Investigation of organic additives in Italian Renaissance devotion stucco reliefs from French collections

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Original article

Investigation of organic additives in Italian Renaissance devotion stucco reliefs from French collections Amra Aksamija a,∗ , Witold Nowik a,∗∗ , Patrice Lehuédé a , Anne-Solenn Le Hô a,b , Marc Bormand c , Anne Bouquillon a,b a

Département recherche, centre de recherche et de restauration des musées de France (C2RMF), 14, quai Franc¸ois-Mitterrand, 75001 Paris, France Chimie-Paris Tech, PSL Research University, UMR 8247 CNRS, institut de recherche de chimie-Paris (IRCP), 75005 Paris, France c Département des sculptures, Musée du Louvre, 75058 Paris Cedex 01, France b

a r t i c l e

i n f o

Article history: Received 1st March 2018 Accepted 26 March 2019 Available online xxx Keywords: Stucco Organic additive Analysis Protein Carbohydrate Siccative oil Beeswax

a b s t r a c t A series of Italian Renaissance stucco artworks from Museum of Fine Arts of Strasbourg and Louvre Museum in Paris were investigated with the aim of revealing and characterizing possible organic additives. As the stucco and its additives are little known but, according to the literature, could be complex and contain many different matters, the methodology was first tuned on the laboratory models prepared with gypsum or gypsum/lime plaster with various quantities of organic matter: animal glue or gum Arabic. The methodology consisted of observation of the surfaces of sampled fragments by optical and electronic microscopy, preliminary investigation of the presence of organic compounds by infrared spectroscopy, then extraction of the organic matters from the samples, preparation, and analysis by gas chromatography. The adopted sample preparation scheme allows screening the substances from various classes of organic products from one sample. The optimized analytical approach was applied to the samples from historical objects, showing the presence of proteins, oils, wax and sugars in reliefs. The interpretation of obtained results is not straightforward because of the low response of these substances in the samples, their possible mixture, or unknown origin. In some cases, the protein matter response varies with the deepness of sampling in the matter of stucco, which could be connected with its layered structure or particular surface treatment. © 2019 Elsevier Masson SAS. All rights reserved.

1. Introduction and research aim Stucco is known since ancient times. Numerous historical documents show that stucco was used by the Romans, foremost as an external covering and coating system in masonry, but also to create decorative ornaments and reliefs [1]. According to Arcolao [2], stucco – plaster obtained with lime and marble dust – was used as filler for cracks, a wall finisher or as decorative modeling material. The most ancient treatises relating to the ancient practice of building: De architectura by Vitruvius (probably written between 30 and 15 BC) and Naturalis Historia by Pliny the Elder (77–79 AD), already

∗ Corresponding author at: NU-ACCESS Northwestern University/Art Institute of Chicago, Center for Scientific Studies in the Arts, Northwester University, 2145, Sheridan road, 60208-3116 Evanston, Illinois, USA. ∗∗ Co-corresponding author at: Pôle peintures murales et polychromie, laboratoire de recherche des monuments historiques (LRMH), 29, rue de Paris, 77420 Champssur-Marne, France. E-mail addresses: [email protected] (A. Aksamija), [email protected] (W. Nowik).

described the technique of stucco. However, they considered gypsum easily perishable material [1]. Stucco was modeled in the ancient world and throughout medieval Europe and Asia with small differences in material composition or technique [3]. Some examples of polychrome and gilded plasterworks from the 13th and 14th century are known in Granada (Spain) [4,5]. Stucco also served as a primer in painting and polychrome sculptures [6]. The technique of stucco was developed during the Renaissance, when stucco work was “rediscovered”, as reported by Vasari [7]. Artists from this period searched for the achievement of an ancient formula for true stucco technique [3]. A recipe written around 1503 by Master Jacopo de Monte St. Saviano describes stucco for making and modeling figures, colouring them and providing water resistance for them [8]. Other recipes for mortar and plasterwork preparations described in literature were attributed to Pirro Ligorio, an architect and coadjutor of Michelangelo for St. Peter’s basilica in Rome [8], and to Andrea Palladio [9]. Mortar used by Michelangelo for ‘‘The Last Judgment” in the Sistine Chapel is reported to be a fresco mortar containing pozzolana as an active hydraulic component [10].

https://doi.org/10.1016/j.culher.2019.03.012 1296-2074/© 2019 Elsevier Masson SAS. All rights reserved.

