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II. Origine des matières premières et préparation du stuc

Geochemical and isotopic provenance determination of gypsum used for stucco artworks: application to Florentine stucco reliefs

Détermination géochimique et isotopique de la provenance du gypse employé pour les œuvres en stuc : application aux reliefs en stuc florentins
Gianluca Gariani, Lise Leroux, Philippe Bromblet, Fabio Fratini et Wolfram Kloppmann
p. 64-74

Résumés

Les roches gypseuses de Toscane et les matériaux dérivés (albâtre, stuc, gesso, etc.) ont souvent été utilisés par les sculpteurs. Des exemples remarquables sont attestés à la Renaissance florentine. À la jonction de deux études scientifiques en cours, portant respectivement sur les reliefs en stuc du xve siècle (dits Madonne di gesso) et sur la provenance des albâtres employés dans des œuvres d’art, nous avons entamé une recherche sur les sources d’approvisionnement en gypse brut en réalisant une prospection géologique dans les alentours de Florence. Une démarche multi-analytique a été adoptée pour identifier des marqueurs de provenance géochimique sur trente échantillons de roche géoréférencés ; les éléments-traces liés à la fois au gypse et aux minéraux synsédimentaires, ainsi que les isotopes du gypse (S, O, Sr) ont pu être testés. Les résultats présentés ici apportent un éclairage sur l’origine géographique des matériaux bruts utilisés pour les reliefs en stuc dans les ateliers florentins.

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Introduction: geological background and previous provenance studies

  • 1 Kloppmann et al., 2017; Cheetham, 2005, p. 360.
  • 2 Gariani et al., 2018.
  • 3 Gariani et al., 2018; Beaugnon et al., 2019.
  • 4 See article by G. Gariani, F. Beaugnon et al. in this volume.
  • 5 Marchand, 2007; Gettens, 1954.
  • 6 Cat. Exhib. Cento, 2007, p. 213-219; Guillet, 2017.
  • 7 Cat. Exhib. Cento, 2007, p. 213-219; Guillet, 2017.
  • 8 Cat. Exhib. Cento, 2007, p. 213-219.

1Until the last decade, and despite their frequent usage, the determination of geological sources of gypsiferous rocks used in artworks has been understudied. This is partly due to the complications and challenges that reside in the characterisation of evaporitic rocks1. Previous works2 on a corpus of gypsum-based stucco reliefs (also called Madonne di gesso) after renowned Renaissance Masters, raised questions on the provenance of their raw material. Despite their common Florentine attribution, discrepancies in their geochemical composition3 pointed toward a diversity in the raw material used for these artworks4. Since gypsum deposits are widespread in Tuscany, it is reasonable to assume that raw materials for Florentine workshops were quarried and supplied locally. The few available sources mention that the alabaster and gypsum quarries in the surroundings of Volterra and Siena supplied the Florentine manufacturing5 . Nevertheless, many questions about this topic are still open and poorly investigated6. Considering the studied stucco reliefs, the main hypotheses consists in the existence of Tuscan workshops using regional raw material supply, but other production centres across Italy are equally plausible7. Artists such as Donatello or Antonio Rossellino worked also in Padua, Ferrara and Bologna among other cities8. Therefore, the raw material from various geological formations could have been quarried either in Tuscany or elsewhere, depending on the manufacturing site. Moreover, during the 19th century, the diffusion of late copies, or even forgeries, broadens the field of possibilities to the scale of Europe, without excluding Tuscany where a continuous local production had existed during centuries. These hypotheses conducted our interest toward the geological gypsum sources in the proximity of Florence in search of a method to distinguish the provenance of the raw material as a crucial contribution to the geographical attribution of artworks.

  • 9 Nusara et al., 2017; Testa et al., 1996.
  • 10 Chang et al., 1998.

2Gypsum (CaSO4,·2H2O) is indeed the most abundant mineral in sulphated evaporitic rocks. The best-known Tuscan variety is microcrystalline (alabastrine) gypsum9, but it occurs also with a large diversity of other facies, which impacts both the geochemical composition and associated accessory minerals, notably synsedimentary anhydrite, dolomite, calcite, clay minerals, and celestine10.

  • 11 Costagliola et al., 2001; Gale et al., 1988; Playà et al., 2005, 2012.
  • 12 Kloppmann et al., 2014 and 2017.
  • 13 Leroux et al., 2018.

3So far, the few studies addressing the geo-sourcing of gypsum in cultural heritage mainly concerned microcrystalline gypsum (alabaster), investigating the use of Sr, S, O, isotopic signatures to discriminate different geological sources11. Multi-isotope fingerprints were used as tracers to link ancient extraction areas to alabaster artworks produced over five centuries (13th-17th), leading to a rather comprehensive database of both quarries and sculptures12 and showing the importance of survey and sampling individual historical quarries. A pilot study was also conducted on the use of trace elements on quarry material and alabaster artworks but the results obtained by ICP-AES analysis were not sufficiently sensitive or discriminatory to be used as deposit-specific parameters13.

