Metallic and Translucent Decorative Layers: Analytical and Historical Insights from the Medieval Sculptural Complex of the Refectory of San Salvador de Oña (Burgos, Spain)

Abstract

The Monastery of San Salvador de Oña (Burgos) is a Benedictine site that has undergone substantial modifications since its foundation in the 11th century and preserves a significant corpus of Medieval, Renaissance, and Baroque artistic remains. Among these, the refectory stands out as a particularly distinctive ensemble, exhibiting sculptural influences from the Burgundy region and serving as a notable example in terms of structure, craftsmanship, and decoration. Material characterization analyses of this ensemble have not only identified the range of pigments present but also documented metallic materials and applied decorative elements, providing the basis for a proposed chronological framework for the various pictorial strata and stages. A detailed examination of the metallic materials and their overlaying layers has facilitated a comprehensive analysis focused on materiality, manufacturing techniques, and methods of application, while also situating the decoration within its historical, artistic, and cultural context.

1. Introduction

Until the late 16th century, the presence of polychromy and metals in sculptural works—both in stone and wood—as well as in architectural elements, liturgical furniture, and luxury objects, was a widely disseminated decorative practice. During the Medieval period, this was not considered a mere aesthetic addition but rather a culminating phase in the creation of these works, conferring uniqueness, visual richness, and significant artistic prestige.

Chromatic ornamentation on stone involved complex technical interventions, incorporating a range of materials, from mineral pigments to metals such as gold and tin, whose application produced luminous effects and reflective surfaces. Among the most characteristic decorative techniques are colored varnishes, applied as surface layers to intensify the chromatic effect and enhance durability.

Through an interdisciplinary approach, this article aims primarily to explore the metallic materials and chromatic coatings on stone within the architectural ensemble of the refectory of the Monastery of San Salvador de Oña (Burgos, Spain), considering material, historical, technical, and artistic aspects. A first integral reading of the ensemble is proposed from an art-historical perspective, serving as a preliminary step toward establishing a chronological sequence for the different stages of manufacture of its metallic and decorative layers. Secondly, the study seeks to highlight the historical and cultural significance of the materials used in metallic and pictorial decoration, considering their physicochemical and aesthetic properties, as well as the working methods derived from their manufacture and the application techniques employed during the Medieval period.

The study also contributes to technical and pictorial knowledge of Medieval stone sculpture by analysing the remains of metals and colored varnishes in Oña and comparing them with techniques and procedures documented in Medieval treatises and recipes. This allows for the identification of patterns of diffusion and use across the Iberian Peninsula and the assessment of whether these standards were followed as guides or teaching resources in monastic centres. Finally, the research aims to establish artistic, material, and technical analogies between the Monastery of San Salvador de Oña and other sites in Europe and southern Iberia, considering political and geographical relationships between the 12th and 14th centuries, based on the analysis of extant remains.

A mixed-method approach was employed, combining direct examination of visible remains, stylistic analysis of ornamental motifs, critical review of documentary sources, and a series of scientific analyses in specialized laboratories. The collection of microsamples from different points within the ensemble enabled a detailed characterization of metallic and pictorial materials, identifying beaten gold leaf and tin applications with decorative functions.

Analytical techniques included optical microscopy with halogen and ultraviolet lighting, scanning electron microscopy (SEM) with backscattered electron detection (BSE) and energy-dispersive X-ray spectroscopy (EDX), as well as methods such as gas chromatography–mass spectrometry (GC-MS), X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FTIR). These procedures allowed for the thorough identification of both organic and inorganic components involved in the decorative process.

It is important to note the scarcity of technical–artistic sources that accurately describe tin applications and related decorations prior to the development of applied brocades in the 15th century. This limited documentation contrasts with the widespread historical use of the technique, documented in regions ranging from the Eastern Mediterranean and North Africa to Western Europe.

Material analysis reveals careful aesthetic and functional planning at every stage of decoration, employing costly materials such as gold and certain colored varnishes. Overall, the study enhances understanding of the technical, symbolic, and cultural context of the ensemble, highlighting its material complexity and value as a testimony to an artistic tradition largely overlooked in conventional historiography.

Consequently, this article represents a significant advance in the study of metallic and chromatic coatings in European Medieval sculpture, not only due to the large number of preserved strata—all Medieval—but also because of the state of conservation, extensive presence of tin and gold, and technical quality. Moreover, the interdisciplinary methodology, combining historical, technical, and artistic documentation with material characterization, creates a synergy that transcends the limits of each discipline, providing a more comprehensive understanding of not only a single object but also its broader historical, artistic, and cultural context.

2. A Historic–Artistic Approach to the Artwork and Manufacturing

2.1. The Monastery of San Salvador de Oña and the Sculptural Complex of the Refectory

The Monastery of San Salvador de Oña has been studied for decades due to its historical significance, from multiple perspectives, including documentary, economic, and historical approaches, with key publications by Faci Lacasta (1977), García González (1989), Ruiz Gómez (1990), Blasco Martínez (1992), Diago Hernando (2004), and Rojo Díez (2009), among others [1,2,3,4,5,6]. Between 2011 and 2012, in connection with the millennium anniversary of its foundation, several important works were published in two compilations edited by Rafael Sánchez Domingo: Oña. Un milenio, Actas del congreso internacional sobre el Monasterio de Oña, 1011–2011 (2012) and San Salvador de Oña: mil años de historia (2012) [7,8]. These joint publications included numerous studies covering all chronologies and fields, allowing a comprehensive understanding of monastic life in Oña from its geographical context and foundation to the present day. Following earlier traditions, these compendia emphasized architectural and artistic studies, focusing on building renovations and some paintings on canvas. However, until recently, no in-depth study had been undertaken on the multiple Medieval polychromies preserved within the monastery.

Currently, the monastic complex, founded in the 11th century and no longer in active use, comprises various spaces reflecting diverse architectural styles and periods. Nevertheless, architecture from the 11th to the 16th centuries is characterized by coherence among spaces, functionality, and aesthetic ornamentation [9]. To explore the monastery’s most significant remains chronologically, it is essential to examine the construction phases carried out during the Medieval period. This allows for the connection of existing works with the abbots and historical events that shaped the monastery’s material reality. Based on studies by historians, artists, and architects, such as Enrique Herrera Oria (1917), Arzalluz (1950), Juan del Álamo (1950), Antonio de Yepes (1960), Pilar Silva Maroto (1974), Isabel Oceja Gonzalo (1986), Santiago Olmedo Bernal (1987), Ernesto Zaragoza Pascual (1994), Fernando Gutiérrez Baños (2005), José Luis Senra Gabriel y Galán (1992, 1995, 2012), Elena Martín Martínez de Simón (2012), and Cambero Lorenzo (2019) [10,11,12,13,14,15,16,17,18,19,20,21,22,23], it is possible to suggest a sequence of construction stages for the building and ornamentation of the most significant works between the 11th and 16th centuries Cuesta & Pazos [24] (p. 60), and Cuesta [25] (p. 339). Of the six existing phases, only three fall within the period of manufacture and decoration of the refectory ensemble, namely:

• Phase 3: From Abbot García (1032) to Abbot Pedro Pérez (1259–1271).
• Phase 4: From Abbot Pedro García (1272–1287) to Abbot Sancho Díaz (1381–1419).
• Phase 5: From Abbot Pedro de Briviesca (1419–1452) to Abbot Juan Manso (1479–1495).

The carved and polychromed stone ensemble of Oña (Burgos, Spain), which is the focus of this article, is highly distinctive, with no direct structural or decorative parallels within the Iberian Peninsula, but it shows clear artistic affinities with Burgundian and Cluniac art. It was carved in 1141 of the Hispanic era. One of the pieces considered a model or inspiration for these Oña arcades is the choir screen of the stone church at Cluny, preserved in fragments at The Cloisters, Metropolitan Museum of New York, and at the Musée d’Art et d’Archéologie de Cluny. Burgundian influence is evident in multiple decorative motifs, including sawtooth patterns and polylobed archivolts, found in other sites in the Saône-et-Loire department, such as the interior of Cluny, local houses, the Basilica of Paray-le-Monial, and the Cathedral of Saint-Lazare in Autun. Some decorative details, such as rosettes, also appear at major sites like the Puerta de Platerías of the Cathedral of Santiago de Compostela.

