Methodology to Evaluate the State of Conservation of Historical Plasterwork and Its Polychrome to Promote Its Conservation

Citation: Torres-González, M.; Alejandre, F.J.; Alducin-Ochoa, J.M.; Calero-Castillo, A.I.; Blasco-López, F.J.; Carrasco-Huertas, A.; Flores-Alés, V. Methodology to Evaluate the State of Conservation of Historical Plasterwork and Its Polychrome to Promote Its Conservation. Appl. Sci. 2022,12, 4814. https://doi.org/10.3390/ app12104814 Academic Editor: Asterios Bakolas Received: 29 March 2022 Accepted: 6 May 2022 Published: 10 May 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). applied sciences Article Methodology to Evaluate the State of Conservation of Historical Plasterwork and Its Polychrome to Promote Its Conservation Marta Torres-González 1,2,* , Francisco Javier Alejandre 1, Juan Manuel Alducin-Ochoa 1, Ana Isabel Calero-Castillo 3, Francisco Javier Blasco-López 1, Ana Carrasco-Huertas 3 and Vicente Flores-Alés1 1Department of Architectural Construction II, Universidad de Sevilla, Av. Reina Mercedes, 4, 41012 Seville, Spain; [email protected] (F.J.A.); [email protected] (J.M.A.-O.); [email protected] (F.J.B.-L.); [email protected] (V.F.-A.) 2Department of Civil Engineering, Architecture and Georesources, CERIS-IST, University of Lisbon, 1049-001 Lisboa, Portugal 3Paint Department, Universidad de Granada, 18014 Granada, Spain; [email protected] (A.I.C.-C.); [email protected] (A.C.-H.) *Correspondence: mtorr[email protected] Abstract: This work presents a methodology for the assessment of ancient plasterworks based on traditional inspection techniques, such as organoleptic tests and chemical characterization, and also on digital tools, such as photogrammetric surveys, thermography images, and measurement of ambient conditions with thermohygrometers. This method allows not only defining the alterations detected and establishing the state of conservation but also digitalizing the plasterworks to preserve the heritage and replicate the model if necessary (i.e., replacement of a detachment piece), drawing a hypothesis of the original hidden design of the plasterwork, and conducting a chronological study about the polychromies used over time. In some cases, the assessment has shown that the liming and repolychrome interventions to which plasterworks have been subjected and the powdery state of some areas do not ensure the possibility of the complete polychromy restoration, only its preservation by taking conservation and maintenance measures. Keywords: gypsum plasterwork; visual inspection; weathering; preservation; architectural heritage; thermography; XRD; polychromies; virtual reconstruction 1. Introduction In recent decades, a great effort has been made in the field of preservation and conservation of built heritage. This is important not only for protecting the history of the city but also for improving tourism, one of the main sources of income in European countries. In fact, social networks are a sales display for tourism and play an important role in choosing a travel destination [ 1 ]. Thus, destination management organizations (DMOs) together with the governments are promoting preventive conservation and maintenance plans for preserving architectural heritage and restoration interventions. In this sense, methods, policies, and adequate planning for building preservation are being developed to preserve buildings’ cultural heritage, slow down their degradation, maintain adequate functionality, and provide information regarding the priority of intervention [2,3]. The literature provides a wide background on the field of ancient plasterworks, which were traditionally used as decorative coatings and are thus a part of cultural and architectural heritage, and their polychromies and anomalies to identify and prevent the causes of alteration and establish a plan of strategies, maintenance, and preservation according to the evaluation. The Real Alcázar of Seville (RAS) is a palatine complex whose construction began in the 11th century [ 4 ], and it is a great sample of a wide variety of plasterwork corresponding Appl. Sci. 2022,12, 4814. https://doi.org/10.3390/app12104814 https://www.mdpi.com/journal/applsci Appl. Sci. 2022,12, 4814 2 of 18 to different styles. In 1931, it was declared an Asset of Cultural Interest (RI-51-0001067 according to the Law 16/1985, of 25 June, on Spanish Historical Heritage), and some decades later, in 1987, the UNESCO declared the whole complex of the Cathedral, the RAS, and the Archive of the Indies of Seville as a World Heritage Site (http://whc.unesco.org/en/list/383 (accessed on 1 February 2022)). The RAS is constituted by different buildings, including the noticeable Palace of King Pedro I (1356–1366) due to the compositional richness of its decorations, such as