  • 14 Sarti, Testa, 1994; Dinelli et al., 1999; Bossio et al., 1999.
  • 15 Barbieri, Masi, 1976; Dean, 1978; Testa et al., 1996; Bossio et al., 1994; Lugli, Testa, 1993.
  • 16 Bossio et al., 1999.
  • 17 Claypool et al., 1980; McArthur et al., 2001; Kampschulte, Strauss, 2004; Bernasconi et al., 2017.
  • 18 Kushnir, 1982.
  • 19 Kloppmann et al., 2014, 2017, 2018, 2021.

4The geological age of the depositional episodes must be taken into account. In Tuscany, Miocene (Messinian) evaporites are dominant, outcropping in two extensional basins in central Tuscany, i.e. the Fine Basin14 and the Volterra Basin15, but punctually Triassic formations also exist16. A strong isotopic contrast between these Triassic and Messinian evaporites would be expected, given the well-constraint variations of marine sulphur, oxygen and strontium over geological time scales17. Nevertheless, experience from the studies of alabaster used for European artworks has clearly shown that isotopic variations of seawater is not the only factor to explain the variety of signatures observed for historical quarries. Indeed, paleogeography, recycling of earlier evaporites and diagenesis will strongly influence the isotopic characteristics of gypsum and anhydrite deposits18. Such local conditions contribute to the specific isotopic signatures for individual historical quarries even of the same geological age. So, their localisation and characterisation are crucial for provenance studies for sulphate-based artistic materials19.

Research aims

5Geo-referenced samples were collected in Tuscany thanks to a geological survey in selected quarries possibly exploited during the Renaissance. The analytical strategy was based on three main objectives:

  • to test and verify the use of minor and trace elements (related to gypsum or synsedimentary minerals, such as clay minerals) as provenance markers for Tuscan gypsum rocks;

  • to apply isotopic multi-tracing, already well established for European alabaster quarries, on Tuscan gypsum rocks;

  • to perform a feasibility test of the isotopic tracing on a raw gypsum crystal sampled from a Florentine stucco relief.

  • 20 See article by G. Gariani, F. Beaugnon et al. in this volume.

6Elemental compositions of the gypsum rock samples were also compared to the data previously acquired on the material of 24 Florentine Madonne di gesso20.

Materials and methods

Geological survey and geo-referenced samples

  • 21 Vasari, 1550 (ed. 2008); Marchand, 2007; Gettens, 1954.

7A non-exhaustive geological survey was carried out in three different Tuscan sites (fig. 1), to provide references of material likely used in 15th century Florence (fig. 2). We focused on occurrences located close to Florence, Volterra and Siena which are often mentioned for supplying raw material during the concerned historical period21. These sampling sites are located near Volterra (V), Castellina Marittima (CM) and Gambassi (G), in the southwest area of Florence. Overall, 30 geo-referenced samples were collected (17 from Messinian outcrops of Volterra and Castellina Marittima; 13 from the Triassic outcrop of Gambassi). Table 1 reports information about these three quarries.

Fig. 1. Gypsum quarry of Gambassi, Tuscany (Italy)

Fig. 1. Gypsum quarry of Gambassi, Tuscany (Italy)

© G. Gariani.

Fig. 2. Extract of the geological map of Tuscany (modified from Carmignani, 2013) showing the locations of the investigated gypsum quarries

Fig. 2. Extract of the geological map of Tuscany (modified from Carmignani, 2013) showing the locations of the investigated gypsum quarries

Table 1. Main information about the three Tuscan quarries sampled

Table 1. Main information about the three Tuscan quarries sampled
  • 22 Cat. Exhib. Cento, 2007, p. 213-219; Gariani, 2019.

8To test the feasibility and the potential use of multi-isotopic signature on stucco (i.e. gypsum based plaster) reliefs, we investigated a Nativity after Donatello and B. Bellano (RF 1191) owned by the Louvre Museum. During the study for restoration, a raw gypsum crystal (5 mm size) present as inclusion in this stucco was sampled. The Florentine origin of the Nativity model is often debated; many replicas are scattered over other Italian regions so that it can be considered as an excellent starting point for methodological testing22. Before analysis, the gypsum crystal was carefully cleaned of all residues of the plaster matrix in which it was embedded.

Geochemical analyses

  • 23 Gariani, 2019.

9For each of the 30 samples, powders were obtained and partly conditioned as pellets. Both PIXE (Particle induced X-ray emission), and LA-ICP-MS (Laser ablation-inductively coupled plasma-mass spectrometry) measurements were performed for bulk elemental analysis. The LA-ICP-MS results will be mainly considered in the following, due to a larger data set. To allow a reliable comparison, the experimental set-up and parameters are the same used for previous analyses on artworks23.