Following the discovery of the ensemble embedded in a wall in 1969, it was removed and placed in the north wing of the cloister until 2008, when it was relocated to the chapterhouse.

This architectural relief currently consists of nineteen elements: five arches carved in white dolomitic stone, of which two are partially damaged—one lost and another presumed to be embedded in the southern wall; four square capitals with remnants of impost blocks broken into five fragments; a small frieze with traces of carved inscription; and four thin slabs bearing remnants of paint and Kufic script [9] (p. 10). Concerning the carving stage, the arches exhibit different decorations on their archivolts, which are consistent across the ensemble, progressing from the outer edges toward the lost central arch. Ornamentation increases in complexity from the simple, raised flat archivolts of Arch 1 to the external trilobed leaf archivolts of Arches 2 and 5, contrasting with interior sawtooth archivolts. Finally, Arches 3 and 4 feature contoured, riveted braids on the exterior, with angular archivolts adorned with pinecone motifs. The spandrels of the arches are completed with two types of rosettes: one with a core of thin, stylized leaves, and another combining smooth and grooved leaves, retaining an interior of small, smooth, stylized leaves covered with gold and polychromy in various colors. These rosettes are encircled by a band perforated with evenly spaced punched holes (Figure 1).

Regarding the capitals and impost blocks currently present in the ensemble (Figure 2), it is important to note that only four of the original eight have survived. During the 16th-century renovations and the reduction of the walls in the 19th century, four other elements were lost, along with the possible original bases and columns. The surviving elements accompany the ensemble, following the style of the arches in terms of complexity, with the simplest located at the far left and the most elaborate positioned at the centre of the composition.

Regarding the iconography of the ensemble, it is worth noting only the presence of figurative remnants in the lunettes of the arcades, rendered in a Gothic style that suggests a later dating than the carving of the piece itself, with Gutiérrez Baños dating it to the early 15th century [10]. The figures depicted in the lunettes are partially mutilated at their lower sections and exhibit an advanced state of degradation, preventing precise identification at present. Since its extraction, historiography has catalogued the piece as a representation of the Last Supper due to certain depicted objects, the location of the piece, and its interpretation within the Benedictine order.

The clarification of historical, artistic, and formal issues related to the architectural and sculptural remains of the refectory ensemble, along with the study of material evidence of the polychromy, has allowed the proposal of three distinct decorative phases for this work, dated as follows: (1) 1141–1200; (2) 1332–1360; and (3) 1430–1460. However, among the results obtained from material characterization studies, the most significant finding is the detection of metallic sheets of tin and gold, as well as limited traces of copper varnishes in decorative phase 2.

2.2. Metals

2.2.1. Tin Leaf

Tin is a metallic chemical element extracted from the mineral cassiterite in the form of tin dioxide. It is solid and oxidizable at room temperature, naturally occurring in two allotropic forms: α and β. α-Tin lacks metallic properties and has a powdery, non-metallic gray appearance that remains stable below 13.2 °C, although it is more brittle. β-Tin, by contrast, has a whitish metallic appearance, conducts electricity, and remains stable above 13.2 °C, becoming malleable at higher temperatures. The whitish tin suitable for use as decoration on artistic objects, or β-tin, was already noted by Cennino Cennini in the chapter “XCIC On how to make gilded tin and how to apply fine gold with said gilding”: “Take a board […] well-polished and rub it with tallow. Place white tin on it” [26] (p. 142).

Due to these characteristics, when found as a decorative element in artworks, tin often presents conservation challenges associated with its physicochemical properties, such as oxidation leading to brown tones if not coated with a suitable protective layer. It corrodes easily in the presence of chlorine and hydrochloric acid, is soluble in sulfuric acid, and oxidizes with nitric acid without forming nitrates [27] (p. 4). Furthermore, under prolonged cold conditions, β-tin or white tin transforms into α-tin or gray tin in a phenomenon known as “tin pest,” causing the metal to disintegrate into powder. This transformation occurs only in highly pure tin, while impure metals may undergo this change below 0 °C.

The production of tin requires a complex process, with temperatures approaching 230 °C, although some authors suggest slightly higher temperatures [27] (p. 15), [28]. During manufacture, tin often occurs in alloys with other metals, most commonly copper and lead, either due to an insufficiently refined process or the intentional addition of small amounts of lead to increase material stability [29] (pp. 31–32).

Tin’s high malleability allows it to be made into sheets as thin as 1/5000 inch [27] (p. 1), equivalent to 5.08 µm—thicker than gold or silver leaf. As Nadolny notes in multiple studies, tin’s malleability is lower than that of gold or silver, resulting in visibly thicker sheets, which should therefore be referred to as tin sheets or tin leaves [30,31].

Regarding sources, historical records cite European deposits in locations such as Lusitania, Gallaecia, and the Casiterides islands. While activity at these sites declined during the Roman period, it resumed in the 16th century, and some mines may have supplied workshops at Santiago de Compostela Cathedral, remaining in use through the Middle Ages [32] (p. 280), [33,34].

Studies of tin as a decorative element in contemporary literature have generally focused on applied brocade, leaving other metallic decorations, such as molded applications or coatings, underexplored [35] (p. 410), [36] (pp. 68–69), [37] (p. 136), [38] (p. 91). Innovative study methodologies, such as those by Rodríguez López and Bazeta Gobantes at the University of the Basque Country in 2014, illustrate the diverse types of metallic applications included under the designation of tin brocade in relief, noting its use by European painters since the 8th century [39].

From the 9th century onward, manuscripts such as the Lucca Manuscript and other treatises on artistic technology—Mappae Clavicula [40], Diversarum Artium Schedula by Theophilus [29], Heraclius’ De coloribus et artibus Romanorum, Pier Saint Audemar’s treatise [41], and Cennino Cennini’s Libro dell’Arte [26]—describe the production and use of cut and applied metals as a decorative technique. However, none of these sources provides guidance on the full or partial coating of a stone sculpture with tin prior to decoration with polychromy or colored varnishes.

2.2.2. Gold Leaf and Gilding Techniques

The metal most commonly used for producing metal leaf since Antiquity has been gold, followed by silver and other metals and alloys, depending on the financial means of the patrons. This manufacture is possible due to gold’s physical properties, as it is the softest, most malleable, and ductile metal. It remains unaltered by most chemicals, making it impervious to the passage of time, oxidation, or environmental factors. However, it is soluble and sensitive to certain compounds such as cyanide, mercury, chlorine, bleach, or aqua regia, composed of hydrochloric and nitric acids.

High-quality gold production to obtain thin sheets of the precious metal has been carried out in two ways since Antiquity. The primary method is the beating of pure gold coins from legal tender reserves. Many of the coins used in this process included Muslim dinars or besants, Castilian doblas, Barcelona silver diners, maravedís, or reales. In the Middle Ages, leaf beating was performed manually using a non-stick material derived from ox intestines, producing a large quantity of gold leaf with thicknesses up to 0.006 mm or 6 µm [42] (p. 264), [29] (pp. 29–30).

However, since the Bronze Age, another more complex method for obtaining pure metals has been known and extensively used in the Middle Ages: cupellation [43,44,45]. The cupellation system is based on the principle that precious metals such as gold and silver do not chemically react or oxidize like other metals they may be alloyed with in nature. By melting them at high temperatures, the purer metals remain separated from less stable metals, which form residues or other alloys adhering to the walls of the vessels or cupels. These vessels are made from “porous substances poor in silica, such as bone ash or marl, specially designed to absorb liquid lead oxide as it forms, but not the silver metal” [44] (p. 208), [29] (pp. 96–97).