plasterworks [5,6], ceramic tiles [7,8], and carpentry [9,10]. Numerous rehabilitations, repairs, and interventions have been undertaken over time, and the plasterworks are the decorative elements that present the highest level of alteration from these interventions, highlighting especially the liming and repolychrome [ 6 ] that implicate the distortion and loss of relief and original color. The Courtyard of the Maidens, located in the Palace of King Pedro I, is one of the most representative spaces of the RAS, housing approximately 30% of the plasterwork of the ground floor. These plasterworks are located in partially covered outdoor areas, which is why they are subject to more severe conditions that accelerate their deterioration; therefore, they are in the worst state of conservation, having undergone numerous interventions throughout the history of the RAS [11,12]. This work focuses on digitalizing and evaluating the state of conservation of one of the most relevant elements as a representative of the whole, the plasterwork located on the spandrel (a spandrel is a space between two arches or between an arch and a rectangular enclosure) that decorates the entrance door to the Hall of the Ceiling of Carlos V (hereinafter, STCV) [ 13 ]. It is worth highlighting the convoluted design of the spandrel (Figure 1) [ 13 , 14 ], with a geometric arrangement and decorative motifs similar to those present in the Hall of Toledans, taking into account that the plasterworks are both contemporary and made by Toledan alarifes (bricklayers and other craftsmen working on decorative plasterworks) in the 14th century [14,15]. Appl. Sci. 2022, 12, x FOR PEER REVIEW 3 of 20 Figure 1. (A) Ground floor of the Palace of King Pedro I showing the location of the main courtyard and the entrance to the STCV. (B) Current state of the plasterwork of the entrance to the STCV. Figure 2. Images of the floral pattern on the background (authors’ own images). The vast majority of studies on ancient gypsum were focused on determining its physical, mechanical, mineralogical, and microstructural characterization by sampling and analyzing [17–23] or on the proposal of methodologies comprising the survey, evaluation, and inspection of the conservation status of a building before the repair, replacement, and rehabilitation of gypsum plasterworks [24–27]. In recent years, the study of plasterwork has been implemented from the approach of the prescription of useful life [28] and the study of the durability of the material. Detailed knowledge of traditional execution techniques [29], the history of the building [12,30,31], and the characterization of the plasterwork and its polychromies is essential to prevent the causes of alteration and establish maintenance and preventive conservation plans [32]. It is also relevant when addressing any restoration intervention to choose materials that are compatible with the original ones [33] and/or distinguishable [34], as well as to determine the most suitable cleaning, fixing, and replacement techniques [35–37]. Thus, the study of plasterwork has been recently implemented from the approach of the prescription of useful life [28] and the study of the durability of the material. Figure 1. ( A ) Ground floor of the Palace of King Pedro I showing the location of the main courtyard and the entrance to the STCV. (B) Current state of the plasterwork of the entrance to the STCV. The space of the spandrel is quite complex, presenting great geometric and compositional peculiarities, composed of a carved lower plasterwork [ 16 ] with vegetal motifs on a background of ataurique to which an exterior lattice is superimposed with a meticulous Appl. Sci. 2022,12, 4814 3 of 18 decoration of thin minisebka and losange, and a deep draft that alludes to the caliphal decoration (Figure 2). To date, it is unknown why the initial carved geometry is almost hidden, and it is also unknown if both plasterworks were executed together or if they are from different eras. Appl. Sci. 2022, 12, x FOR PEER REVIEW 3 of 20 Figure 1. (A) Ground floor of the Palace of King Pedro I showing the location of the main courtyard and the entrance to the STCV. (B) Current state of the plasterwork of the entrance to the STCV. Figure 2. Images of the floral pattern on the background (authors’ own images). The vast majority of studies on ancient gypsum were focused on determining its physical, mechanical, mineralogical, and microstructural characterization by sampling and analyzing [17–23] or on the proposal of methodologies comprising the survey, evaluation, and inspection of the conservation status of a building before the repair, replacement, and rehabilitation of gypsum plasterworks [24–27]. In recent years, the study of plasterwork has been implemented from the approach