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From the second half of the 19th century, the stucco practice turns to the prefabricated serial decorative elements. The cement dust and other additives were used for its preparation allowing a reduction in the time needed for its manufacturing. This matter was used in decorative and finishing work inside the buildings and on plaster walls and ceilings [2]. Stucco is a mixed material, which does not have a well-defined composition. Many various recipes for stucco exist, showing that it is similar to mortars and plasters. Basically, stucco is made of: • plastic materials, which become solid after chemical transformation, such as lime or gypsum. But can also contain: • fillers, which help to maintain the constant volume of the mixture after drying and can modify physical characteristics (cohesion) of plaster, such as: sand, marble dust, pozzolana, and gravel; • additives, which modify physical and chemical characteristics of stucco (plasticity, setting time, adhesion, humidity resistance, mechanical resistance, brilliance, etc.). They can be mineral or organic. Many compounds were reported regarding organic additives in stucco, depending on the period and geographic region. According to Berner and Weber [11] the most important organic additives were different types of glue such as fish glue, skin glue, rabbit skin glue, bone glue, parchment size, gum arabic, mucilaginous plants and grains extracts, vinegar or curd with whey, blood or eggalbumin, and wine lees. The addition of glue water seems to have been very usual, as it retards the setting of plaster [3]. To increase the gloss of an already well-polished surface, lipid materials were added, such as different kinds of oils and fats (olive-, walnut- and linseed-oil, pork fat), waxes and soaps [11]. Other organic additives like milk, vegetal resins, and fig juice were also reported [1]. According to the results obtained from previous works on ancient stucco, gelatin and eggs are often found in stucco artworks and sometimes polysaccharides, lipids, and soaps as stearates of calcium or magnesium [12–15]. The animal glue and Pinaceae resin were very recently found in the material somewhat similar to stucco: the bonding mortars from Hierapolis (1st -3rd C., Asia Minor) [16]. The characterizations of organic additives in stucco have been sparsely published. The main difficulty for the analysis is the low content of organics, compared to the mineral part, often as traces if they are even really present. There are some extraction procedures for isolating the organic material from the inorganic matrix. Montana et al. [15] and Ronca [17] extracted lipids with a chloroform-methanol mixture and then analysed them by thin-layer- and gas-chromatography. Sugars and proteins were extracted with 0.1 N NaOH and analysed via the colorimetric method. Other methods of extraction, developed for paintings, merit to be quoted. As they deal with characterization of organic binder of mineral pigments, they could be in fine adapted to stucco analysis purposes. Corso et al. [18] reported combined extraction method of polar and non-polar organic compounds present in samples of mural paintings. After corresponding sample preparation procedure, free amino acids were analysed by LC-MS/MS, monosaccharides by GC-FID and GC-MS and lipids by GC-MS. This method is interesting because wall paintings can contain very low amounts of binder according to the support porosity, technique, and ageing phenomena. However, the protein hydrolysis step was done using enzymes, which is more complicated to set up than the chemical digestion. Another interesting extraction procedure following multi-step isolation of various classes of organic compounds applied to basic paint samples was published by Lluveras et al. [19]. An analysis of extracted lipids, sugars and proteins was performed by GC-MS, after specific hydrolyzes and derivatizations. We

decided to basically adapt this approach to the analysis of organic additives in stucco, introducing some modifications and optimizing it for our purpose. The aim of this work was the characterization of the organic additives present in the stuccoes from serially produced devotional reliefs in Tuscany during the Renaissance period (15th–16th C.). These artworks were molded after the originals sculpted in terracotta, bronze or marble, made by artists like Ghiberti, Desiderio da Settignano, Luca della Robbia, Verrocchio, and others and usually painted. They were diffused in Florence, throughout the whole of Italy, as well as abroad. A significant number of such stucco artworks attributed to Florentine artists is present in French public museum collections. However, their material composition had not yet been studied. Stucco samples were first observed under SEM-FEG to bring out their fine micro-structure and to check if any amorphous organic compounds could be detected. The preliminary analysis was performed by FTIR, to verify the presence of organics in samples and to characterize in the same time some elements corresponding to the mineral composition (sulfates, carbonates, silicates). The presence of different mineral compounds can have an influence on the efficiency of procedures of sample preparation for GC, which thus need to be adapted. Then organic matter was extracted from samples and fractionated giving different families of compounds, which need distinctive preparation protocols prior to GC analyses. The analysis protocol had first been performed on the stucco mock-ups prepared in laboratory, before being applied to the artistic samples of the stucco artworks. The composition and preparation of the stucco mock-ups was inspired by ancient recipes and traditions found in literature [11,13]. Detailed recipes used for stucco mock-ups preparation as well as their analyses are presented in “Supplementary material”. Samples of real Renaissance stucco came from objects conserved in the sculptures department of the Louvre Museum in Paris and Museum of Fine Arts of Strasbourg collections. The precise mineral composition of these objects was recently established [20].

2. Materials and methods Stucco samples were observed using electron microscopy with SEM-FEG instrument JSM-7800F (Jeol Ltd., Tokyo, Japan), with the external secondary electron detector. Prior to analysis, samples were placed on double-sided adhesive tape and fixed with carbon lacquer, then coated with platinum layer 1.6 nm thick. Considering the high porosity of samples, this thickness of platinum layer proved insufficient to avoid the effect of charge and it was necessary to work using low electron beam energy of 2 keV (intensity 0.1 nA). Every sample was observed under optical stereomicroscope using several magnifications (from 5 to 50) to verify the heterogeneity of material, then the different layers and grains were carefully scratched or removed using a tweezers and a scalpel prior to analysis. Fourier-Transform Infrared spectroscopy analysis on solid samples was performed on Spectrum 2000 FTIR instrument (Perkin Elmer, Wellesley (MA), USA) with DTSG detector. The spectra (16 scans) were recorded with resolution of 4 cm−1 in transmission mode with a Diamond Anvil Cell (High Pressure Diamond Optics, Inc., Tucson (AZ), USA). Gas chromatography with mass spectrometric analysis was performed on GCMS-QP2010 gas chromatograph (Shimadzu, Kyoto, Japan) hyphenated with mass spectrometer working in electron impact mode (70 eV). Separations were done on the CP-Sil 8CB low bleed/MS column (length: 30 m, ID 0.25 mm, OD: 0.39 mm) purchased from Agilent Technologies (Middleburgh, Netherlands). The carrier gas (He, purity 99, 995%) was used with a constant flow rate