Gilding, used since Antiquity to cover and beautify surfaces and in the Middle Ages as a method to imitate Eastern goldsmithing, was not treated merely as a color tone, but carried a transcendent meaning. As Doerner notes, “gold alluded to the supernatural, to the compact light of the sun over our earth” [42] (p. 263), often serving as a dogmatic reflection of God’s divine light in cultic and decorative objects. It was in this medieval period, up to the 15th century, that numerous altarpieces and paintings systematically included gold in backgrounds, associating it with divinity.

Within medieval gilding techniques, a particularly characteristic method identified by its binders and specific application process is gilding with mordant.

Gilding with Mordant

Gilding with mordant is a procedure for applying gold leaf to any surface—in this case, previously polychromed stone—achieving durable finishes suitable even for exteriors, though with less shine and laminar metallic appearance than water gilding. First, the support must be isolated using a hydrophobic product such as a pigment-lacquer, linseed oil sealer, or linseed oil varnish bound to the base pigments of the pictorial substrate.

Next, a varnish or mordant called “mixtion” must be applied, which “does not dry by absorption of the ground, but upon contact with the air (oxidation), and in a determined time” [46] (p. 133), following Cennino Cennini’s instructions: “oak the tip in the mordant and apply it […] ensure that the brush is never overloaded […] it is advisable that the strokes be as fine as hairs […] Before continuing, you must wait for a while […]” [26] (p. 188). This mordant or “mixtion” was prepared from a pigment and dried or cooked linseed oil, to which lead or copper pigments were added as siccatives, accelerating drying along with pigments present in the painter’s palette. The cooked mixtion or sisa mixed with these lead siccative pigments created a transparent adhesive liquid, allowing the gold leaf to adhere properly. When the mordant “chirped” or “whistled” when touched, it indicated readiness for gold leaf application. This allowed the mixtion to retain some adhesive strength without the gold leaf sinking into the oil layers or failing to adhere [26] (pp. 188–189).

Powdered Gold

Powdered gold is made by crushing gold leaf with salt and an insoluble liquid medium, such as honey, on a grinding stone. Once the gold leaf is broken into fine flakes by the salt grains, the mixture is washed with water [42] (p. 267), [47] (pp. 131–132). These gold flakes were applied with brushes and bound with mordants based on varnish, dewaxed shellac, Arabic gum, or egg white [26] (p. 199).

2.3. Color-Bearing Materials

2.3.1. Verdigris, Copper Oleates, and Resinates

Throughout history, it is common to find both natural and synthetic materials used as pigments in pictorial layers or as a base for coloring corlas. One of the key pigments for making these corlas is copper-derived compounds, whether verdigris or cardenillo, and copper resinates or oleates.

Copper pigments, verdigris or cardenillo, are among the most complex materials in the art world, known since Antiquity and mentioned in the writings of Theophrastus, Dioscorides [48], Pliny [49], and Vitruvius [50] under the names aerugo or aeruca [51,52]. These terms were used to designate different products formed on the surface of copper, exhibiting blue-green and green hues [53]. The term verdigris is commonly used specifically for copper acetate products, and its manufacture has been described in artistic technology treatises for centuries. Multiple examples of different preparation methods can be found in the Mappae Clavícula, De diversis artibus by Theophilus, De coloribus et artibus romanorum by Heraclius, the manuscripts of Jean le Begue, and the Bolognese Manuscript, also known as Segreti per Colori, in chapters four and seven [41]. All these recipes follow a common procedure, similar to the production of lead white, using liquids with high pH to corrode a metal—in this case, copper—which is stored covered in a container for a variable period. Other substances, such as salt or honey, can be added to these liquids during verdigris production, affecting the chemical reaction and slightly altering the pigment’s tones.

Chemically, verdigris refers to copper acetates with varying compositions, ranging from green to blue tones. Depending on this composition, acetates can be classified as basic, with a pH below 7, or neutral. Basic verdigris forms through the action of acetic acid, water vapor, and air on copper and its alloys, producing up to five types of acetates spanning blue and green shades [54] (p. 10). Neutral verdigris [Cu(CH₃COO)₂·H₂O], formed from the dissolution of basic verdigris with strong acetic acid, is a dark green pigment [26] (p. 100).

The pigment’s low durability relates to a property inherent to verdigris: both the basic and neutral acetates shift in color from blue-green to green during the first month after application, with the type of verdigris and the binder used being decisive for the final color.

These complex copper pigments may also be used in different concentrations with other binding media to create alternative pigments, such as resinates—products formed with resins—or oleates, mixtures of pigments with oils, resulting from combining proteins with these acetates. Considerable debate surrounds copper resinates and related products [55]. As Hermann Kühn notes, “the common name given to transparent green glazes colored by copper salts of resinous acids” [56]. Until the 1960s, the term resinate was often misapplied to any translucent green layer observed under a microscope [54] (p. 11). Subsequent research showed that microscopy alone is insufficient to make this determination, requiring gas chromatography and mass spectrometry (GC-MS) to correctly identify the compounds, as traditional recipes often include additives such as linseed oil, beeswax, resin, or alum. Similar pictorial films to resinates can also be produced by mixing verdigris with drying oils, oil–resin mixtures, or even protein-based media. Švarcová, Hradil, Hradilová, and Čermáková note:

“In mural paintings, copper oleates, resinates, and/or proteinates appear rare, although some samples have been observed, for example, in a medieval mural at the monastery of Kadaň, Czech Republic. In other cases, such as the green parts of Persian oil murals from the Safavid period (1501–1736), it is unclear whether the copper oleates in the transparent green layers were intentionally prepared or formed over time between the copper compounds used as pigments and the binding media”

[54] (p. 11).

2.3.2. Colored Varnishes or Corlas

Colored varnishes, or corlas, are translucent layers made of resins and tinted essences, applied over metallic foils with the purpose of altering their hue without diminishing the characteristic brilliance of these materials. This allows for subtle color modifications, enhanced reflections, or the maintenance of the original metallic sheen while adjusting the hue. Stefanos Kroustallis notes:

“The most frequently used coloured varnish was yellow to imitate gold, applied on silver or tin sheets. The term may also extend to transparent red and green lacquers applied over gold and silver”

[57] (p. 132).

These layers primarily aim to reproduce the appearance of gold or gemstones at a lower cost, typically applied over metals such as silver or tin. Yellow varnishes may include additional tones to modify the underlying metal colour, creating varied tonal ranges enhanced by light reflection.

International terminology for these layers varies, as Luis Ángel de la Fuente Rodríguez explains:

“Different names are used in other languages: in Portugal, barniz Martín; in Italian, meccha; in French, glacis; in English, glazé; in German, lüsterfarben; and in Latin America, barniz chinesco. The terms corla and meccha refer to colored glazes over metallic bases, while glazé, glacis, and lüsterfarben refer to coloured glazes over any pictorial surface, including metallic ones (in these latter languages, a specific term seems absent)”

[58] (p. 265)

Therefore, from this point onward, and given the support under study—the metal sheets—we will adopt the terminology corla, rather than any more general term. To understand the use and finish of corlas, it is necessary to know their composition, beginning with the binder. There is a clear transition in the materials used for gilded corlas in European medieval art, beginning in the 11th and 12th centuries with the use of tin and musivum gold, later replaced by silver in the 13th century, which coexisted with the imitation of gemstones and the emergence of applied brocade.

Not only did the base materials over which corlas were applied evolve, but also the binders and colorants. The binders most commonly used in gilded corlas in Antiquity and the Early Middle Ages were water-based, composed of glues and egg, whereas in the transition to the Late Middle Ages and up to the 15th century, protein-based binders and linseed oil were increasingly employed [58] (p. 272). It is in the late medieval period that oil-based corlas appeared, completing their recipe with resins and colorants to create mixtures adapted to the desired color. These oil corlas were produced using cooked oils, to which colored resins were added hot, along with colorants or pigments in shades ranging from greens to blues, passing through reds and yellows.