of the prescription of useful life [28] and the study of the durability of the material. Detailed knowledge of traditional execution techniques [29], the history of the building [12,30,31], and the characterization of the plasterwork and its polychromies is essential to prevent the causes of alteration and establish maintenance and preventive conservation plans [32]. It is also relevant when addressing any restoration intervention to choose materials that are compatible with the original ones [33] and/or distinguishable [34], as well as to determine the most suitable cleaning, fixing, and replacement techniques [35–37]. Thus, the study of plasterwork has been recently implemented from the approach of the prescription of useful life [28] and the study of the durability of the material. Figure 2. Images of the floral pattern on the background (authors’ own images). The vast majority of studies on ancient gypsum were focused on determining its physical, mechanical, mineralogical, and microstructural characterization by sampling and analyzing [17–23] or on the proposal of methodologies comprising the survey, evaluation, and inspection of the conservation status of a building before the repair, replacement, and rehabilitation of gypsum plasterworks [ 24 – 27 ]. In recent years, the study of plasterwork has been implemented from the approach of the prescription of useful life [ 28 ] and the study of the durability of the material. Detailed knowledge of traditional execution techniques [ 29 ], the history of the building [ 12 , 30 , 31 ], and the characterization of the plasterwork and its polychromies is essential to prevent the causes of alteration and establish maintenance and preventive conservation plans [ 32 ]. It is also relevant when addressing any restoration intervention to choose materials that are compatible with the original ones [ 33 ] and/or distinguishable [ 34 ], as well as to determine the most suitable cleaning, fixing, and replacement techniques [ 35 – 37 ]. Thus, the study of plasterwork has been recently implemented from the approach of the prescription of useful life [28] and the study of the durability of the material. Sometimes, the scarce and imprecise documentation about the plasterwork execution and the multiple interventions carried out over time makes the evaluation difficult, and in this kind of work, it is necessary to adopt adequate and regular maintenance strategies for plasterwork preservation. In the specific case of the RAS, characterization work has been carried out on plasterworks [ 5 , 38 – 41 ] and their polychromies [ 42 – 44 ], restoration work has been performed on the plasterworks [ 37 , 45 – 48 ], specific study methodologies have been implemented through nondestructive testing (NDT) [ 49 – 52 ], and a model to predict the useful life of the plasterworks was developed [ 28 ] to obtain an extensive database that will support future intervention works in the heritage. 2. Research Aim and Novelty The main objective was to establish a methodology to characterize the plasterwork and polychrome and to determine the state of conservation of historical plasterwork. It is organized in the following stages: (i) elaboration of planimetry, photogrammetry, and 3D modeling, (ii) visual inspection (e.g., fissures, cavities, stains) and diagnosis by NDT (thermography and rebar locator for nails), (iii) mapping of anomalies, (iv) characterization of plasterwork and its polychromies using various instrumental analysis techniques (XRD, stereoscopic and optical microscopies, and electronic SEM-EDAX) to determine their composition and identification of historical and modern pigments to establish a hypothesis of the evolution of polychromy from the 14th century to the present day, and (v) recording the ambient conditions (i.e., relative humidity and temperature) during a long-term period. In this regard, the novelty of this paper is the combination of all the techniques available to date, namely traditional organoleptic inspection and scientific tests, with new techniques Appl. Sci. 2022,12, 4814 4 of 18 developed for the understanding of the architectural heritage and the assessment of the state of conservation. This work demonstrates the possibilities of a multidisciplinary study and highlights the need to obtain results and contrast them through the application of different techniques promoting information feedback. This fact is relevant for obtaining the best general analysis of the case study to establish a maintenance plan or guidelines for restoration. An additional objective was to obtain a digital model of the spandrel to study the geometry and the current state of conservation and generate a database linked to this model that contributes to establishing the best conservation and maintenance measures applicable to other similar architectural elements. 