Isotope analysis

  • 24 Kloppmann et al., 2017.
  • 25 Kloppmann et al., 2014.

10In addition to five previously published results from Volterra and Castellina Marittima24, a sample from Gambassi was analysed by Continuous-flow isotope ratio mass spectrometry (CF-IRMS) for S and O, and Thermal ionisation mass spectrometry (TIMS) for Sr. The previously described detailed analytical protocol25 requires a minimum quantity around 20 mg, corresponding to a tiny flake of around 2 mm3.

Results

Geochemical results

  • 26 See article by G. Gariani, F. Beaugnon et al. in this volume.

11Minor elemental compositions (expressed as oxides) are grouped by quarry and main lithofacies in table 2 and reported (fig. 3 a-b) with supplementary data from previous artworks analyses26. Figure 3 reveals a certain inter and intra-variability between Tuscan quarries.

Fig. 3 a-b. a. Histograms showing minor oxide contents in the geological samples regrouped by quarry. Red dotted lines indicate the mean content (2 +1.2) of minor oxides measured in 24 Florentine stucco reliefs. b. Minor oxide contents measured in these 24 Florentine stucco reliefs

Fig. 3 a-b. a. Histograms showing minor oxide contents in the geological samples regrouped by quarry. Red dotted lines indicate the mean content (2 +1.2) of minor oxides measured in 24 Florentine stucco reliefs. b. Minor oxide contents measured in these 24 Florentine stucco reliefs

Table 2. Minor oxides composition measured by LA-ICP-MS on 30 geo-referenced gypsum samples from 3 Tuscan quarries

Table 2. Minor oxides composition measured by LA-ICP-MS on 30 geo-referenced gypsum samples from 3 Tuscan quarries

12Considering Messinian gypsum, the total contents of minor elements are higher for Castellina Marittima than for Volterra. Indeed, microcrystalline gypsum/alabaster is known to represent calcium sulphate with less impurities. Nevertheless, a certain internal variability within the same quarry exists and seems mostly due to the variety of the lithofacies sampled. MgO, SiO2, Al2O3 are predominant in all samples with some K2O and Fe2O3. The Triassic gypsum from Gambassi quarry presents a quite different pattern, globally with higher contents in MgO in comparison to SiO2 and particularly to Al2O3, Fe and K being nearly absent.

  • 27 Gariani, 2019 ; Kockman, Foley, 1987.
  • 28 Gariani, 2019.

13On figure 3 a, the dotted red lines delimit the mean range of minor oxide contents of 24 stucco artworks previously analysed. Stucco reliefs present minor oxide contents between 0.5 wt% and 4.8 wt%, with an average of about 2 wt%. None of the measured samples have MgO contents higher than 1 wt%. All in all, these values are therefore compatible with the contents measured in the studied natural gypsum rocks27. These results support the observation formulated28 according to which a fraction of synsedimentary minerals could be detected within the stucco matrix of the artworks. The observed patterns in the artworks are compatible with most Miocene gypsums but not with the Triassic gypsum of Gambassi, in particular due to the presence of Al-, K-, and Fe-oxides.

Isotope results

  • 29 Kloppmann et al., 2017.
  • 30 Kloppmann et al., 2017.

14The results obtained on the Triassic gypsum of the Gambassi quarry complete previously published data on the Volterra and Castellina Marittima outcrops29, both Messinian. With a δ34S of 15.4 ± 0.3 ‰ vs. V-CDT, a δ18O of 13.0 ± 0.5 ‰ vs. V-SMOW, and a 87Sr/86Sr of 0,707951 ± 0,000008, this quarry has isotope signatures similar to some Triassic alabasters, e.g. of the French Alps30 clearly distinct from the Tuscan alabaster of Messinian age.

15The raw gypsum inclusion sampled from the Louvre stucco relief RF 1191 (type Nativity, after Donatello and B. Bellano) falls, with a δ34S of 23.6 ± 0.3 ‰ vs. V-CDT and a 87Sr/86Sr of 0,708930 ± 0,000006 within the field of Messinian Tuscan gypsum, whereas the δ18O of 11.9 ± 0.5 ‰ vs. V-SMOW is significantly lower.

Discussion

16Results obtained on geological samples will be discussed evaluating inter and intra-variability amongst quarries. Their contents in minor and trace elements will be compared with those of the artworks previously analysed in the same analytical conditions. Finally, results on the case study of the stucco reliefs RF 1191 (The Nativity after Donatello and B. Bellano) will be used to evaluate the usefulness and pitfalls of this approach and particularly the applicability of isotopic fingerprints on stucco artworks.