The use of linseed oil as a medium for applying corlas to tin sheets is a common element in several medieval treatises, such as the Mappae Clavicula [40] (pp. 36,40), De diversis artibus [29] (pp. 31–32), The Montpellier Liber diversarum arcium [59], and the Libro dell’Arte [26] (p. 142). To this oil, other resins widely used in the Middle Ages were added, such as colophony resin, sandarac, and dragon’s blood [60] (p. 52), [57] (p. 383). Regarding the materials used to provide color to these corlas, both pigments and dyes can be found. Pigments impart color to the corla itself, as they possess stability, inherent covering power, and color depth. In contrast, dyes, which lack these properties, must be applied onto a solid, inert element to ensure their durability, such as alum, generating lake pigments.

For yellow corlas, the most widely used in the Middle Ages, pigments include orpiment [40] (p. 59), referenced in previously cited recipes, saffron [29] (pp. 31–32), [40] (pp. 36,44), buckthorn, liverwort, weld or ancorca, turmeric, as well as cooked oils and other substances already possessing the desired yellow color.

For red corlas, since Antiquity and without interruption, dragon’s blood was employed, later joined in the Late Middle Ages by madder root transformed into cochineal lake imported from the East, along with ivy, Brazilwood subsequently processed into verzino lake, and orchil, whose tones range from red to violet. Blue corlas were produced from plant-based colorants such as indigo, woad (Isatis tinctoria), or glasto, and exceptionally from blueberries, which yield a very characteristic violet juice. These colorants were combined with pigments such as azurite or enamel blue, the former being the most traditionally used in translucent paintings over metal sheets.

Green corlas could be made from either plant-based colorants or copper pigments [26] (p. 142), the most common being copper acetate or verdigris [37] (p. 51), and copper resinates [37] (p. 51), used from the 8th to the 16th centuries, when the fusion was carried out with colophony resin.

3. Materials and Methods

The absence of previous studies on polychromy over stone at Oña, as well as the proposal of a study methodology that integrates a multidisciplinary perspective, has shaped the structure of this article, promoting an interdisciplinary approach from the fields of Art History and Cultural Heritage Conservation.

The study of the materials constituting the stone polychromy and metallic decorations proceeds from the analysis of the work itself toward historical–artistic sources, with subsequent feedback from these sources, in order to achieve comprehensive studies that allow identification of the constituent materials and application techniques used in the manufacture of the works. To deepen this technical expertise and the extensive use of materials present in the remains at Oña, various studies from the discipline of Cultural Heritage Conservation–Restoration have been employed, enabling the identification and characterization of different supports, preparations, pigments, binders, and metals used in the polychromy.

From 2014 to 2022, multiple field campaigns were carried out on this complex, in which a total of forty-five samples were collected to study the pictorial–decorative techniques employed, allowing different hypotheses to be proposed regarding the materials, techniques, and pictorial decoration used. These samples were taken from different locations and polychrome ensembles in the Monastery of San Salvador de Oña, following organoleptic studies under natural light, UV light, and raking light, as well as observations using a Nix^® Mini2 colorimeter (Nix Sensor Ldt, Ontario) and a small USB-connected digital microscope (Bresser GmbH, Gutenbergstr. 2, DE – 46414 Rhede), as well as photographic studies with a Canon Reflex Camera EOS 500D IIS (Canon Spain, FNAC, Madrid, Spain). These preliminary studies, along with measurements of the pieces and photography, allowed the potential sampling areas for the forty-five samples to be identified, establishing the following key criteria:

• The search for materials showing the greatest stratigraphic integrity.
• The presence of a greater sequence of layers.
• The presence of different colors and surface finishes.
• Areas with evident material deterioration.
• Zones with the same color or finish on the same sculptural areas of the corresponding paired arches.

All of this aims to obtain the most complete possible view of the decorative plane of the ensemble, as well as to propose possible decorative sequences throughout the centuries, the distribution and patterns of decoration and painting, and the potential uniqueness of decoration in the mirror arches (2–5 and 3–4).

Of all the samples collected, only twenty-six have been analysed to date by two specialized laboratories in material characterization, which, through various scientific techniques, have provided data on the elemental, physical, stratigraphic, and technical composition of this ensemble.

These samples are coded following a systematized and standardized identification system that has been maintained in all field campaigns. This cataloguing allows identification not only of the artworks and the samples extracted from them, but also of their location within the monastery through a grid system established for the different rooms (Figure 3). This code allows the identification of:

• The main independent spaces (Cloister CL, Chapter House SCP, Refectory RFT)
• Sections of the church are divided into southern and northern sections (TR1/2/3/4-CP1: section 1-4, chapel 1, north; TR1/2/3/4-CP2: section 1-4, chapel 2, south), as well as other church spaces without divisions (Atrium AT; Main Chapel CM; Altar AL)
• The type of the heritage objects (CPT: Capital; F: Frontal; ARC: Arch)
• The nature of the extracted materials (PD: Stone; IMP: Mortars; MATRES: Residual materials; PL: Polychromy)
• The consecutive numbering of the extracted samples (01 to n)

The laboratories selected for analyses during these campaigns were, first, ArteLab S.L., which employed analytical techniques that allowed for detailed reports including data on both the stratigraphic structure of the microsample and specific information on the identified materials (fillers, pigments, binders, metals, etc.) and their state of conservation [61] carried out the following tests on the provided samples: optical microscopy with polarized, incident, and transmitted light; halogen and UV light; Fourier-transform infrared spectroscopy (FTIR); gas chromatography–mass spectrometry (GC-MS); scanning electron microscopy with X-ray energy-dispersive microanalysis (SEM–EDX); micro-RAMAN spectroscopy; and X-ray diffraction (XRD). These analyses and reports were performed by professionals who, due to their expertise in material analysis, were not given precise instructions regarding specific zones or materials to target, which in some cases means that the results, although reliable, were not fully precise or tailored for certain critical points of the study. This occasionally required consulting the CAI as a method to verify specific results, for example, in the case of metal analyses.

Additionally, the services of the CAI of Earth Sciences and Archaeometry at UCM, located in the Faculty of Geological Sciences [62], were employed. The CAI conducted the following tests on the provided samples: optical microscopy and scanning electron microscopy with X-ray energy-dispersive microanalysis (SEM–EDX). The CAI also provided the researcher with the INCA software to interpret the results and analyse the electron microscopy images. Work conducted at this unit was similar to that carried out by ArteLab S.L., though in this case, the researcher had a greater presence. This allowed participation not only in the initial phases of analysis, such as embedding samples in resin, but also in firsthand observation under optical microscopy, selection of magnifications in EDX spectra, and choice of the most appropriate zones for BSE and SEM imaging. This represents an advantage over the ArteLab S.L. data, as it allows for a larger number of measurements, which are extracted and processed individually, with selection of points guided by the researcher’s specific needs. The reports from CAI samples are based on INCA software, enabling post hoc extraction of data and images from the exact areas where measurements were performed. Therefore, CAI results provide a greater quantity of data, which have been processed and interpreted through graphs and weight-percentage data for each measurement, and while not as exhaustive as ArteLab S.L., they have enabled the creation of more complete images and stratigraphies.

All these analyses, complemented by organoleptic and microscopic studies with UV, halogen, raking, and digital light in situ, along with comparison with historical sources, have provided data and images that allowed the determination of the polychrome sequences of the ensemble, the identification of materials, and possible decoration methods.

4. Results

The results provided by the laboratories on the twenty-six analysed samples allowed the identification of metallic sheets in eighteen of them, within this ensemble, revealing the presence of this metallic decoration on 80% of the surface. This was corroborated by in situ studies using organoleptic observations and digital microscopy (

Table 1. Identification of samples and metallic materials present in the Oña refectory ensemble. Source: Author.