3. Material and Methods According to this case study, the methodology followed in this work comprised these stages: (i) characterization of the plasterwork and its polychromies, (ii) planimetric survey and 3D model, (iii) visual inspection and NDT, (iv) damage mapping, and (v) ambient condition monitoring. 3.1. Characterization of Plasterwork and Its Polychromies Identifying the mineral phases of the plasterwork allows the determination of its general composition, as well as the presence of impurities and anomalous phases, which will allow obtaining more details about the elaboration process [ 19 ]. A PANalytical X ’Pert Pro diffractometer with X ’Celerator solid-state linear detector—whose potential radiation is CuK α 45 Kv and intensity is 40 mA—has been used. The mineralogical characterization by X-ray diffraction (XRD) was performed using a continuous sweep between 3 ◦ and 60 ◦ of 2 θ , 20 s of measurement time in each step, and a sweep speed of 3 ◦ of 2 θ per minute. Xpowder software was used for the data processing and identification of compounds. For mineralogical characterization, two microsamples of the spandrel were taken from the floral decoration carved in the background and from the external network. On the other hand, five microsamples of the most representative polychromies were taken by applying the following instrumental techniques: • Stereoscopic microscopy. It is the first analysis carried out on the samples and serves to select those that can provide more information. It does not require specific preparation, and it allows a preliminary analysis in which the existing strata, as well as their characteristics (grain size of some pigments, insulating layers, metal sheets, or the morphology of the support), can be identified. A Nikon smz 1000 stereoscopic microscope with DS-U3 Digital integrated camera was used. • Optical microscopy. Thin stratigraphic sections were prepared by impregnation with low-pressure methacrylate resin, then cut with a diamond disc, and finally polished to study stratigraphy under the LEICA DM 750P microscope. • Scanning electron microscopy (SEM-EDX). A Zeiss SUPRA 40 VP High Resolution Variable Pressure Scanning Electron Microscope (FESEM) equipped with an X-Ray Dispersive Energy (EDX) microanalysis system with an X-Max 50 mm large surface detector was used. 3.2. Planimetric Survey and 3D Model The methods proposed by Cotrim et al. [ 24 ] and Gleeson [ 25 ] for the survey and inspection of plasterwork were followed in the first stage due to their quick and low-cost process for establishing the degradation condition of the elements under analysis [ 53 ]. A historiographic study was carried out to put the evolution of the plasterwork in context and to find information about the way of execution, restorations, and interventions performed in the spandrels. A thorough visual inspection of the area was carried out, and the necessary graphic and photographic documentation was obtained to obtain a detailed planimetric survey [54]. The photogrammetric model of the spandrel was obtained from the point cloud thanks to the Agisoft METASHAPE Professional software v.1.7.1. Subsequently, 3D modeling Appl. Sci. 2022,12, 4814 5 of 18 and sculpting of the floral geometries were carried out. These processes, along with the final rendering, were performed in Blender v.2.93, while the 3D texturing was performed in Adobe Substance Painter v.7.4.1. The photogrammetry model corresponds to the left spandrel because it is the one that preserves the lower relief visible on most of the surface, which allowed obtaining the information necessary to carry out the planimetry of this area at high resolution. This survey included a hypothetical recreation of the pattern that is currently hidden under the sebka network. Specific software packages were used for this stage: ASRIX for the restitution of the images and AUTOCAD 2020 and PHOTOSHOP CC2015 for the planimetric survey and mapping work. 3.3. Visual Inspection and NDT A high-performance thermal FLIR T840 camera, with a thermal sensitivity of <30 mK at 30 ◦ C (42 ◦ lens) and a resolution of 928 × 696 pixels, and the Flir Tools and ThermaCAM Researcher Pro-2.10 software packages were used to obtain thermographic images, without the help of lamps or luminic perturbation systems, that complement the visually obtained data. This technique allows us to detect discontinuities, possible nonvisible detachments in coatings, and the presence of moisture by representing apparent surface temperatures [ 55 – 58 ]. The detection of cavities was carried out by striking with a rubber hammer and acoustic evaluation to delimit the affected areas [59]. Original iron nails are common in this type of decoration, as they were probably