Provenance information by means of mineral chemistry

  • 31 Gariani, 2019.

17Part of the investigation is based on the elemental composition (minor elements with respect to calcium and sulphates), most likely related to the presence of secondary/accessory minerals in the material used for artworks, and its usefulness for provenance determination. A first assessment was therefore made on the amounts of minor oxides and their possible correlation with syndepositional minerals of the gypsum rocks. One of the main discriminating criteria observed for stucco casts31 was indeed the presence of clay minerals whether naturally occurring in the raw gypsum or added during the making of the plaster. The hypothesis of natural occurrence needed characterisation of raw geological gypsum. Bivariate scatter plots (fig. 4 a-b) of three minor oxides (SiO2, Al2O3 and K2O) show the correlation between gypsum from quarries and the 24 analysed artworks (range of values indicated by the red dotted lines in fig. 4 a-b). Data points of stucco artworks are also visible in fig. 4 c-d.

Fig. 4 a-d. Scatter plots showing: a. SiO2 vs Al2O3 and b. K2O vs Al2O3 in geological samples; c. SiO2 vs Al2O3 and d. K2O vs Al2O3 in stucco artworks of Florentine attribution. The red dotted areas in fig. 4 a-b represents the range of values shown in fig. 4 c-d

Fig. 4 a-d. Scatter plots showing: a. SiO2 vs Al2O3 and b. K2O vs Al2O3 in geological samples; c. SiO2 vs Al2O3 and d. K2O vs Al2O3 in stucco artworks of Florentine attribution. The red dotted areas in fig. 4 a-b represents the range of values shown in fig. 4 c-d
  • 32 Caillère et al., 1982.
  • 33 See article by G. Gariani, F. Beaugnon et al. in this volume.

18The two Messinian gypsum quarries show overlapping values and similar, excellent correlations (R2>0.9) of both SiO2 and K2O with Al2O3. Clay minerals and other silicates are likely present in these raw gypsums. The Gambassi quarry stands out from the two others by presenting higher MgO contents (fig. 3 a). We can also observe a lack of correlation of SiO2 and Al2O3 for Gambassi (fig. 4 a), pointing to a predominance of quartz over clay minerals in the silicates32. The similarities of the Messinian quarries of Volterra and Castellina Marittima can be related to their common depositional age and lithology with predominant pure microcrystalline gypsum. The wider dispersion of the Castellina Marittima data can be explained by the larger number of samples and facies collected and analysed. Florentine stucco artworks show similar ranges of values and ratios of minor oxides as the Messinian quarries, Volterra and especially Castellina Marittima. The results also seem to confirm that the level of minor oxides found in stucco artworks can be linked to mineral impurities (i.e. clay minerals) in raw material rather than to additives in the formulation of the stucco33.

Trace elements

19Regarding the use of trace elements as geochemical markers, figure 5 represents the box charts of the values of Ti (a) and Sr (b) contents (expressed in ppm). These trace elements can be related both to synsedimentary minerals and to the gypsum itself.

20Ti contents (fig. 5 a) vary from values of few ppm to around 200 ppm. While Volterra and Gambassi samples are very closely regrouped within a narrow range of values (lower than 25 ppm), samples from Castellina Marittima present a higher mean Ti content close to 50 ppm and a wider dispersion with maximum values of 220 ppm.

Fig. 5 a-b. a. Whisker plot of Ti (expressed in ppm). The red dotted line represents the mean Ti content in the 24 stucco artworks (Gariani, 2019), while the blue star represents those measured in the sample from RF 1191 relief; b. Whisker plot of Sr contents. The dotted rectangle represents the range of data reported in literature on Tuscan occurrences (Dinelli et al., 1999; Barbieri, Masi, 1976; Cortecci et al., 1981)

Fig. 5 a-b. a. Whisker plot of Ti (expressed in ppm). The red dotted line represents the mean Ti content in the 24 stucco artworks (Gariani, 2019), while the blue star represents those measured in the sample from RF 1191 relief; b. Whisker plot of Sr contents. The dotted rectangle represents the range of data reported in literature on Tuscan occurrences (Dinelli et al., 1999; Barbieri, Masi, 1976; Cortecci et al., 1981)
  • 34 Chang et al., 1998; Playà, Rosell, 2005; Dinelli et al., 1999; Testa et al., 1996; Franceschi, Loca (...)
  • 35 Cortecci et al., 1981.
  • 36 Playà et al., 2012.
  • 37 Playà, Rosell, 2005.