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of 1.0 mL/min for all analyses. All injections were done into the split/splitless injector in splitless mode. For analysis of amino acids, the injector was set at 260 ◦ C. The oven program temperature started at 100 ◦ C for 1 min, then raised 5 ◦ C/min and finished at 300 ◦ C for 5 min. The chromatograms were acquired in SIM (Selected Ion Monitoring) mode, choosing the following m/z ions: 158, 184, 198, 200, 218, 232, 260, 288, 292, 302, 314, 336 which are characteristics for respective amino acids: Ala, Gly, Val, Leu, Ile, Nor, Pro, Ser, Thr, Phe, Asp, Glu, Hpr and Lys. For analysis of monosaccharides and uronic acids, the injector was set at 260 ◦ C. The analyses were performed with an optimized multi-segment temperature program, starting from 90 ◦ C for 1.5 min, then with the 10 ◦ C/min gradient, then isothermal plateau at 120 ◦ C for 3 min, gradient of 2 ◦ C/min up to 190 ◦ C, then with the gradient of 10 ◦ C/min up to 295 ◦ C kept for 8 min. The chromatograms were acquired in SIM mode, selecting the m/z ions 204 and 217 as specific fragments for monosaccharides and uronic acids. For analysis of fatty acids, terpenes, hydrocarbons and alcohols (lipids, resins, waxes) the injector was set at 310 ◦ C. The general purpose linear temperature gradient program was used. It began with isothermal conditions at 80 ◦ C for 1 min, then raised 5 ◦ C/min and finished at 325 ◦ C for 5 min. The chromatograms were acquired in TIC (Total Ion Chromatogram) mode. 2.1. Renaissance stuccoes and sampling procedure Sampling campaigns were carried out on the Renaissance stucco artworks in the Department of Sculpture of Louvre Museum and in Museum of Fine Arts of Strasbourg (Fig. 1a). Eighteen pieces were considered for this study (Table 1). They are mostly reliefs of a thickness of few centimeters over the entire piece. Some of them are hollow with thick walls, but there are also those which are rather thin (less than a cm). Samples were systematically taken on the back of the artwork, free from any paint, and in two different forms: a fragment from the surface (a few mm2 , depth 1 to 2 mm) obtained with a micro-burin and a powder (about 100 mg) obtained by micro-drilling with a tungsten carbide mini-drill (Fig. 1b). However, some artworks (Inv 239, Inv 246 and RF 1169) were very thin and fragile, so it was not possible to take a fragment sample and in the case of those artworks, samples were taken only in powder form. The collected samples for organic analysis are stored in aluminium foil to minimize external contamination. To verify that the samples are representative for the artwork material, some heterogeneous artworks were sampled twice or even three times on specific areas: RF 564, RF 588, Campana 19 from Louvre Museum, and INV 507 from Museum of Fine Arts of Strasbourg. RF 1191 was chosen to test the presence of several layers of stucco in the same artwork. This relief was sampled twice by micro-drilling to two successive depths in the same place: 0–5 mm and 5–10 mm. About forty samples were prepared and analysed. A list of objects and samples is set in Table 1. 2.2. Sample preparation for GC-MS analysis The extraction protocol is adopted from literature [19] for extraction of organic compounds from inorganic matrices. Some changes were made with the aim of improving extraction procedures for our samples. The final sample preparation scheme is shown in Fig. 2. The only changes in the original protocol we adopted from literature were made in the preparation of sugar [21] and lipid/wax/resin fractions [22]. The aliquot of 1.5 mg of powder was taken from each sample for the analysis following the complete scheme.