placed during execution or added in later periods to solve fixing problems [ 60 ]. A metal detector model ETI-H0385 was used to locate them. This instrument is very useful for detecting the location and depth, which allows one to evaluate possible future damage caused by corrosion, which generates expansive oxides [ 61 ] and can trigger the detachment of fragments of plasterwork. In this way, not only the visible oxidations, such as a reddish patina, are studied, but also areas susceptible to future deterioration due to the oxidation of the metal inside the plaster are documented. NDT applied in other plasterwork studies, such as the measurement of hardness or surface humidity [ 49 ] and the realization of readings with the georadar [ 51 ], could not be carried out due to the high relief presented by the spandrel and the difficult geometry. Additionally, an exhaustive visual inspection of the polychromies was carried out to establish the quality of decoration execution and the current state of conservation, for which specific solubility tests were carried out by rotating a swab impregnated with distilled water to verify the stability of the polychromy [62,63]. 3.4. Damage Mapping Before any intervention, it is necessary to list the alterations present in the elements and draw them in a detailed planimetry. This damage mapping consists of indicating the main alterations and visible damage or the data obtained by the NDT on an image or a plan [59]. This will make it possible to characterize the state of conservation and establish the severity of plasterwork degradation, as well as to determine the criteria and priorities to be followed in the maintenance or restoration intervention. 3.5. Ambient Conditions For the determination of temperature and relative humidity, an EL-USB 2 LCD thermohygrometer model with two registration channels was used. In this case, the measurements were carried out over a period of one year, from 20 February 2020 to 20 February 2021, at intervals of one hour uninterrupted with a data logger placed in the southeast gallery of the Courtyard of the Maidens at a height of 3.00 m. For the statistical study and analysis of environmental records, the indications presented by Torres-González et al. (2021) were followed [52]. Appl. Sci. 2022,12, 4814 6 of 18 4. Results and Discussion 4.1. Characterization of Plasterwork and Its Polychromies 4.1.1. Plasterwork Figure 3shows the diffractogram of sample (i) of the latticework and sample (ii) belonging to the floral background, whose locations are indicated in Figure 1[6]. Appl. Sci. 2022, 12, x FOR PEER REVIEW 7 of 20 Figure 3. Diffractogram of samples (i) and (ii): gypsum (GYP); calcite (CAL); quartz (QTZ); anhydrite (A). 4.1.2. Identification of Pigments After visual inspection, the polychromies have been classified into two groups: historical polychromies that could be original and modern polychromies that would correspond to repaints executed in the 19th century as indicated by the chronologies of the industrial pigments identified in Table 1 and Figure 4. Table 1. Summary of the results obtained through microscopy analysis. Sample Identification Images Obtained under Optical and Stereoscopic Microscopes Identified Materials PD-I Fine gold on mixtion. Metallic layer of very pure fine gold, appearing in minimal proportions of silver in some spectra on the basis of chromium yellow and lead. This layer of color appears on a 30 μm thick layer of lime. PD-II Emerald green polychromy identified by the characteristic spherulitic forms, applied on the whitewashed layer. Figure 3. Diffractogram of samples ( i ) and ( ii ): gypsum (GYP); calcite (CAL); quartz (QTZ); anhydrite (A). The presence of gypsum as the major mineral is attributable to the hydration of the hemihydrate used for the execution of the plasterwork. Calcite has its origin in the liming carried out in 1805 and 1816 [ 64 ]. The identified quartz and anhydrite at the trace level indicate a careful selection of the raw material, the gypsum stone, and the correct dehydration of it. Thus, it can be concluded that the plasterwork was executed with gypsum pastes with a high degree of purity [ 65 ], corroborating the results obtained in other plasterworks previously analyzed from the RAS [ 5 ]. These previous works determined that the original pastes of the plasterwork of the Courtyard of the Maidens were composed mainly of gypsum dihydrate, distinguishing other mineral phases in a minority proportion [6,39]. 