Sample	Laboratory	Location	Type of Polychromy by Structure
			P.1	P.2.	P. 3	P. 4
			Tin Foils and Corlas	Tin Applications on Tin Foils and Corlas	Tin and Gold Leaf	Gold Leaf
			-Whitewashed -Corla -Tin -Mordant -Pictorial Layers -Preparation -Pre-Fixing -Pore Filler -Stone	-Polychromy -Tin -Mordant -Polychrome/Corla -Tin -Mordant -Pictorial Layers -Preparation	-Golden leaf -Mordant -Corla -Tin Leaf -Seat Cover -Pictorial Layers -Preparation -Pre-Pairing -Pore Filler -Stone	-Golden Leaf -Mordant -Pictorial Layers -Preparation -Pre-Fixing -Pore Filler -Stone Support
F-PL05	ArteLab S.L.	Arch 5				X
F-PL07	ArteLab S.L.	Arch 5	X
F-PL08	ArteLab S.L.	Arch 5	X
F-PL14	ArteLab S.L.	Arch 1	X
F-PL22	CAI UCM	Arch 1	X
F-PL23	CAI UCM	Arch 4	X
F-PL24	CAI UCM	Arch 1	X
F-PL25	CAI UCM	Arch 4			X
F-PL27	CAI UCM	Arch 3		X
F-PL30	CAI UCM	Arch 1			X
F-PL31	CAI UCM	Arch 1			X
F-PL32	CAI UCM	Arch 1	X
F-PL33	CAI UCM	Arch 3		X
F-PL34	CAI UCM	Arch 1	X
F-PL35	CAI UCM	Arch 1	X
RFT-P23-PL02	ArteLab S.L.	Impost block 1	X
RFT-P25-PL01	ArteLab S.L.	Impost block 3	X
RFT-P29-PL03	ArteLab S.L.	Capital 3	X

).

Three different types of materials associated with the metallic decorations have been identified through these studies: tin, which covers the largest part of the ensemble’s surface; gold, located in selected areas; and finally, copper resinates, material remnants of the corlas applied over the tin sheets.

4.1. Presence of Tin and Tinning

In the Oña refectory ensemble, multiple analyses have determined that tin is distributed over 80% of the total surface (Figure 4), through the material identification of the metal in seventeen samples taken from the five main arches and three of their adjoining pieces (

Table 2. Composition of the microsamples with tin leaves. Source: Author.

Code	Arch	Intake Zone	Layer	Thickness	Composition (%Weight)
MSSO-SCP-F-PL07	5	Blue-gray coloration of the sawtooth archivolt.	5	0–5 µm	Sn, Ca, Pb, Cl, Cu, Fe 1
MSSO-SCP-F-PL08	5	Turquoise blue interstices of the sawtooth archivolt.	4	0–5 µm	Sn, Ca, Pb, Cl, Cu 1
MSSO-SCP-F-PL14	1	Smooth inner archivolt with brown and blue scale patterns.	5	0–10 µm	Sn, Pb, Cl, Cu 1
MSSO-SCP-F-PL22	2	Interstices of the archivolt of the saw-toothed architrave have a turquoise blue coloration.	1	4.83 µm	C (7.43); Cu (4.27); Sn (53.11); O (35.19)
MSSO-SCP-F-PL23	4	Braided archivolt with turquoise blue coloration.	2	5–17 µm	C (6.74); Cl (0.79); Sn (58.77); O (33.80)
MSSO-SCP-F-PL24	2	Outer archivolt of the sawtooths colored blue-grey.	4	5.8–9.2 µm	C (7.43); Sn (53.18); Pb (2.70); O (36.05)
MSSO-SCP-F-PL25	5	Left-hand flanged rosette.	3	17.5 µm	C (5.85); Cl (1.17); As (0.34); Sn (52.02); Pb (10.13); O (30.49)
MSSO-SCP-F-PL27	3	Cut out and polychrome the flower of the third projection of the angled archivolt.	5	5–15 µm	C (13.86); Cu (3.43); Sn (30.16); Pb (6.16); O (46.39)
MSSO-SCP-F-PL30	2	Central zone of the archivolt of sawtooth with superficial gold leaf.	2	12.8–44.3 µm	C (11.76); Cl (0.62); As (0.53); Sn (35.41); Pb (9.86); O (41.81)
MSSO-SCP-F-PL31	2	Right-hand flanged rosette shows small areas of residual tin traces	1	11.6–13.4 µm	C (16.01); Si (0.57); Cl (1.12); Ca (2.24); Fe (6.99); Sn (15.11); Pb (7.14); O (50.82)
MSSO-SCP-F-PL32	1	Intercolumn with brown and blue rhombus design	4	16.2 µm	C (5.54); Cu (2.15); Sn (60.65); O (31.66)
MSSO-SCP-F-PL33	3	Cut out and polychrome flower of the third projection of the angled archivolt.	5	5–13 µm	C (17.46); Sn (25.26); Pb (3.68); O (53.60)
MSSO-SCP-F-PL34	2	Intrados of the arch, covered by vermilion pigment and with some laminae with brownish colorations close to black.	2	28.1 µm	C(5.17); Cl (1.22); Sn (54.94); Pb (9.35); O (29.32)
MSSO-SCP-F-PL35	1	Black color with whitewashing of the exterior archivolt with blue and green duplicate tones.	5	7.26 µm	C (7.39); Sn (57.44); O (35.17)
MSSO-RFP-P23-PL02	Impost block 1	Right side of the crest of Arch 1, brown coloration.	7	0–5 µm	Sn, Ca, Cl, Pb, Si, Al, Cu, Fe 1
MSSO-RFP-P25-PL01	Impost block 3	Left side of the crest of Arch 3. It presents a red superficial coloration.	3	0–15 µm	Sn, Ca, Cl, Pb, Cu, Fe, Si 1
MSSO-RFP-P29-PL03	Capital 3	Left side of the vegetal stems of the capital of Arch 3, shows a turquoise blue coloration.	5	0–5 µm	Sn, Ca, Cl, Pb, Fe, Si, Al 1

¹ In the analyses carried out by ArteLab S.L., the %Weight of the Composition column is not available, as this data was not provided by the laboratory.

).

• Arch 1: Three samples (MSSO-SCP-F-PL14/PL32/PL35) with laminar thicknesses between 0 and 16.2 µm were analysed, showing the presence of tin, lead, copper, carbon, oxygen, and chlorine, the latter associated with metal corrosion.
• Arch 2: Five samples (MSSO-SCP-F-PL22/PL24/PL30/PL31/PL34) were analysed with variable thicknesses between 4.83 and 44.3 µm. Chlorine and lead predominate, playing a key role in metal deterioration. All samples contain a high amount of organic material, likely residues of the binder, reflected in the spectra by the high proportions of carbon and oxygen. Lead is also consistently present, suggesting the possibility of an intentional or incidental alloy with tin, although this remains uncertain due to the metal’s manufacturing and refining process. The presence of chlorine particles indicates advanced contamination from soluble salts and environmental pollutants, contributing to corrosion and degradation [27] (pp. 5,21,266).
• Arch 3: Two samples (MSSO-SCP-F-PL27/PL33) with thicknesses of 5–15 µm show tin, carbon, and oxygen, with noticeable concentrations of copper and chlorine that may affect metal stability.
• Arch 4: One sample (MSSO-SCP-F-PL23) with a thickness of 5–17 µm shows chlorine contamination, likely causing oxidation of the sheet.
• Arch 5: Three analysed samples (MSSO-SCP-F-PL07/PL08/PL25) with thicknesses of 0–15 µm show tin, chlorine, lead, copper, and soil contamination in the underlying layers, possibly from the base layer.
• Capitals and impost blocks: In the capitals and impost blocks (MSSO-RFP-P23-PL02/P25-PL01/P29-PL03), variable thicknesses of 0–15 µm were detected, along with contamination by chlorine, lead, and other elements, indicating open manufacturing processes susceptible to cross-contamination and exposure to environmental agents promoting structural deterioration.

It is noteworthy that a common feature among all analysed samples is the variable thickness of the sheets, ranging widely from 0 to 44.3 µm, although most samples, except those with severe deterioration, show standard values between 0 and 10 µm. Another factor linking most of the samples is the presence of certain metals alongside tin, mainly lead and copper. These elements may occur together with tin due to the refining process, impurities present from extraction, or intentional alloys designed to improve tin’s final properties, as in the case of lead. Finally, the presence of chlorine detected in the metallic layers and much of the remaining strata in the samples analysed by SED-EDX at CAI indicates the formation of soluble salts, resulting from both moisture rise and atmospheric contamination on surface areas.