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3. Results and discussion 3.1. SEM-FEG analysis SEM-FEG analysis allows studying microstructural properties which are at first hand important for the mineral part of stucco, but may also give significant information about the presence of organic matter. Here, only samples in fragment form were studied, without any other preparation, given that powder samples are not suited for this kind of analysis. The majority of samples were presented by small fragments of a few millimeters taken from the surface of artworks and their fracture surface was studied by SEM-FEG. On some samples, the surface corresponding to the surface of the object was also observed. The gypsum crystals are easily observed in all samples in different but typical forms, as needles, stars and tablets (Fig. 3) and their size is about a few microns. In some sample fragments, but especially those from an area near the surface, a kind of filamentous structure with characteristic nodules of few microns were found. Their particular shape is characteristic of microorganisms or fungal spores (Fig. 4). This contamination, which can be seen on many objects (Table 2), may affect the conservation of organic matter in stucco, but also be detected along with original matter at molecular level, which could lead to possible misinterpretation of obtained analytical results. 3.2. FTIR analysis The first analysis of molecular structure of stucco artwork samples was performed by FTIR. Analysis was carried out in transmission mode in diamond cell. FTIR analysis was used to characterize the mineral matrix of the stucco artwork samples and to detect the eventual presence of organic compounds (Table 2). Prior to FTIR analysis, each sample was observed under optical stereomicroscope. Generally, it was possible to distinguish three different zones in fragments: white or beige coloured inner part, very porous and fragile intermediate part containing different kinds of orange to brown coloured encrusted particles, and a brown to grey coloured superficial layer. FTIR spectra also reveals three different groups of spectra: • interior matter being gypsum; • different grains of silicate or carbonate minerals in intermediate gypsum layer and; • a superficial layer containing organic substances, like lipids, proteins or polysaccharide compounds (Fig. 5). The FTIR spectra of sample from relief RF 588, displays sharp absorption bands around 1620 cm−1 and around 1712 cm−1 , which can be attributed to drying oils, together with the bands around 2848 and 2918 cm−1 , characteristic for C-H bond in organic compounds. For the sample from relief RF 564, two absorption bands around 1651 and 1541 cm−1 correspond to amide I and amide II absorption bands, which occur in protein compounds, and again C-H bands around 2848 and 2918 cm−1 . A large absorption band around 3300 cm−1 corresponds to N-H stretching, and is usually present on IR spectra of amides [23]. The results of FTIR analysis performed on stucco artwork samples show traces of different fillers such as calcium carbonate, different clay and silicate minerals, and also organic compounds such as lipids, proteins and polysaccharides [24] in amounts detectable by FTIR. FTIR analysis on Campana 20 and Inv 244 (fragment, powder 1 and powder 2) did not reveal the presence of organic matter

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Fig. 1. Examples of analysed stuccoes and their sampling. a. Virgin and the Child (Inv 239) from Museum of Fine Arts, Strasbourg; b: sampling by drilling on Virgin and the Child after Ghiberti (RF 786) (powder), Department of Sculpture, Louvre Museum, Paris and; c: Sampling by micro-burin on Nativity after Donatello (RF 1191).

Fig. 2. General scheme of sample preparation for analysis of organics present in stucco samples.

(Table 2). The cellulose or polysaccharides are present in almost every plant and thus not specific and maybe unintentionally added, which could be an explication for Inv 246 (powder) where FTIR analysis revealed the presence of polysaccharide or cellulose in the sample. For the samples Inv 246 (powder) and Inv 247 (fragment), some organic matter like polysaccharides or lipids was detected by FTIR analysis. 3.3. GC-MS analysis The results obtained with historical stucco samples are shown in Table 2. The interpretation of results obtained for historical stucco samples was done qualitatively. However, as internal standards were added in analysis of amino acids and monosaccharides respectively, a calculation of relative responses were done in the aim of tentative comparison of the results obtained for each of these classes. 3.3.1. Protein fraction We obtained positive responses for the presence of amino acids in nearly all the samples (Fig. 6) and estimation by relative response of a series of amino acids reported to Nor standard were used to range the samples and make the difference between them (noted as

+, ++ and +++, Table 2). The whole population of results was ranged in decreased ratio of the sum of relative amino acid responses (see Fig. 4 in Supplementary material). The population was separated in arbitrary way in three parts according to response level and the ¨ signs were attributed to each sub-population. The subnumber of +¨ ¨ corresponds probably to the level of contamination population +¨ of samples or preparation chain by proteins. The sub-population ¨ ¨, represented by few samples and characterized by high amino +++ acid relative response could signify the addition of proteinaceous material to the stucco. It seems difficult to postulate the classification of intermediary “++” sub-population responses. However, we will principally consider in our further discussion the samples belonging to these two groups. Six artworks, on eighteen studied, were classed in “+++” group: INV 247, INV 507, RF 564, Campana 19, RF 1191 and Campana 16. Following artworks reached at least “++” group: RF 588, INV 242, INV 362, INV 586, RF 896, INV 246. For INV 507, RF 564, Campana 16 and Campana 19, the differences in amino acids response are observed between superficial fragments and in-depth sampled powder. It is also important to note that in only one sample, fragment from RF 897, the presence of amino acids was not detected, except the internal standard (norleucine). In GC-MS analysis of proteins, the characterization and distinction of proteinaceous materials are generally based on the relative

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amounts of certain amino acids [19,25,26]. The comparisons of relative responses of respective amino acids detected in the samples are made with current standards of proteinaceous materials (gelatin, casein and egg white), with particular attention to the presence or the absence of hydroxyprolin (Hpr), the specific marker of gelatin. The samples containing Hpr are supposed to contain at least gelatin, present in the animal skin glue, but do not exclude the simultaneous presence of other proteins. Thus, gelatin could be present in twelve stucco artworks on eighteen studied. In two art-

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works (Campana 20 and RF 786), the chromatographic response of Hpr seems to be very high in comparison with the internal standard (Nor) (see Fig. 5 in Supplementary material). In the contrary, in the case of artworks INV 242 and INV 244, the response of Hpr is very weak in comparison with internal standard, which could indicate that gelatin is may be absent in corresponding samples. This conclusion should be considered with caution, because the recovery of Hpr is one of most challenging among all amino acids in GC analysis. The following amino acids: glycine (Gly), leucin (Leu), glutamic

Table 1 Studied objects and their general characteristics. Object

Provenience, name and inventory number Collection

Stylistic analogies with other artworks

After Mino da Fiesole, woman’s bust (Sainte Catherine de Siena?), RF 588 Paris, Louvre Museum, Department of Sculptures