21Amongst trace elements primarily related to gypsum, Sr is often reported as particularly useful for provenance information34. Sr concentrations of the 30 analysed samples are reported in figure 5 b. Overall, results fall into a range of values compatible with the highly variable Sr concentrations reported in literature for gypsiferous rocks of Tuscany35. The dispersion within all quarries is quite important and contrasts are not strong enough to differentiate the three deposits. It is known that Sr can be present in two main forms, substituting Ca in gypsum crystals, or as SrSO4 (celestine). This unfortunately biases the use of this element as a provenance tracer36 since the presence or absence of celestine grains in the tiny samples obtainable from artworks may considerably impact Sr contents. A solution to avoid this bias is to use strontium isotopes instead of concentrations, knowing that the Sr-isotopic composition of syngenetic celestine is similar to that of gypsum37.

Results on Tuscan quarries based on multi-isotopic signatures

  • 38 Claypool et al., 1980; McArthur et al., 2001; Kampschulte, Strauss, 2004; Bernasconi et al., 2017.
  • 39 Kloppmann et al., 2017.
  • 40 Barbieri, Masi, 1976; Cortecci et al., 1981; Dinelli et al., 1999.

22The main factor explaining the observed contrasts between the Tuscan quarries exploiting Messinian and Triassic evaporites are the isotopic variations of seawater at the geological time-scale38. Even though, as stated earlier, erosive (dissolved or particular) input contributes to the strontium budget in isolated or partly isolated basins, so that non-marine strontium will lead to isotope signatures specific for each (sub-)basin or even quarry. Furthermore, specific geochemical basin environments, e.g. reducing conditions, together with the recycling of older evaporites, induce isotope contrasts between quarries, including sulphur and oxygen isotopes. As shown by previous works on historical European alabaster, it is necessary to identify as accurately as possible and characterise the historical exploitations39. It is not possible to solely rely on seawater values, and even on non-specific regional literature data from geo-scientific studies, as, in our case, on Tuscan evaporites40.

23The Triassic Gambassi gypsum shows isotope signatures for S, O and Sr that are similar to French alpine Triassic alabaster quarries (fig. 6 a-b), clearly distinct from the Messinian evaporites of the Volterra and Castellina Marittima quarries. This should make it possible to identify local use of the Gambassi gypsum for sculpture or plaster production.

Fig. 6 a-b. a. δ34S versus 87Sr/86Sr ratio; b. δ34S vs δ18O of Tuscan quarries compared to principal selection of major deposits previously identified to have delivered alabaster for 14th to 16th century sculpture in W Europe (Kloppmann et al., 2017, 2021). The results obtained on the selected stucco artwork (RF 1191) of the Louvre Museum are labelled “STUC RF 1191”

Fig. 6 a-b. a. δ34S versus 87Sr/86Sr ratio; b. δ34S vs δ18O of Tuscan quarries compared to principal selection of major deposits previously identified to have delivered alabaster for 14th to 16th century sculpture in W Europe (Kloppmann et al., 2017, 2021). The results obtained on the selected stucco artwork (RF 1191) of the Louvre Museum are labelled “STUC RF 1191”

Summary of the tested provenance tracers

24The sourcing of raw material used in Tuscan stucco reliefs is at an early stage and the present study only provides the main geochemical and isotopic features determined for three quarries and, as such, attributions require caution. Nevertheless, this pilot study could lay the basis for more extensive research. Studied samples from Volterra are mostly of quite pure alabaster (avg content of 1-2 % of impurities), hence with relatively small minor oxide and trace element contents, systematically lower or equal than in the other two quarries. Isotopic results are in agreement with data of Miocene gypsum alabaster, congruent with the Messinian age of this deposit. Hence, Volterra raw gypsum could not be distinguished from material from Castellina Marittima, also Messinian. However isotopic fingerprints allow to separate both Volterra and Castellina Marittima from Triassic Gambassi gypsum.

  • 41 Gariani, 2019.

25The quarry of Castellina Marittima, characterised by a large variability of facies, presents a wider dispersion of data for minor oxides and trace elements. It is worth noticing that values of microcrystalline gypsum samples superpose with those of the Volterra quarry. Castellina is the only studied quarry where such a broad panel of facies of raw material was sampled, which consequently explains the higher variability of Ti and other trace elements41.

  • 42 Kloppmann et al., 2017.

26A higher MgO content and silica values not related to Al2O3 seem to be distinctive features of the Gambassi quarry. Multi-isotopic fingerprints work well in separating this Triassic source from the two Messinian ones. Even if only one sample was tested, consistency of the result is supported by the agreement with some other Triassic sources42.

Feasibility of the application on Florentine stucco artworks

27The main challenge was to test the method for provenance determination on representative Renaissance stucco reliefs previously studied. Some of the investigated geochemical and isotopic markers were selected trying to ascertain compatibility or determine the most probable source of the raw material used among the investigated Tuscan quarries. However, since the current knowledge and data on other quarries with historical and geographical significance for this artistic production is limited, the results of this comparison will need to be further confirmed by more detailed investigation.