The application of tin sheets as a metallic finishing decoration is carried out by depositing the sheets onto a layer with a high organic content, mainly oils, which facilitates adhesion without burnishing. The surfaces are decorated with colored varnishes.

Currently, these tin sheets are visible in certain areas of the Oña refectory ensemble through simple organoleptic examinations, where the laminar superposition, application wrinkles, and cuts in the sheets can be observed. Based on the studies conducted, the location of this metal on the surfaces and elements of the ensemble has been identified, which currently shows advanced oxidation in brown tones (Figure 5).

To analyse the tinning of this ensemble, the paired arches were compared from the ends to the centre, as their decoration is similar in structure and composition, as demonstrated by laboratory analyses. Thus, Arch 1 will be studied separately due to its unique paired arch, while Arches 2–5 and 3–4 will be analysed together.

In Arch 1 (Figure 6), the identification of tin in the spandrels and the two archivolts is based on samples PL14, PL32, and PL35. These are complemented by PL01 and PL15, which were examined via digital microscopy, confirming the presence of the same material. Tin presence in the rings, leaves, and petals of the rosettes was identified in situ through visual inspection and with tools such as magnifying lenses and digital microscopes. The only areas where this metal could not be identified are the intrados of the arch and the interstices of the petals and leaves of the rosettes.

Arches 2 and 5 feature tin-based decoration that, like the previous arch, covers nearly the entire element (Figure 7). Eight samples from Arches 2 and 5—PL07, PL08, PL22, PL24, PL25, PL30, PL31, and PL34—have been analysed and confirm the presence of this metal in the architectural decorations. These are complemented by sample PL20, studied via digital microscopy, where tin can be observed between layers of coloration. The presence of tin in the spandrels, vegetal friezes, and the bands framing them has been verified through visual examination and photographic contrast studies. The only areas lacking tin, identified in situ, are the gaps between the archivolts of the vegetal frieze. In Arch 2, a metal layer was also found on the intrados, beneath a layer of vermilion belonging to a later polychromy, although it is unknown whether this application extends to the rest of the arches or the entire intrados of this module.

Arches 3–4 are partially sectioned by their interior parts, showing an extensive distribution of tin, absent only in the intrados of the structure, the inner parts of the interlaced archivolts, and the pinecone motifs located within (Figure 8). The samples taken from these arches are PL27, PL33, and PL23, all from the interlaced archivolt. The area from which sample PL27 was taken contains trilobed flowers with slight relief, visible under raking light. Analysis of this sample at the CAI did not yield conclusive results regarding the binders or organic materials in the ground layer beneath the tin leaf. Therefore, one could hypothesize that these decorative elements might represent an application of tin leaf with volume or a primitive applied brocade. However, the inconclusive results regarding the possible binders do not allow confirmation of this hypothesis.

The braided archivolt, as well as the spandrel and rosette, also show tin presence, identified through organoleptic and microscopic examinations of six additional samples. In these, the tin layers appear thicker than the gold layers due to existing corrosion, which is confirmed by the brownish tones.

After all organoleptic and visual examinations, it can be observed that the tin currently present in Oña exhibits certain characteristics consistent across all samples. Its laminar presence is fragile, fracturing easily and always showing regular, sharp edges due to the oxidation it has undergone. As a result of metal oxidation, it is always seen with a brown tone, sometimes turning dark brown or blackish. All tin samples show evidence of having suffered significant generalized deterioration at some point in their history, possibly due to prolonged exposure of the ensemble to adverse natural conditions, combined with the loss of the metal’s covering material, allowing these agents to affect the metal directly. Finally, it is noteworthy that these degraded sheets are considerably thicker than the thin gold leaf applied on top, which in many cases even serves as a support.

4.2. Presence of Gold and Types of Gilding in Oña

In Oña, gold is found only in specific areas of the refectory ensemble, located solely on the serrated archivolts of Arches 2 and 5 and the rosettes of the spandrels. In total, four gold-bearing samples have been analysed in the laboratory, with results summarized in

Table 3. Composition of the microsamples with gold leaves. Source: Author.

Code	Arch	Intake Zone	Layer	Thickness	Composition (%Weight)
MSSO-SCP-F-PL05	5	Sawtooth archivolt	6	Less than 0.5 µm	Au 1
MSSO-SCP-F-PL25	5	Left-hand flanged rosette	3	3.23 µm	C (12.87); Au (47.11); O (40.02)
MSSO-SCP-F-PL30	2	Sawtooth archivolt	4	4.63 µm	C (20.38); Ca (4.42); Fe (4.82); Au (11.54); O (58.84)
MSSO-SCP-F-PL31	5	Left rosette	3	2.65 µm	C (21.09); Au (20.26); O (58.65)

¹ In the analyses conducted by ArteLab S.L., the %Weight in the Composition column is not available, as this data was not provided by the laboratory.

:

• Arch 2: one sample (MSSO-SCP-F-PL30) with a laminar thickness of 4.63 µm, showing the presence of gold, carbon, calcium, and iron, the latter coming from the gold leaf’s ground layer.
• Arch 5: three samples (MSSO-SCP-F-PL05/PL25/PL31) with laminar thicknesses ranging from less than 0.5 µm to 3.23 µm, showing pure gold in very thin and sometimes severely deteriorated layers.

It can be observed in all the samples that the gold leaf is evidently deteriorated, with some showing no continuous layer. This suggests that the extremely thin gold leaves, their delicate nature, and the high presence of chlorine—both as soluble salts and environmental contaminants—may have degraded their integrity, causing flaking and loss of film stability.

As Cennini notes, very thinly beaten gold leaf is appropriate for this type of gilding. This is what we find in sample MSSO-SCP-F-PL05, with a thickness of <0.5 µm. In this sample, a fine gold leaf acts as the final finish on the serrated archivolt of Arch 5. Beneath this thin gold, there is a base layer dominated by organic material, likely linseed oil mixed with white lead and earth pigments, indicating this technique. In the EDX spectrum of sample PL05, lead particles are present, showing predominance of lead white and earths, as described in Recipe CLII of Libro dell’Arte, replacing minium with lead-based pigments.

The partial presence of gold in several analysed samples suggests the possible use of shell gold rather than gold leaf. This hypothesis arises from small traces of gold in samples PL25, PL30, and PL31, located on the rosettes and serrated archivolts of Arches 2 and 5, which are visible organoleptically. The application of gold powder over a tin base prepared with a layer of lead white and earth pigments could explain the current golden appearance. The lack of a laminar appearance in BSE-EDX microscopy for these samples, contrasted with PL05, where a complete laminar layer is observed in stratum 6, also supports this. However, a highly eroded metal leaf may lose its laminar appearance. Therefore, while gold leaf seems the simplest explanation, the use of shell or powdered gold cannot be ruled out.

4.3. Corlas in the Refectory Ensemble

In Oña, analyses by ArteLab S.L. identified linseed oil as the binder used for both metal adhesion and pictorial layers. This drying oil was applied with pigments of various types in all pictorial layers, with copper-based pigments (verdigris) being among the main materials. This indicates extensive use of neutral copper acetates throughout the monastery’s decorative phases in the Middle Ages.

Six samples were taken from this ensemble, revealing translucent products derived from verdigris. Only in one case could linseed oil be conclusively identified as the binder. Although the precise binder type cannot always be determined, EDX analyses from CAI show a high organic content, with carbon ranging from 17.14–25.14 wt% and oxygen from 20.11–68.56 wt% (

Table 4. Composition of the microsamples with copper oleates or resinates. Source: Author.