After Mino da Fiesole, work in marble in Washington National Gallery, inv 1943.4.71. Other copies are known

Florence, first half of 15th century, Virgin and the Child, Inv. 244 Strasbourg, Museum of Fine Arts

No

No history

Lorenzo Ghiberti’s workshop, Virgin and the Child, RF 896 Paris, Louvre Museum, Department of Sculptures

Other copies are known

1930’s Then 2014

Past conservationrestoration treatments

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Table 1 (Continued) Object

Provenience, name and inventory number Collection

Stylistic analogies with other artworks

Past conservationrestoration treatments

Florence, first half of 15th century, Virgin and the Child, Inv. 246 Strasbourg, Museum of Fine Arts

No

No history

After Lorenzo Ghiberti, Virgin and the Child, RF 786 Paris, Louvre Museum, Department of Sculptures

Other copies are known, including Inv 239

2009

After Lorenzo Ghiberti, Virgin and the Child, Inv. 239 Strasbourg, Museum of Fine Arts

Other copies are known, including RF 786

No history

Circle of Luca della Robbia, Florence, Virgin and the Child, Inv. 247 Strasbourg, Museum of Fine Arts

No

No history

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Table 1 (Continued) Object

Provenience, name and inventory number Collection

Stylistic analogies with other artworks

Past conservationrestoration treatments

After Donatello and Bellano, Nativity, RF 1191 Paris, Louvre Museum, Department of Sculptures

Other copies are known

2017

Florence (?), first quarter of 16th century, Virgin and the Child carried by angels, RF 564 Paris, Louvre Museum, Department of Sculptures

Other copies are known

2013

After Benedetto da Maiano, Virgin and the Child, Inv. 507 Strasbourg, Museum of Fine Arts

Other copies are known

No history

After Antonio Rossellino, Virgin and the Child, type Vienne, Campana 16 Paris, Louvre Museum, Department of Sculptures

Other copies are known

Undated old-one. Then probably in the 1930’s

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Table 1 (Continued) Object

Provenience, name and inventory number Collection

Stylistic analogies with other artworks

Past conservationrestoration treatments

After Antonio Rossellino, Virgin and the Child with Saint Jean-Baptist andangels, type Santa Maria Nuova, Campana 19 Paris, Louvre Museum, Department of Sculptures

Other copies are known, including Inv 362

After 1934

After Antonio Rossellino, Virgin and the Child with Saint Jean-Baptist and angels, type Santa Maria Nuova, Inv. 362 Strasbourg, Museum of Fine Arts

Other copies are known, including Campana 19

No history

After Antonio Rossellino, Virgin and the Child with candelabras, Campana 20 Paris, Louvre Museum, Department of Sculptures

Other copies are known

Undated old-one

After Benedetto da Maiano, Virgin and the Child, RF 1169 Paris, Louvre Museum, Department of Sculptures

After a work in marble in Washington National Gallery, Kress collection. Other copies are known

1934

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Table 1 (Continued) Object

Provenience, name and inventory number Collection

Stylistic analogies with other artworks

Past conservationrestoration treatments

After Desiderio da Settignano, Virgin and the Child known as Madonna of Turin, RF 897 Paris, Louvre Museum, Department of Sculptures

Other copies are known

Undated old-one

After Desiderio da Settignano, Virgin and the Child known as Madonna of Turin, Inv. 586 Strasbourg, Museum of Fine Arts

Other copies are known

No history

After Donatello, Virgin and the Child known as Madonna Pazzi, Inv. 242 Strasbourg, Museum of Fine Arts

Other copies are known

No history

acid (Glu), aspartic acid (Asp) and serine (Ser) displayed the largest variation of their relative response from one sample to another, suggesting the presence of another protein material aside from gelatin. Glu and Asp may suggest that casein or egg were also added into the stucco mixture. In the case of Gly, the interpretation should be more careful because the estimation of relative amount of this amino acid can be also be increased by presence of microorganisms, as bacteria and fungi [25]. The obtained results make it difficult to identify with precision the protein material found in these samples of stucco artworks. On the one hand, there probably could be a mixture of two or more different protein materials in the original recipe for stucco.

On the other hand, there are other hypotheses which can give a possible explanation. The degradation of proteins, the possibility of contamination with other protein materials than usually supposed and undocumented restoration work or development of microorganisms can have an influence on the amino acids ratio and thus make the right protein identification impossible. The protein profile modification by the presence of proteins from microorganisms [27] could be especially true since observation under SEM-FEG revealed their presence. The microorganisms are the source of different classes of organic compounds, mainly proteins, lipids, and sugars as well as secondary metabolites. Nonetheless, the presence of relatively high responses

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Fungal growth (SEM-FEG)

Proteins (GC-MS)