4.1.2. Identification of Pigments After visual inspection, the polychromies have been classified into two groups: historical polychromies that could be original and modern polychromies that would correspond to repaints executed in the 19th century as indicated by the chronologies of the industrial pigments identified in Table 1and Figure 4. Appl. Sci. 2022,12, 4814 7 of 18 Table 1. Summary of the results obtained through microscopy analysis. Sample Identification Images Obtained under Optical and Stereoscopic Microscopes Identified Materials PD-I Appl. Sci. 2022, 12, x FOR PEER REVIEW 6 of 19 as a reddish patina, are studied, but also areas susceptible to future deterioration due to the oxidation of the metal inside the plaster are documented. NDT applied in other plasterwork studies, such as the measurement of hardness or surface humidity [49] and the realization of readings with the georadar [51], could not be carried out due to the high relief presented by the spandrel and the difficult geometry. Additionally, an exhaustive visual inspection of the polychromies was carried out to establish the quality of decoration execution and the current state of conservation, for which specific solubility tests were carried out by rotating a swab impregnated with distilled water to verify the stability of the polychromy [62,63]. 3.4. Damage Mapping Before any intervention, it is necessary to list the alterations present in the elements and draw them in a detailed planimetry. This damage mapping consists of indicating the main alterations and visible damage or the data obtained by the NDT on an image or a plan [59]. This will make it possible to characterize the state of conservation and establish the severity of plasterwork degradation, as well as to determine the criteria and priorities to be followed in the maintenance or restoration intervention. 3.5. Ambient Conditions For the determination of temperature and relative humidity, an EL-USB 2 LCD thermohygrometer model with two registration channels was used. In this case, the measurements were carried out over a period of one year, from 20 February 2020 to 20/02/2021, at intervals of one hour uninterrupted with a data logger placed in the southeast gallery of the Courtyard of the Maidens at a height of 3.00 m. For the statistical study and analysis of environmental records, the indications presented by TorresGonzález et al. (2021) were followed [52]. Table 1. Summary of the results obtained through microscopy analysis. Sample Identification Images Obtained under Optical and Stereoscopic Microscopes Identified Materials PD-I Fine gold on mixtion. Metallic layer of very pure fine gold, appearing in minimal proportions of silver in some spectra on the basis of chromium yellow and lead. This layer of color appears on a 30 μm thick layer of lime. PD-II Emerald green polychromy identified by the characteristic spherulitic forms, applied on the whitewashed layer. PD-III 20 μm thick emerald green polychromy on a 30 μm thick layer of red earth and cinnabar. Fine gold on mixtion. Metallic layer of very pure fine gold, appearing in minimal proportions of silver in some spectra on the basis of chromium yellow and lead. This layer of color appears on a 30 µm thick layer of lime. PD-II Appl. Sci. 2022, 12, x FOR PEER REVIEW 6 of 19 as a reddish patina, are studied, but also areas susceptible to future deterioration due to the oxidation of the metal inside the plaster are documented. NDT applied in other plasterwork studies, such as the measurement of hardness or surface humidity [49] and the realization of readings with the georadar [51], could not be carried out due to the high relief presented by the spandrel and the difficult geometry. Additionally, an exhaustive visual inspection of the polychromies was carried out to establish the quality of decoration execution and the current state of conservation, for which specific solubility tests were carried out by rotating a swab impregnated with distilled water to verify the stability of the polychromy [62,63]. 3.4. Damage Mapping Before any intervention, it is necessary to list the alterations present in the elements and draw them in a detailed planimetry. This damage mapping consists of indicating the main alterations and visible damage or the data obtained by the NDT on an image or a plan [59]. This will make it possible to characterize the state of conservation and establish the severity of plasterwork degradation, as well as to determine the criteria and priorities to be followed in the maintenance or restoration intervention. 