Code	Arch	Intake Zone	Layer	Thickness	Composition
MSSO-SCP-F-PL08	5	Turquoise blue interstices of the sawtooth archivolt.	6	50 µm	Cu, Ca, Pb, Cl 1
MSSO-SCP-F-PL22	2	Interstices of the archivolt of the saw-toothed architrave have a turquoise blue coloration.	2	16.6–38.8 µm	C (24.84); Cu (7.11); O (68.03)
MSSO-SCP-F-PL23	4	Braided archivolt with turquoise blue coloration.	3	44.4 µm	C (24.32); Cl (1.16); K (0.28); Cu (7.51); O (66.74)
MSSO-SCP-F-PL27	3	Cut out and polychrome flower of the third projection of the angled archivolt.	6	48.2 µm	C (21.57); Cu (16.74); O (61.69)
			3	22.3–42.5 µm	Cu (79.89); O (20.11)
MSSO-SCP-F-PL32	1	Intercolumn with brown and blue rhombus design	5	35.8–58.1 µm	C (24.14); Cl (0.54); Ca (0.48); Cu (8.36); O (66.59)
			3	51.8–132 µm	C (25.14); Cu (6.31); O (68.56)
MSSO-SCP-F-PL33	3	Cut out and polychrome flower of the third projection of the angled archivolt	3	44.1 µm	C (17.14); Cu (29.71); O (53.15)

¹ In the analyses carried out by ArteLab S.L., the %Weight in the Composition column is not available, as this data was not provided by the laboratory.

). These materials—resinates or oleates—and their specific use in the refectory are dictated by their consistent location over lighter-colored layers or white metal leaves, in this case, tin. This strategic placement suggests that their application was intentional, aimed at enhancing luminosity in a translucent, inherently light-absorbing, dark layer.

These Onian resinates or oleates that coat the tin, due to their translucent character and protective and chromatic function, can be considered as corlas, although their color varies—from the more traditional yellows to the copper characteristic green. However, the organic composition of these corlas, as well as the degradation suffered by tin at temperatures below 18 °C, contributes to the formation of fragile and delicate layers that are easily affected by external agents [41] (pp. 160–162). Consequently, it is common for these layers to be lost over time, resulting in the pulverization or oxidation of the underlying metal, which necessitates the application of thicker and more opaque coatings to mask such alterations. These severe deteriorations have made it nearly impossible in Oña to detect corlas with an organic base, except for the use of this copper resinates or oleates found in the samples.

5. Discussion and Hypotheses

The decorative ensemble of the refectory of San Salvador de Oña reveals a technical and symbolic complexity that transcends its aesthetic function. The extensive presence of noble materials such as gold and tin, as well as surface decorations or the artistic transmission of techniques and materials in a private-use space, raises fundamental questions about the meaning, purpose, and ideological context of these interventions.

5.1. Metal Leaves: Tin and Gold

The use of metal leaves as a decorative element is not unusual in the art world, as it dates back to Antiquity. However, in the Middle Ages, their use—sometimes associated with polychromy—was mostly limited to specific decorative motifs, volumetric metallic applications [39] (p. 954), [63] (pp. 331–334), or textile imitations on stone or wood sculptures, and did not appear extensively in large-scale works. In the case of tin, this textile-imitation technique, known as applied brocade [35,36,37,38], is not represented in the remains found at Oña. One must look beyond the evidence of tin use in Christian art between the 12th and 16th centuries to find its normalized application as a base for architectural and artistic decoration in contemporary sites, such as those from Nasrid art [58,64] (pp. 268–269), or in Northern Europe, such as the sculptures of the Painted Portal of Lausanne Cathedral.

Analyses of samples from the Monastery of San Salvador de Oña have verified the extensive presence of tin (on 80% of the ensemble’s surface), as well as gold and possible gold dust. This allows us not only to examine the application techniques used in the Middle Ages but also to reflect on the aesthetic intent and the reasons for their placement in the refectory. From the reforms of 1332 onward, Oña presents new and unique materials not previously seen in this site, making it the only ensemble in the monastery with this material decoration.

The use of tin on 80% of the ensemble demonstrates not only a deep knowledge of materials but also of how to apply and decorate them, since, in some cases, tin serves as a base for gilding found on rosettes and sawtooth patterns. This technique, employed in the Middle Ages, is referenced by Cennini and documented in notable sites such as the Alhambra in Granada [65] (pp. 246,258).

Regarding gilding, the use of this highly pure metal carried significant symbolic meaning throughout the Middle Ages, not only in Christian contexts. It symbolized wealth and divinity and was reserved for high-status decorations such as heraldic emblems, Gothic phylacteries, and other elements [66] (p. 131). This precious material was widely used on all types of supports for its color, luminosity, and durability, obtained in high purity by hammering coins, notably in sites such as the Patio de las Doncellas of the Reales Alcázares in Seville, where its purity reaches nearly 100% [66] (pp. 135–136).

However, in Oña, gold was not always applied directly onto a mordant or gesso; in some cases, certain rosettes display this metal applied over tin leaves (Figure 9). This technique is only referenced in Nasrid contexts, such as the Alhambra, and despite the artistic influence of Granada on other Iberian sites like the Reales Alcázares of Seville, no traces of this gilding method are found in the Hispano–Christian context [64] (p. 90), [67] (pp. 169–172).

5.2. The Corlas

The Oña ensemble appears to be an ideal case study for understanding not only the extensive use of tin as a decorative layer but also the potential superficial layers that may have complemented it in the late medieval period with a wide range of colors. However, at present, only highly oxidized or corroded metal layers and some residual traces of binders are preserved, which allows us to hypothesize about the loss of varnishes or protective films applied over this metallic layer, commonly referred to as corlas.

Although we currently lack conclusive data on the actual corla layers in Oña, there is evidence of oily and resinous binders such as linseed oil or colophony resin, which suggests the possibility of such decoration. It can be hypothesized that the Oña ensemble featured a medieval decoration based on tin overlaid with corlas of various colors, similar to those observed today in the final polychromy of the piece: red, black, green, or turquoise blue, and yellow. The application of corlas over a lighter and more uniform tin base facilitates the distribution of light and shadow [60] (pp. 64,66–67), and its use to produce a glazed green effect was widespread across Europe, as seen in the Chartreuse de Champmol in Dijon, dated 1385 [31] (p. 178). It is also worth noting, as Jorge Rivas indicates, that there is evidence of “the use of copper resinates over malachite layers” [68] (p. 398), suggesting that the application of these resinous and oily copper compounds over lighter layers of other mineral pigments was well known in the Middle Ages [60], not only in Europe but also in Al-Andalus [64] (p. 88).

Following the hypothesis of potential Nasrid influence on the use of materials and decorative techniques in Oña, based on the studies of Ana García Bueno, there are some precedents in Nasrid art where tin leaves were overlaid with a mixture of oil-based verdigris, creating a copper oleate. One of the best examples of corled tin is a polychrome wood fragment catalogued as no. 6690 in the Alhambra Museum [69].

In the case of the Oña refectory ensemble, it can be hypothesized that these colored corla layers exhibited different tones, with green being the predominant color. However, this is not the only color present: visual inspection of the current opaque layers shows greens or turquoise blues, blacks, and reds, which may represent restorations using lead white or similar organic corla pigments. To verify this hypothesis, it is necessary to examine the metallic strata to locate residual pigments that may have survived in these layers. Due to the organic and degradable nature of most materials traditionally used in corlas, only pigments detectable via scanning electron microscopy with SEM-EDX can be traced, as this method allows identification of the elemental composition of the compounds. For a more systematic and visual analysis of these components, the following table records the sample locations, detected elements, and the possible colors and corlas used (

Table 5. Proposal of possible color varnishes in the refectory complex. Source: Author.