Campana 16 fragment Campana 16 powder Campana 19A powder Campana 19B fragment Campana 19B powder Campana 19C powder Campana 20 fragment Campana 20 powder Inv. 239 powder Inv. 242 fragment Inv. 242 powder Inv. 244 fragment Inv. 244 powder 1 Inv. 244 powder 2 Inv. 246 powder Inv. 247 fragment Inv. 247 powder Inv. 362 fragment Inv. 362 powder Inv. 507 fragment Inv. 507 powder 1 Inv. 507 powder 2 Inv. 586 fragment Inv. 586 powder RF 1169 powder RF 1191 fragment RF 1191 powder (depth 0-5 mm) RF 1191 powder (depth 5-10 mm) RF 564 II fragment in the head RF 564 II fragment on the back RF 564 II powder RF 588-2 fragment RF 588-2 powder 1 RF 588-2 powder 2 RF 786 fragment RF 786 powder RF 896 fragment RF 896 powder RF 897 fragment RF 897 poudre

Dark-brown spots

Cellulose or other polysaccharides Lipids Proteins or lipids Cellulose, resins, lipids Proteins Lipids Proteins (traces) Proteins or lipids -

-

+ +++ + + +++ ++ + + + ++ + + + + ++ +++ ++ ++ ++ ++ + +++ ++ + +

Brown and black grains

White and homogeneous White to beige powder Dark-brown and orange grains Dark matter that crumbles easily

Light-brown grains Greyish surface layer; brown grains White crystals, greyish surface layer, dirty Black and rigid surface crust ; pieces of inlaid wood

Greyish thin surface layer

Orange-brown grains

Greyish surface layer ; traces of fibers or hairs Blackened surface layer

Very thin glossy layer on some places Heterogeneous beige layer with blackened surface Dark-brown and orange grains

-

+

+ +

-

Other compounds (GC-MS) (if detected)

Polysaccharides Polysaccharides

Polysaccharides

Beeswax

+

+ –

+ + +

+ +++ ++ +++ + ++ + ++ + + ++ + +

Animal fat (traces) Siccative oil (traces)

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Object and sample

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Table 2 Samples and results of their characterization. Empty space in column “Observations” means that the sample was homogeneous. In FTIR column, the symbol “-” means that in those samples organic matter was not detected. In the column “SEM-FEG” analysis, the empty space means that these samples were not analysed (all powder samples except Inv 246 where FTIR analysis revealed the presence of organic matter) by SEM-FEG, and symbol “-” means that in these samples no fungal growth was observed.

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Fig. 3. Image SEM-FEG of sample from stucco RF 897, gypsum crystals present in form of tablets.

obtained for proteins in some of our samples does not seem to correlate with the presence and abundance of observed spores and fungal growth. For example, in samples from INV 507, INV 362 and RF 588-2, the observed fungal growth was abundant and the relative response of detected amino acids after GC-MS analysis is also important. In the contrary, in the samples from artworks INV 242, INV 586 and INV 247, no fungal growth is observed but relative response of detected amino acids is nevertheless high (respectively denoted “++” and “+++” for INV 247, Table 1). In the other cases where the fungal growth is observed, relative response of detected amino acids is even very low (denoted “+” for INV 244, RF 786 and RF 897). According to the information we gathered, some of the stucco artworks studied in this paper were restored once or several times in the past. In some cases, the reports mention cleaning and dusting of an artwork’s surface, but for many others, there is any information available (see Table 1). It is possible that restorative work may have led to a contamination or degradation of the original organic compounds. Thanks to results obtained in protein fraction analysis from the samples taken from the surface (fragment) and in-depth of the material (powder), the investigated objects may be tentatively organized in three groups (Table 3). The first group “fragment > powder” (roughly ratio > 2:1) represents samples where proteins give higher response for the samples taken at the surface, including obviously matter located some millimeters in deep. Second group “fragment ∼ powder” represents samples which show likely similar responses of proteins and third group “fragment < powder” (roughly ratio < 1:2) represents samples where the response of proteins on the surface is relatively poor.

Table 3 Artworks distributed in groups according to the ratio of amounts of proteins between the surface (fragment) and the depth (powder) of the stuccos. Group

fragment > powder

fragment ∼ powder

fragment < powder

Object

RF 564 II RF 896 Inv. 242 Inv. 247 Inv. 586

RF 786 Inv. 244 Campana 20

RF 1191 Inv. 507 Campana 16 Campana 19 RF 897

The artworks, Campana 19 and Inv 507 were sampled in different areas. Campana 19 was sampled in three different areas noted as A, B, and C. The Inv 507 was sampled in two different areas and two sampled powders show the difference in color: one is white and the other is beige. The obtained results show that relative response of amino acids varies between sampling points: higher chromatographic response of amino acids was detected in powder samples B and C from Campana 19 than in sample A. A similar situation is observed in Inv 507 where the beige sample seems to give higher response. RF 1191 was sampled in the same area but in two depths (0–5 mm and 5–10 mm). The powder taken at 5–10 mm depth gets higher relative response for amino acids than that taken from the superficial part (0–5 mm). These different levels of response in amino acid analysis in different deep of sampling in stucco matter may suggest that several artworks could present a possible superposition of stucco layers, a migration of organic matter added during manufacturing, or deposition / removal of proteins during post-production treatment or cleaning. These suppositions are based on an estimation presented above. However, it could be perfectible. More in-depth investi-

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Fig. 4. Image SEM-FEG of sample from stucco INV 507; microorganisms present on the surface. Magnification 5000×, acceleration potential 2 kV, white bar corresponds to the reference distance of 1 ␮m.