28As it has been shown, an overall compatibility with Tuscan sources can be found for the corpus of artworks considering the level of minor oxides and their correlation, as well as trace elements that could be mainly related to synsedimentary mineral phases associated to gypsum. The high geochemical variability between and within quarries does not allow a clear and unambiguous separation or attribution. Other geochemical tracers related to gypsum, such as Sr contents, allow to ascertain a compatibility with all the three gypsum quarries considered, but are not conclusive for a clear provenance determination. The studied stuccoes have more pronounced geochemical similarities with the non-microcrystalline gypsum samples collected in the Castellina Marittima quarry, but it must be emphasized that it could be due to the fact that a larger number of samples with a higher variety of facies was collected in this quarry.

29So far, the multi-isotopic signature appears to be the most conclusive and convincing provenance tracer. A test was carried out on a raw gypsum crystalline inclusion sampled in a stucco relief after Donatello and B. Bellano (RF 1191, Louvre Museum). Figure 6 shows the results of this first attempt of the use of multi-isotopic signature for a gypsum based stucco object.

30For δ34S and 87Sr/86Sr, the raw gypsum inclusion from the artwork RF 1191 falls in the field of the Volterra and Castellina Marittima quarries.

  • 43 Chiba, Sakai, 1985.
  • 44 Chiba et al., 1981.
  • 45 Kloppmann et al., 2008, 2011.

31The δ18O value falls within the range of values compatible with Miocene occurrences in Tuscany found in the literature but is clearly depleted in 18O with respect to the Tuscan Messinian quarries. Whereas δ18O values of sulphates are considered as stable and not prone to water-sulphate isotopic exchange at temperatures below about 200 °C43, important isotope fractionation is possible at higher temperatures44. The possibility of isotopic exchange of solid sulphates with constitutional water contained in gypsum or pore water pre-sent during the plaster production still needs to be investigated in further studies. On the other hand, a study showed very similar isotopic ranges for δ34S and δ18O for so-called “Paris plaster” and the Ludian gypsum of the Paris basin (raw material45) so that no significant isotope fractionation during plaster production has been postulated for both elements.

  • 46 Usdowsky, 2001.

32Concerning sulphur, the study of Usdowsky in 200146, concluded on the absence of isotopic fractionation upon de-hydration and re-hydration of gypsum in a temperature range of 100 to 1100 °C upon plaster and mortar production.

33Concerning 87Sr/86Sr, there is no significant isotope fractionation induced by natural processes due to the tiny mass difference between both isotopes. Consequently, 87Sr/86Sr and δ34S can be considered as reliable tracers of plaster/mortar/stucco provenance.

34Supplementary evidence might be the additional information given by Ti, correlated to gypsum-associated clay minerals. For the artwork RF 1191, the average Ti content, about 170 ± 50 ppm (fig. 5 a) is so far only compatible with values measured for the Castellina Marittima quarry. The Messinian quarry is indeed the only one of the three Tuscan quarries presenting such a high Ti dispersion. Further and in-depth confirmations are obviously required, but this could be a complementary indicator suggesting the provenance of the material from the Castellina Marittima area.

Conclusions

35The main objective of this work was to test a methodology for the provenance determination of gypsum, which is a key aspect for the attribution of Florentine Masters’ stucco reliefs. To identify possible provenance markers, it was decided to carry out an investigation on 30 geological samples from three occurrences close to Florence. Based on previous studies, two main geochemical methods were investigated, i.e. minor and trace element and isotope analyses (S, O, Sr). The outcomes pointed out how obtaining a straightforward provenance discrimination can be quite challenging.

36Microcrystalline gypsum (i.e. alabaster) can be easily distinguished from other varieties of gypsum because of its lower content in minor oxides and trace elements. Even tough overlapping cannot be avoided, partial quarry recognition can be achieved using concentrations of minor oxides and trace elements. The Gambassi quarry presents a higher average of MgO content while Castellina and Volterra show similar Al2O3 vs SiO2 correlations. Castellina is the only quarry with high Ti contents, maybe due to the larger variety of gypsum facies studied.

  • 47 Kloppmann et al., 2017.

37The use of multi-isotopic provenance fingerprints of S, O and Sr in sulphate, was applied for the first time on plaster-based artworks (stucco) and their attribution to Tuscan gypsum deposits. As expected from literature, results allowed to well separate Triassic and Messinian raw materials. Nevertheless, distinction may be less clear when dealing with outcrops of same depositional age, as shown for Volterra and Castellina Marittima. Even if tested on a limited number of samples this method has to be considered as solid, at least when combining 87Sr/86Sr with δ34S, considering the large data base already published47.

