν O-H, 1646 cm-1linked to ν C=O, 1327 cm-1 attributed to ν C-O and 798 cm-1 corresponding to δ C-H) in samples 1S, 3S, 6E and 4W. This material probably results from the mineralization process of ancient organic consolidation treatments [36] more than from biological attack [37,38].
Finally, soiling is the most widespread deterioration morphology in the whole building (Figure 5e). It appears as a tenuous blackish-grey layer (DL between 1 and 4) that confers a dirty appearance to the stone.
Extensive SEM investigations revealed, through SEM-BSE images, peculiar micro-damage patterns in the Carrara marble samples from the ground floor columns. These are micro-cracks concentrated only in the superficial crystals clearly guided by the cleavage planes of the mineral (Figures 7f).
Biological colonization
Lichens and fungi colonizations (DL between 1 and 2) are mainly developed on Istrian stone (Figure 8a,b) with a homogeneous distribution on the three façades (Figure 9). Rarely, moss was identified and when present are visually impactful (Figure 8c); they were found mainly in the walkways between the balustrades of the two wings of the building and therefore they cannot be mapped on the elevations. More in detail, five different microorganisms were detected by OM investigation, as follows below.
- Caloplaca saxicola (Hoffm.) Nordin [39]
Crusty lichen with lecanorine apothecia having bright orange disc and slightly lighter margins (Figure 8d). The thallus is placodomorphic and pale yellow in colour. Chemical test with potassium hydroxide (KOH) solution gives a positive result (K+) [19].
- Candelariella aurella (Hoffm.) Zahlbr [39]
Crusty lichen with yellow apothecia and blurred thallus. Some apothecia are covered with a blackish patina, while others have brownish branching. Chemical test with potassium hydroxide solution (KOH) gives negative result (K-).
- Meristematic fungi
Blackish patina (Figure 8c) and brownish branching due to meristematic fungi, whose hyphae are arranged in chain and brown colour indicates the presence of melanin [40]. This biological material often appears in combination with lichens and algae (Figure 8e).
- Moss
It refers to plant belonging to the bryophyte division and more specifically to the class of mosses. Under the stereomicroscope it is possible to observe the gametophyte, i.e. the green part of the moss consisting of leaflets, and the sporophyte of which we can see the capsules, or urns, containing the spores, which are carried on top by filaments known as setae [40].
- Green Algae
Investigation of greenish patinas identified green algae cells combined with cyanobacteria colonies and a few fungal hyphae (Figure 8f) [40].