3.5. Ambient Conditions For the determination of temperature and relative humidity, an EL-USB 2 LCD thermohygrometer model with two registration channels was used. In this case, the measurements were carried out over a period of one year, from 20 February 2020 to 20/02/2021, at intervals of one hour uninterrupted with a data logger placed in the southeast gallery of the Courtyard of the Maidens at a height of 3.00 m. For the statistical study and analysis of environmental records, the indications presented by TorresGonzález et al. (2021) were followed [52]. Table 1. Summary of the results obtained through microscopy analysis. Sample Identification Images Obtained under Optical and Stereoscopic Microscopes Identified Materials PD-I Fine gold on mixtion. Metallic layer of very pure fine gold, appearing in minimal proportions of silver in some spectra on the basis of chromium yellow and lead. This layer of color appears on a 30 μm thick layer of lime. PD-II Emerald green polychromy identified by the characteristic spherulitic forms, applied on the whitewashed layer. PD-III 20 μm thick emerald green polychromy on a 30 μm thick layer of red earth and cinnabar. Emerald green polychromy identified by the characteristic spherulitic forms, applied on the whitewashed layer. PD-III Appl. Sci. 2022, 12, x FOR PEER REVIEW 7 of 19 PD-IV and PD-V Artificial ultramarine blue on a layer of natural azurite. Figure 4. Polychromies present in the spandrel: golden layer (PD-I), emerald green (PD-II), artificial ultramarine blue (PD-III), earth red (PD-IV), and azurite (PD-V). Figures 5 and 6 correspond to the SEM-EDX images of the selected samples PD-I and PD-II. Table 2 shows the results of the elemental chemical composition, and Figures 7 and 8 show the XRD diffractograms of the selected samples PD-III and PD-V. 20 µm thick emerald green polychromy on a 30 µm thick layer of red earth and cinnabar. PD-IV and PD-V Appl. Sci. 2022, 12, x FOR PEER REVIEW 7 of 19 PD-IV and PD-V Artificial ultramarine blue on a layer of natural azurite. Figure 4. Polychromies present in the spandrel: golden layer (PD-I), emerald green (PD-II), artificial ultramarine blue (PD-III), earth red (PD-IV), and azurite (PD-V). Figures 5 and 6 correspond to the SEM-EDX images of the selected samples PD-I and PD-II. Table 2 shows the results of the elemental chemical composition, and Figures 7 and 8 show the XRD diffractograms of the selected samples PD-III and PD-V. Artificial ultramarine blue on a layer of natural azurite. Figures 5and 6correspond to the SEM-EDX images of the selected samples PD-I and PD-II. Table 2shows the results of the elemental chemical composition, and Figures 7and 8 show the XRD diffractograms of the selected samples PD-III and PD-V. Modern Polychromies PD-I. Fine gold in yellow of chromium and lead: Chromium yellow is a synthetic mineral pigment used throughout the 19th century [ 66 ], after the construction of the ground floor of the Palace of King Pedro I (RAS), identified in preliminary investigations as executed between the years 1805 and 1816 [64]. PD-II. Emerald green (copper acetoarsenite—Cu 3 As 2 O 3 Cu(C 2 H 3 O 2 ) 2 ): The presence of this pigment in the PD-II sample, characteristic both for the spherulitic forms and for the presence of arsenic (As) and copper (Cu) in the composition, also provides a very specific chronology to this study, because in Spain it was used mainly between the last third of the 18th century and the beginning of the 19th century [66–68]. PD-IV. Artificial ultramarine blue: X-ray diffraction analysis of PD-5 and PD-6 samples and optical microscopy of the surface or outer layers verified that the pigment used in both cases was artificial ultramarine due to the presence of Na, Al, Si, O, and S. This pigment also indicates a very specific chronology since it was used since the early 19th century [ 68 ]. Appl. Sci. 2022,12, 4814 8 of 18 Below this layer, in both cases a copper carbonate is identified, typical of a natural azurite that probably corresponds to the original polychromy [69]. Appl. Sci. 2022, 12, x FOR PEER REVIEW 8 of 20 PD-III 20 μm thick emerald green polychromy on a 30 μm thick layer of red earth and cinnabar. PD-IV and PD-V Artificial ultramarine blue on a layer of natural azurite. Figure 4. Polychromies present in the spandrel: golden layer (PD-I), emerald green (PD-II), artificial ultramarine blue (PD-III), earth red (PD-IV), and azurite (PD-V). Figures 5 and 6 correspond to the SEM-EDX images of the selected samples PD-I and PD-II. Table 2 shows the results of the elemental chemical composition, and Figures 7 and 8 show the XRD diffractograms of the selected samples PD-III and PD-V. Figure 5. SEM images of PD-I taken in backscattered electron mode (BSE) (left) and secondary electron mode (SE). Numbers 1 to 10 correspond to points analysis whose results are in Table 2. Figure 4. Polychromies present in the spandrel: golden layer (PD-I), emerald green (PD-II), artificial ultramarine blue (PD-III), earth red (PD-IV), and azurite (PD-V). Appl. Sci. 2022, 12, x FOR PEER REVIEW 8 of 20 PD-III 20 μm thick emerald green polychromy on a 30 μm thick layer of red earth and cinnabar. PD-IV and PD-V Artificial ultramarine blue on a layer of natural azurite. Figure 4. Polychromies present in the spandrel: golden layer (PD-I), emerald