38The approach was tested to study the provenance of a stucco artwork attributed to Donatello and followers (RF 1191, The Nativity). Results obtained on a gypsum crystal preserved during burning showed an overall compatibility of the raw material of this artwork with Messinian Tuscan sources. The interpretation is complicated by the fact that other Italian sources with similar isotopic signature cannot be excluded (e.g. the Messinian deposits in Sicily, and particularly in Emilia Romagna, which could be of some interest for devotional reliefs in Northern Italy). Nonetheless, the outcomes are promising, in particularly for Sr and S isotopes. For further applications on stucco reliefs, a more attentive experimental study on fractioning effects affecting δ18O would be useful, including material processing, external contaminations or ageing. In perspective, it will also be important to acquire minero-petrographic and geochemical data of other Tuscan and Italian quarries with historical significance. This first application opens promising perspectives for an optimisation of this approach in providing valuable information on stucco artworks and their attribution.

This work was supported by the Paris Seine Graduate School Humanities, Creation, Heritage, Investissement d’Avenir ANR-17-EURE-0021 – Foundation for Cultural Heritage Science. Isotope analyses were performed by the Bureau de Recherches Géologiques et Minières (BRGM) in Orléans in the framework of a collaboration with the AlbatREE research project (N° PATRIMA AAP 2013 07). The AGLAE team is gratefully acknowledged for PIXE measurement. Jean-Christophe Lebannier and Guirec Querré (CReAAH UMR 6566 Centre de Recherche en Archéologie, Archéosciences et Histoire, University of Rennes) are gratefully acknowledged for LA-ICP-MS measurements. Felice Tirabasso (quarry manager in Castellina Marittima for the Knauffcompany), and Gabriele Marasco in Volterra are thanked for their help to collect samples in historical alabaster extraction sites in Tuscany (Italy).