gation, using validated quantitative approach, should be done to confirm our first observations. 3.3.2. Polysaccharide fraction GC-MS analysis of polysaccharide fractions show the presence of monosaccharides: glucose (Glu), xylose (Xyl), arabinose (Ara), rhamnose (Rha) and galactose (Gal) in three samples (INV 507 fragment and INV 244 fragments and powder). The obtained chromatogram for polysaccharide fraction of INV 507 fragment is presented in Fig. 7. The monosaccharides Ara, Rha, Gal and Xyl could come from a plant gum but considering their small relative response comparing to the internal standard (Man) present in the samples, the source of these monosaccharides could not be well characterized. In a fragment sample from INV 507 artwork, cellulosic fibers were observed during preliminary observations under optical stereomicroscope and FTIR analysis. The cellulose is a polysaccharide built by a large number of monomeric glucose units. This could be an explanation for the presence of Glu in the analysed polysaccharide fraction from this object. Xylose (Xyl) is also present in the analysed sample. However, it is a component of the hemicellulose, contained in the majority of plants and not particularly specific to any species. As reported in Lluveras-Tenorio et al. 2012 [28], the presence of Xyl cannot be used to distinguish between Arabic, tragacanth and fruit tree gums when the softwood is present, so the obtained result do not allow the precise identification of saccharide sources. In the same time, in stucco and historical Renaissance frescoes mortars, the cellulosic fibers [29], elm bark [30] and straw [31] are often intentionally added to improve the elasticity and avoid cracking. This result of present monosaccharide composition can be explained by the presence of cellulosic fibers observed in the

sample. Also, there is a possibility that a polysaccharide gum is present in parallel, because of the presence of arabinose and rhamnose, but its presence cannot be clearly confirmed because of their very low relative response.

3.3.3. Lipid/resin/wax fraction The majority of analysed samples contain common fatty acids: palmitic, stearic and oleic acid and their amount are detectable in most samples. Also, the traces of lipids are often present as environmental pollutants. Possible sources of these fatty acids are numerous, including even the animal skin glue which may contain a small amount of animal fat, as it was a case in our mock-ups done with skin glue. No internal standard was added, as we looked especially for markers which are less present as contaminants. However, two samples: a fragment of RF 588 and a fragment of RF 564, show clearly higher response of fatty acids than the rest of samples, which might correspond to the presence of an added fat. In the fragment sample from artwork RF 588 palmitic (C16), stearic (C18) and oleic (C18:1) acid are present as for all other samples, but also azelaic acid (C9, 2COOH), suggesting the presence of traces of a drying oil (Fig. 8). The ratios of acids: azelaic/palmitic (A/P) and palmitic/stearic (P/S) are usually used for identification of drying oils, but in this sample the obtained lipid profile cannot be assigned to a pure drying oil; it seems more as a combination of two lipids (drying oil with an animal lipid contaminant?). The function of this drying oil in stucco remains uncertain because it is not clear if it is added in stucco material or if it is used in surface treatment (e.g. waterproofing). As we do not have information about the past restoration treatments, the presence of the lipid compounds can be related to this practice as well.

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Fig. 5. Spectra of stucco artwork samples: RF 588 (1) and RF 564 (2), both representing superficial layer with characteristic bands of proteins (“P”) and lipids (“L”).

Fig. 6. Chromatogram of the protein fraction in SIM mode: artwork INV 247, fragment sample; Ala: alanine; Gly: glycine; Val: Valine; Leu: Leucine; Ile: Isoleucine; Pro: Proline; Ser: Serine; Thr: Threonine; Phe: Phenylalanine; Asp: Aspartic acid; Hpr: Hydroxyproline: Glu: Glutamic acid; Lys: Lysine; Nor* : internal standard norleucine.

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Fig. 7. Chromatogram of the polysaccharide fraction in SIM mode: artwork INV 507, superficial fragment sample; Ara: Arabinose; Rha: Rhamnose; Xyl: Xylose; Gal: Galactose; Glu: Glucose; Man* : internal standard mannitol.

The second sample, fragment of artwork RF 564, may contain an animal fat, since mostly saturated fatty acids: myristic, palmitic and stearic are present, giving a very high response. In the absence of specific markers, the nature of this fat remains difficult to define. The powder sample taken from artwork RF 1169 gave a chromatogram where several fatty acids are distinguished, but especially palmitic acid (C16) and several long-chain hydrocarbons with an odd-number of carbon atoms (n-C27 being the most abundant), as shown in Fig. 9. This chromatographic profile is characteristic of beeswax treated by alkaline hydrolysis during preparation of the sample for analysis. The waxes are substances that can be used for the surface treatment of some objects. Generally, they are heated prior to application to the support. The relief RF 1169 is very thin and it is possible that the wax has migrated in the depth of the artwork from one of its surfaces. The further study of polychrome part of these artworks may give more information. 4. Conclusion The present work was carried out on the corpora of eighteen artworks and was mainly focused on the organic fraction of stucco. The characterization of this fraction presents two main difficulties: its concentration in the gypsum matrix seems very low and the variety of organic additives can be extremely wide. Also, several matters were actually present in stucco, as it was shown here for the Italian Renaissance stucco reliefs from 15th century. The obtained results show the presence of several classes of organic compounds in the artworks:

• the estimation was based here on the chromatographic relative responses of amino acids and according to that just few objects can be supposed to be made in stucco with protein addition. They were characterized as probably gelatin, because of presence of hydroxyproline, maybe sometimes associated with casein or egg or another uncharacterized protein material. It is hard to say precisely which sample contains the stucco additive and which sample contains proteins from other sources, as contaminants or traces of previous restoration treatments; • in a small number of samples the analyses revealed the possible presence of drying oil (RF 588 and RF 564) and polysaccharides (INV 507 and INV 244). One of them contained beeswax (RF 1169). The more precise interpretation of the obtained results is related to several points that need to be clarified further. The observations under optical stereomicroscope revealed that many of stucco samples have heterogeneous structure, which is confirmed by FTIR analysis. It was possible to distinguish different mineral (gypsum and silicates) and organic (lipids, proteins, polysaccharides) matters present in the same sample. In the case of samples which seem homogeneous (powder), the FTIR analysis was not detected the presence of organic matter, except in the case of one sample (Inv 246 powder). But further analysis by GC-MS shows that in several powder samples, the organic matter gives positive results, even if not detected by FTIR analysis, probably because of dominant mineral matrix. Our first results show that stratigraphy of stucco reliefs might be complex including layer corresponding to the first few or few tenths of millimeters below the surface and coarser layers used as support to hold the whole structure. This stratigraphy could

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Fig. 8. Chromatogram of the lipid/resin/wax fraction in TIC mode: artwork RF 588-2, sample – fragment from the surface; C9, 2COOH – Azelaic acid, C14 – Myristic acid, C16:1 – Palmitoleic acid, C16 – Palmitic acid, C18:1 – Oleic acid, C18 – Stearic acid.

Fig. 9. Chromatogram of the lipid/resin/wax fraction in TIC mode: artwork RF 1169, powder from deep sampling; C16 – Palmitic acid, C18:1 – Oleic acid, C18 – Stearic acid, C24 – Lignoceric acid, n-C23, n-C25, n-C27, n-C29, n-C31 – long-chain hydrocarbons with an odd-number of carbon atoms.

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be responsible for different chromatographic responses of organic matter observed in samples taken from different depths in stucco reliefs. However, these different responses could also result from possible migration of organic compounds, as it was observed on mock-ups containing skin glue and in artwork samples. Finally, the difficulty of interpretation of some GC-MS profiles is probably connected to the alteration of organic compounds, caused by natural ageing, restoration treatments, or biological activity, especially the fungal growth that was observed in some cases. As long as these questions remain open, we will interpret the present results very cautiously. The obtained results show that the characterized corpus of objects is not homogenous from the organic matter point of view. Several reliefs seem to contain proteins, some of them other matters but do not form consistent groups which could display some convergence between the type of matter and aesthetic or formal types of relief. Thus, no characteristic and constant workshop practice was held to the light in this series of studied stuccos. It will be necessary to enlarge stucco series by other objects originating from other geographical areas or from other historical periods to enrich and complement the results obtained in this study. Although a number of above mentioned uncertainties remain, the research work presented in this paper contributes to the better knowledge of artworks that are still left little known. Acknowledgments The authors are grateful to Mrs. Helene Suzini, the restorer of stone materials in Department of Sculpture in the Louvre museum, for her precious help during the preparation of stucco mock-ups. This research was financially supported by the French Foundation for Sciences of Cultural Heritage (LabEx PATRIMA) in the frame of the project ESPRIT (Etude des Stucs Polychromés de la Renaissance ITalienne) EUR-17-EURE-0021. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at https://doi.org/10.1016/j.culher.2019.03.012. References [1] R. Martins Peres, C. Bonin Luis, Damage in Architectural Historic Patrimony – An Approach on “Stucchi” of Facades”, Portugal, XII DBMC International Conference on Durability of Building Materials and Components Porto (2011) (April 12th–15th). [2] C Arcolao, Le ricette del restauro – malte, intonaci, stucchi dal XV al XIX secolo, Marsilio Editori, Venezia, 1998. [3] G. Beard, Stucco and Decorative Plasterwork in Europe, Thames and Hudson Ltd, London, 1983. [4] A.G. Bueno, V.J.M. Flórez, The Nasrid plasterwork at qubba Dar al-Manjara lKubra in Granada: characterization of material and techniques, J. Cult. Herit. 5 (2004) 75–89. [5] A. Andreotti, I. Bonaduce, V. Castelvetro, M.P. Colombini, A. Lluveras-Tenorio, M. Raihane, M. Ibnoussina, A. Buajamidda, Characterization of the organic materials used in the painting of the vaulted ceiling at the Saadian Tomb of Mulay Ahmed Al-Mansour (Marrakech), J. Cult. Herit. 15 (2014) 300–307. [6] C. Cennini, The Craftsman’s Handbook “Il libro dell’arte”, Translated by DV. Thompson Jr, Dover, New York, 1960. [7] G. Vasari, The Lives of the Artists, Translated by JC. Bondanella, Oxford University Press, Reissue edition, 2008. [8] G.T. Robison, Plasterwork generally: a glimpse of its history, in: W. Millar, G. Bankart (Eds.), Plastering Plain & Decorative, Donhead Publishing Ltd, UK, USA, 2009, pp. 1–28.

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