green (PD-II), artificial ultramarine blue (PD-III), earth red (PD-IV), and azurite (PD-V). Figures 5 and 6 correspond to the SEM-EDX images of the selected samples PD-I and PD-II. Table 2 shows the results of the elemental chemical composition, and Figures 7 and 8 show the XRD diffractograms of the selected samples PD-III and PD-V. Figure 5. SEM images of PD-I taken in backscattered electron mode (BSE) (left) and secondary electron mode (SE). Numbers 1 to 10 correspond to points analysis whose results are in Table 2. Figure 5. SEM images of PD-I taken in backscattered electron mode (BSE) (left) and secondary electron mode (SE). Numbers 1 to 10 correspond to points analysis whose results are in Table 2. Historical Polychromies PD-III. Red earth and cinnabar: The PD-III sample has a red layer composed of a mixture of red earth and cinnabar/vermilion [70]. PD-V. Natural azurite (copper carbonate—CuCO 3 ): This pigment appears in the lower stratum of the samples, a result that coincides with previous research about polychromies on the plasterwork of the RAS [6]. The modern polychromies were probably made after 1816 and perhaps between the years 1843 and 1858, a period of time in which an attempt to clean the liming carried out in the years 1805 and 1816 was carried out by intervening in the four galleries that surround the Courtyard of the Maidens [ 64 , 71 ]; however, when they did not achieve a uniform finish, they were repainted with emerald green pigments giving a general color finish to the plasterwork. This fact can also be seen in the low delicacy of the intervention, without following the lines or contours of the plasterwork and painting the background in all its Appl. Sci. 2022,12, 4814 9 of 18 extension, in this case, using emerald green. On the other hand, the golden layers were probably created between 1854 and 1857 [72–74]. Taking into account the results obtained, a hypothesis of the chromatic evolution has been made, and the levels of intervention established are shown in Figure 9. Appl. Sci. 2022, 12, x FOR PEER REVIEW 9 of 20 Figure 6. SEM images of PD-II taken in backscattered electron mode (BSE) (left) and emerald green polychromy identified by the characteristic spherulitic forms. Numbers 1 to 6 correspond to points analysis whose results are in Table 2. Table 2. Chemical composition of PD-I and PD-II determined by SEM-EDX. Sample Strata Elements PD-I Whitewash Sp.1—Al, Si, S, Ca Sp.2—Al, Ca Sp.2′—Al, Si, K, Ca Sp.3—Al, Si, S, Cl, K, Ca Sp.3′—Mg, Al, Si, S, Cl, K, Ca, Ti, Fe Sp.4—Al, Si, S, Ca Gold base. Chrome and lead yellow Sp.5—Mg, Al, Si, S, Pb, Ca, Cr Sp.6—Mg, Al, Si, S, Pb, Ca, Cr Sp.7—Mg, Al, Si, S, Pb Sp.10—Mg, Al, Si, P, Pb, K, Ca, Fe Gold leaf Sp.8—Au, Ag, Ca Sp. 9—Au, Ca PD-II Whitewash Sp.1—Al, S, Ca Sp. 2—Mg, Al, S, Ca Sp.3—Fe, Mg, Al, Si, P, S, K, Ca Emerald green Sp.4—Ba, Fe, As, Al, Si, P, S, Cl, Ca, Cu Sp.5—Cu, As, Cl, Ca Sp.6—Cu, As, Cl, Ca Figure 6. SEM images of PD-II taken in backscattered electron mode (BSE) (left) and emerald green polychromy identified by the characteristic spherulitic forms. Numbers 1 to 6 correspond to points analysis whose results are in Table 2. Table 2. Chemical composition of PD-I and PD-II determined by SEM-EDX. Sample Strata Elements PD-I Whitewash Sp.1—Al, Si, S, Ca Sp.2—Al, Ca Sp.20—Al, Si, K, Ca Sp.3—Al, Si, S, Cl, K, Ca Sp.30—Mg, Al, Si, S, Cl, K, Ca, Ti, Fe Sp.4—Al, Si, S, Ca Gold base. Chrome and lead yellow Sp.5—Mg, Al, Si, S, Pb, Ca, Cr Sp.6—Mg, Al, Si, S, Pb, Ca, Cr Sp.7—Mg, Al, Si, S, Pb Sp.10—Mg, Al, Si, P, Pb, K, Ca, Fe Gold leaf Sp.8—Au, Ag, Ca Sp. 9—Au, Ca PD-II Whitewash Sp.1—Al, S, Ca Sp. 2—Mg, Al, S, Ca Sp.3—Fe, Mg, Al, Si, P, S, K, Ca Emerald green Sp.4—Ba, Fe, As, Al, Si, P, S, Cl, Ca, Cu Sp.5—Cu, As, Cl, Ca Sp.6—Cu, As, Cl, Ca 4.2. Planimetric Survey and 3D Model The graphic survey has made it possible to distinguish the elements incorporated in the background decoration, in spite of the overlapping latticework that draws rhombuses such as sebka and horseshoe arches, respecting the law of the bow and maintaining a similar thickness in the whole [ 76 ]. The complexity of the latticework, the low thickness of its partitions, and the depth of the same can be indicative of a plasterwork executed by carving [16] (Figure 10). Appl. Sci. 2022,12, 4814 16 of 18 References 1. Kim, S.B.; Kim, D.Y.; Wise, K. The effect of searching and surfing on recognition of destination images on Facebook pages. Comput. Human Behav. 2014,30, 813–823. [CrossRef] 2. Prieto, A.J.; Macías-Bernal, J.M.; Chávez, M.J.; Alejandre-Sánchez, F.J. Fuzzy Modeling of the Functional Service Life of Architectural Heritage Buildings. J. Perform. Constr. Facil. 2017,31, 04017041. [CrossRef] 3. Silva, A.; de Brito, J.; Lima Gaspar, P. 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