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Notes

1 Kloppmann et al., 2017; Cheetham, 2005, p. 360.

2 Gariani et al., 2018.

3 Gariani et al., 2018; Beaugnon et al., 2019.

4 See article by G. Gariani, F. Beaugnon et al. in this volume.

5 Marchand, 2007; Gettens, 1954.

6 Cat. Exhib. Cento, 2007, p. 213-219; Guillet, 2017.

7 Cat. Exhib. Cento, 2007, p. 213-219; Guillet, 2017.

8 Cat. Exhib. Cento, 2007, p. 213-219.

9 Nusara et al., 2017; Testa et al., 1996.

10 Chang et al., 1998.

11 Costagliola et al., 2001; Gale et al., 1988; Playà et al., 2005, 2012.

12 Kloppmann et al., 2014 and 2017.

13 Leroux et al., 2018.

14 Sarti, Testa, 1994; Dinelli et al., 1999; Bossio et al., 1999.

15 Barbieri, Masi, 1976; Dean, 1978; Testa et al., 1996; Bossio et al., 1994; Lugli, Testa, 1993.

16 Bossio et al., 1999.

17 Claypool et al., 1980; McArthur et al., 2001; Kampschulte, Strauss, 2004; Bernasconi et al., 2017.

18 Kushnir, 1982.

19 Kloppmann et al., 2014, 2017, 2018, 2021.

20 See article by G. Gariani, F. Beaugnon et al. in this volume.

21 Vasari, 1550 (ed. 2008); Marchand, 2007; Gettens, 1954.

22 Cat. Exhib. Cento, 2007, p. 213-219; Gariani, 2019.

23 Gariani, 2019.

24 Kloppmann et al., 2017.

25 Kloppmann et al., 2014.

26 See article by G. Gariani, F. Beaugnon et al. in this volume.

27 Gariani, 2019 ; Kockman, Foley, 1987.

28 Gariani, 2019.

29 Kloppmann et al., 2017.

30 Kloppmann et al., 2017.

31 Gariani, 2019.

32 Caillère et al., 1982.

33 See article by G. Gariani, F. Beaugnon et al. in this volume.

34 Chang et al., 1998; Playà, Rosell, 2005; Dinelli et al., 1999; Testa et al., 1996; Franceschi, Locardi, 2014.

35 Cortecci et al., 1981.

36 Playà et al., 2012.

37 Playà, Rosell, 2005.

38 Claypool et al., 1980; McArthur et al., 2001; Kampschulte, Strauss, 2004; Bernasconi et al., 2017.

39 Kloppmann et al., 2017.

40 Barbieri, Masi, 1976; Cortecci et al., 1981; Dinelli et al., 1999.

41 Gariani, 2019.

42 Kloppmann et al., 2017.

43 Chiba, Sakai, 1985.

44 Chiba et al., 1981.

45 Kloppmann et al., 2008, 2011.

46 Usdowsky, 2001.

47 Kloppmann et al., 2017.

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Table des illustrations

Titre Fig. 1. Gypsum quarry of Gambassi, Tuscany (Italy)
Crédits © G. Gariani.
URL http://journals.openedition.org/techne/docannexe/image/8927/img-1.jpg
Fichier image/jpeg, 496k
Titre Fig. 2. Extract of the geological map of Tuscany (modified from Carmignani, 2013) showing the locations of the investigated gypsum quarries
URL http://journals.openedition.org/techne/docannexe/image/8927/img-2.jpg
Fichier image/jpeg, 784k
Titre Table 1. Main information about the three Tuscan quarries sampled
URL http://journals.openedition.org/techne/docannexe/image/8927/img-3.jpg
Fichier image/jpeg, 364k
Titre Fig. 3 a-b. a. Histograms showing minor oxide contents in the geological samples regrouped by quarry. Red dotted lines indicate the mean content (2 +1.2) of minor oxides measured in 24 Florentine stucco reliefs. b. Minor oxide contents measured in these 24 Florentine stucco reliefs
URL http://journals.openedition.org/techne/docannexe/image/8927/img-4.jpg
Fichier image/jpeg, 296k
Titre Table 2. Minor oxides composition measured by LA-ICP-MS on 30 geo-referenced gypsum samples from 3 Tuscan quarries
URL http://journals.openedition.org/techne/docannexe/image/8927/img-5.png
Fichier image/png, 74k
Titre Fig. 4 a-d. Scatter plots showing: a. SiO2 vs Al2O3 and b. K2O vs Al2O3 in geological samples; c. SiO2 vs Al2O3 and d. K2O vs Al2O3 in stucco artworks of Florentine attribution. The red dotted areas in fig. 4 a-b represents the range of values shown in fig. 4 c-d
URL http://journals.openedition.org/techne/docannexe/image/8927/img-6.png
Fichier image/png, 190k
Titre Fig. 5 a-b. a. Whisker plot of Ti (expressed in ppm). The red dotted line represents the mean Ti content in the 24 stucco artworks (Gariani, 2019), while the blue star represents those measured in the sample from RF 1191 relief; b. Whisker plot of Sr contents. The dotted rectangle represents the range of data reported in literature on Tuscan occurrences (Dinelli et al., 1999; Barbieri, Masi, 1976; Cortecci et al., 1981)
URL http://journals.openedition.org/techne/docannexe/image/8927/img-7.jpg
Fichier image/jpeg, 184k
Titre Fig. 6 a-b. a. δ34S versus 87Sr/86Sr ratio; b. δ34S vs δ18O of Tuscan quarries compared to principal selection of major deposits previously identified to have delivered alabaster for 14th to 16th century sculpture in W Europe (Kloppmann et al., 2017, 2021). The results obtained on the selected stucco artwork (RF 1191) of the Louvre Museum are labelled “STUC RF 1191”
URL http://journals.openedition.org/techne/docannexe/image/8927/img-8.jpg
Fichier image/jpeg, 169k
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Gianluca Gariani, Lise Leroux, Philippe Bromblet, Fabio Fratini et Wolfram Kloppmann, « Geochemical and isotopic provenance determination of gypsum used for stucco artworks: application to Florentine stucco reliefs »Technè, 51 | 2021, 64-74.

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Gianluca Gariani, Lise Leroux, Philippe Bromblet, Fabio Fratini et Wolfram Kloppmann, « Geochemical and isotopic provenance determination of gypsum used for stucco artworks: application to Florentine stucco reliefs »Technè [En ligne], 51 | 2021, mis en ligne le 15 décembre 2022, consulté le 16 décembre 2025. URL : http://journals.openedition.org/techne/8927 ; DOI : https://doi.org/10.4000/techne.8927

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Auteurs

Gianluca Gariani

Doctorant, C2RMF, Paris – Laboratoire de Physicochimie des Polymères et des Interfaces (LPPI), CY Cergy Paris Université – Fondation des Sciences du Patrimoine, LabEX PATRIMA, Cergy-Pontoise – Chimie ParisTech, PSL Research University, CNRS, Institut de Recherche Chimie Paris (IRCP), Paris (gariani.gianluca1[at]gmail.com).

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Lise Leroux

Ingénieur de recherche, Laboratoire de Recherche des Monuments Historiques (LRMH) – Centre de Recherche sur la Conservation (CRC-USR 3224, MNHN-CNRS-MCC), Paris (lise.leroux[at]culture.gouv.fr).

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Philippe Bromblet

Ingénieur de recherche, Centre interdisciplinaire de conservation et de restauration du patrimoine (CICRP), Marseille (philippe.bromblet[at]cicrp.fr).

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Fabio Fratini

Researcher, Consiglio Nazionale delle Ricerche-Institute of Heritage Sciences (CNR-ISPC), Florence, Italie (fabio.fratini[at]cnr.it).

Wolfram Kloppmann

Ingénieur de recherche, BRGM (Bureau de Recherches Géologiques et Minières, French Geological Survey), Orléans (w.kloppmann[at]brgm.fr).

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