Location	Sample	Elements	Color of Varnishes	Possible Pigment Used
Arch 1	PL14	Cu, Cl	Green	Verdigris
	PL32	C, Cu, O, Cl	Green/Blue	Verdigris/Azurite
	PL35		Black Green	Charcoal Verdigris
Arch 2	PL22	Cu, Pb, Cl, C, O	Green	Verdigris
	PL24	Pb, C, O, Cl	Orange yellow	Red lead
	PL30	As, Pb, C, O, Cl	Yellow	Orpiment
	PL31	Pb, C, O, Ca, Cl	Orange yellow	Red lead
	PL34	Pb, Cl, C, O	Orange yellow	Red lead
Arch 3	PL27	Cu, Pb, C, O, Cl	Green	Verdigris
	PL33	Pb, C, Cl	Black	Charcoal
Arch 4	PL23	Pb, As, Cu, Cl, C, O	Yellowish green	Verdigris + Orpiment
Arch 5	PL07	Pb, Cu, Fe, Ca, Cl, Si, Mg	Green	Verdigris
	PL08	Pb, Ca, Cu, Cl	Green/Blue	Verdigris/Azurite
	PL25	Pb, As, Cl, C, O	Yellow	Red lead + Orpiment
Impost block 1	P23-PL02	Ca, Cl, Pb, Si, Fe, Cu, Al	Orange	Red lead +Earths/Ochre
Impost block 3	P25-PL01	Ca, Cl, Pb, Fe, Si, Cu	Orange	Red lead + Earths/Ochre
Capital 3	P29-PL03	Ca, Fe, Cl, Pb, Si, Al, Cu	Brownish green	Verdigris + Earths

).

It could be proposed that in the Oña ensemble, the presence of corla layers in different tones—with pigments such as orpiment, azurite, copper resinates or oleates, and red lead [58] (pp. 277–281)—reflects a clear Nasrid influence in the use of metallic materials and decorative techniques, such as gilding on tin or tin as a surface coating.

However, in the context of Oña, one may ask: where does this influence, distant from the Christian sphere, originate? The use of materials and techniques with a clear Nasrid imprint should be directly related to the reign of Pedro I over the kingdom of Castile and his preference for Islamic culture [70]. His reign was marked by both major military crises and significant cultural exchanges, materialized through architectural and iconographic interventions in key spaces such as palatial and monastic sites, employing decorative resources to create a distinctive style with strong Andalusí influence [71] (p. 309).

We can infer that the Monastery of San Salvador of Oña, linked from its foundation to the comital power, may have had close ties to the Royal Patronage during Pedro I’s reign, adopting a funerary chaplaincy. This is corroborated by the figure of Lope Ruiz (1350–1381) [72], who “was born in Oña, son of the noble Juan Ruiz de Oña. […] He served as chaplain to Alfonso XI and Pedro the Just. During his abbacy, he witnessed the monastery of Oña being sacked by the Prince of Wales, who had come to aid Pedro I the Cruel, after a conflict with him. He died in 1381” [17] (p. 565).

Nasrid influence was not only adopted culturally at the Castilian court through figures such as Ibn al-Jaṭīb [73] or Ibn Khaldūn [74] (p. 33), but also in forms, symbols, and legitimizing structures from epicentres such as the Reales Alcázares of Seville. From its Islamic origins, this city offered numerous palaces and solid artistic elements that the monarch leveraged to establish a clear legitimizing purpose for his reign. The palace established the figure of the monarch as the centre of the cosmos while creating a complex iconographic system of lauds, praises, and decorative elements using sophisticated techniques. Craftsmen from Granada, Toledo, and Seville—specialists in wood, plaster, and polychromy—were employed [74] (p. 35). It is also important to remember that until the 15th century, itinerancy of artists’ workshops was common [75] (p. 9), [76], making it plausible that one or more specialized workshops travelled north to carry out the decoration of this ensemble.

Thus, the Monastery of San Salvador of Oña shows a clear connection between Abbot Lope and the Castilian king, complemented by the evident presence of material traces and artistic techniques of unmistakable Nasrid influence. This allows us to consider that this Burgos monastic centre functioned as another focal point within the Castilian kingdom, where the monarch promoted a legitimizing image over his illegitimate brother, Enrique II [9].

6. Conclusions

In the first place, the study of the refectory of the Monastery of San Salvador de Oña faces the lack of prior documentation regarding the polychromy of the ensemble and the use of metallic decorations in Spanish medieval sculpture of the 14th century. From a historical perspective, existing studies have mainly focused on architectural, artistic, or iconographic analyses of the monastery, leaving a gap in the technical understanding of its pictorial and metallic strata. This work has undertaken a critical review of historical sources and medieval treatises, establishing a theoretical framework that allows for the interpretation and contextualization of the analytical results obtained. However, this framework must be considered flexible, since the empirical data provided by stratigraphy and material characterization may offer information that surpasses or modifies what is described in the historical sources. The combination of bibliographic review and direct study of the ensemble thus allows for the generation of well-founded and verifiable hypotheses regarding the chronology and sequences of application of the metallic and decorative layers.

Secondly, the materials used in Oña —tin, gold, resinates, and glazes— acquire historical and artistic significance from various perspectives. Medieval treatises and recipe books, such as the Mappae Clavicula, De Diversis artibus, or Cennini’s Libro dell’Arte, provide essential information on manufacturing processes, composition, and sequences of application. They allow us to understand what knowledge was transmitted and what the artisans’ aesthetic and technical goals were. However, these sources present limitations: their indications are geographically specific, chronologically bounded, dependent on the economy and the availability of materials, and do not always reflect the actual practice of each workshop. For this reason, the artisan’s expertise is key, as it adapts, interprets, and sometimes replaces these guidelines, adjusting thicknesses, compositions, and techniques according to the needs of the support, the work, and the context. The itinerancy of workshops and intercultural exchange, evidenced in the circulation of specialists and techniques between Castile and the rest of the Iberian kingdoms, as well as between northern and southern Europe, reflects an active exchange of knowledge that combined technical, iconographic, and aesthetic expertise.

Thirdly, the laboratory study of the samples analysed in this research has allowed for precise identification of the stratigraphic arrangement and the composition of the metallic and pictorial layers. It was established that tin covers approximately 80% of the surface, with thicknesses varying between 0 and 44.3 µm, with a predominant standard thickness of 0–10 µm. Gold appears in certain selected areas, such as the sawtooth edges of Arches 2 and 5 and the rosettes of the spandrels, with thicknesses ranging from <0.5 µm to 4.63 µm. Copper resinates and oleates present in glazes contribute to the luminosity and protection of the underlying metal. The combination of SEM-EDX, FTIR, RAMAN, XRD, and organoleptic analyses made it possible to establish the decorative sequence: application of tin on preparatory layers with linseed oil and pigments, superimposition of colored glazes, and, in some cases, coverage with gold leaf or powdered gold. This approach has made it possible to reconstruct how the different materials interact and were assembled, as well as to formulate hypotheses about the organic glazes that have disappeared but may once have existed in Oña.

Lastly, the materials and techniques identified, although common, do not point to genuinely indigenous techniques, but rather to the outcome of a network of cultural and technical transmission. Monasteries such as Oña functioned as centres of both reception and production of artworks. This pattern coincides with what is documented in Castiñeiras’ studies on monastic workshops and cathedral schools [76,77,78]. Although the use of tin and gold was common in the Middle Ages for decorations on portals, tympana, or sculptures in wood and stone, there are no records in Europe of coverage as extensive as that found in Oña. In this way, we are confronted with an extensive use of tin with evident applications of cut metal, dated to the first half of the 14th century. This chronology corresponds to an early stage in the use of applied metals in Spanish decoration, traditionally dated to the 15th century, and it corroborates the chronologies indicated both by Nadolny and Roberto Amieva in their theses, where they state that the use of tin in relief can be dated to moments prior to 1300 [31] (p. 171), [37] (p. 136).

The presence in this monastic context of materials and techniques of possible Nasrid origin, such as copper resinates applied over tin or certain iconographic elements of the ensemble, such as the frieze with Kufic script of praises, directly recalls the artistic context of Pedro I and the Royal Alcázars of Seville. Thus, it is crucial to consider how Oña in the 14th century was inscribed within a historical, cultural, and artistic period that drew directly from this unique context, combining Nasrid influences from Granada with Christian tradition, applied through the legitimizing program of Pedro I and, by extension, the figure of his chaplain Lope Ruiz, Abbot of Oña.

Therefore, it can be said that both the Monastery of Oña and the refectory ensemble enjoyed great relevance during the Middle Ages, not only due to the material investment made in them, but also because of the technical effort and artistic influence reflected, making this particular piece a national benchmark, with no similar examples found to date.