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Article

Documenting a Graffiti Tag by Muelle, a Pioneer of Graffiti Art in Spain

by
Teresa Rivas
1,*,
Alberto Santos-Hermo
2,
Laura Andrés-Herguedas
1 and
Jose Santiago Pozo-Antonio
1
1
CINTECX, GESSMin Group, Department of Natural Resources and Environmental Engineering, School of Mining and Energy Engineering, University of Vigo, 36310 Vigo, Spain
2
Escuela Universitaria CEU de Magisterio, University of Vigo, 36214 Vigo, Spain
*
Author to whom correspondence should be addressed.
Heritage 2026, 9(1), 23; https://doi.org/10.3390/heritage9010023
Submission received: 4 December 2025 / Revised: 27 December 2025 / Accepted: 4 January 2026 / Published: 9 January 2026
(This article belongs to the Special Issue Advances in Non-invasive Multi-analytical Approaches for the Study of Painted Artworks)
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Abstract

This paper presents the results of the historical and artistic documentation and the conservation assessment of a tag by Muelle, a pioneering Spanish graffiti artist, in the city of Vigo (Galicia, Spain). Attribution of the tag is primarily based on evidence of the artist’s presence in the city in the late 1980s, such as by different graffiti elements in the form of tags. To increase the understanding of the technique and materials used, the tag and its substrate were documented and examined by digital photography and hyperspectral imaging. Microsamples of the tag and substrate were analyzed by X-ray diffraction, Fourier transform infrared spectroscopy, optical microscopy, and scanning electron microscopy to characterize the materials and assess the tag’s state of conservation. This work enabled us to document the tag within the context of its creation and to establish hypotheses regarding the intention behind it. We confirmed that the tag has been weathered by degradation processes that typically affect contemporary urban art, with scaling caused by deterioration of the substrate being the most important type of alteration. The results of this multidisciplinary study will be key to identifying the most appropriate intervention measures to ensure the tag’s conservation and the transmission of its legacy.

1. Introduction

Contemporary murals, which form part of the increasingly valued field of neo-muralism [1], have become symbols of pride and identity in cities worldwide. These large-scale artworks can enhance the aesthetic quality of urban spaces, particularly in socially challenged or economically depressed areas. Indeed, urban festivals, such as the Cidade da Cor (City of Colour) festival held in Vigo (Galicia, Spain) [2], can transform cities into open-air museums. Many Spanish urban artists (e.g., Liqen, DOA OA, and NOVE NOEL) are now widely recognized for their distinctive styles. Graffiti artists are now gaining increasing prestige and thanks to various municipal initiatives, such as urban art festivals, they can create their artwork through official commissions and contracts. However, in the past, graffiti writing was carried out spontaneously, without authorization or institutional recognition, and was often regarded as an act of vandalism rather than as artistic expression. These early graffiti elements mainly consisted of brief messages or signatures—known as tags—following the dynamics described in Getting Up [3] and are linked to self-affirmation and territoriality, in which “being seen” was the main aim. These practices laid the foundations for a movement that would eventually evolve into contemporary muralism [4,5]. Therefore, it is important to acknowledge and value the pioneers who, with limited resources and driven by a strong need for expression and belonging, paved the way to establishing what is now one of today’s most common forms of urban art.
The conservation of contemporary murals only began to attract research interest in the past decade, despite the increasing presence and significance of these pieces. Initial research on this topic focused on identifying the types of deterioration present, by analyzing samples taken from murals or more independent pieces and on identifying the factors (both natural and anthropogenic) that contribute to the degradation [6,7,8,9,10,11,12,13]. Some of these studies arose within the framework of a pioneering research project on urban art conservation funded by the European Union Conservation of Art in Public Spaces [14]. The findings of these studies confirmed the importance of the effects of various factors on the conservation of contemporary mural paintings: the conditions of their creation (whether commissioned or not, influencing, e.g., the preparation of the substrate) and the variables that define the urban environment (particularly the soluble salts, atmospheric pollutants, and vandalism) and its ever-changing nature, possibly leading to the complete disappearance of the original paintwork. Once the most important factors were identified, research then focused on characterizing the materials used (the paints and substrates, refs. [6,8,12,15,16,17,18,19,20,21,22] and on defining effective methods of direct intervention (cleaning) and preventive conservation, such as protecting the paintings against the most significant deterioration factors: solar radiation, humidity, and vandalism [23,24]. In some studies, mock-up samples have been subjected to accelerated ageing tests with various weathering agents, such as UV-A, UV-B, UV-C, solar radiation, and gaseous pollutants (SO2 and NOx), under laboratory controlled conditions of temperature and relative humidity. Use of this technique has enabled researchers to assess the susceptibility of paintings to deterioration in relation to their composition and to determine the influence of the substrate on the deterioration process [7,25,26,27,28,29,30,31,32,33,34,35]. The findings of these studies enable artists to select the most suitable paints and protectors to use in different environments. They also enable the promoters of this type of work to select sites in urban environments where the environmental parameters are the most favourable for the conservation of the artwork and to prepare the substrates correctly, if the artwork is expected to endure.
Juan Carlos Argüello Garzo (Madrid, 1965–1995), known as Muelle, is considered a pioneer of graffiti art in Spain. Muelle was active between the early 1980s and early 1990s in the local graffiti scene in Madrid. His influence led to the consolidation of the flechero style, characterized by signatures (tags) that include strokes ending with arrowheads (flechas). This style later became strongly associated with artists such as Remebe, Glub, and Tifón. It formed the basis of the Graffiti Movement [36] and gave rise to hip-hop graffiti, deposing the pioneers and becoming consolidated at a national level by the mid-1990s.
The figure of Muelle has aroused great interest in Vigo since the discovery of an example of his iconic tag in Montera Street in the city of Madrid. Muelle’s tags are often painted in different colours, in profiles similar to hip-hop graffiti elements. The recovery of the tag in Montera Street was promoted by the Observatorio de Arte Urbano, a platform dedicated to documenting, analyzing, and preserving urban art (such as murals and graffiti), while promoting the recognition of this type of art as a part of cultural heritage through research and citizen participation [37]. The platform organizers sought cultural heritage protection status for the tag, specifically BIC (in Spanish Bien de Interés Cultural, Asset of Cultural Interest): this is the maximum level of protection provided for fixed property (such as land, buildings, and monuments) in Spanish legislation, as a recognition of, e.g., artistic, historical, architectonic, archaeological, or ethnographic importance. The conservation state of the tag is documented in the Conservando Muelle blog [38]. Microsamples of the tag were also obtained in order to characterize the component materials and the deterioration forms present. Adhesives (Paraloid B72, BevaD8S, synthetic material such as crepe, etc.) were applied in order to stabilize the underlying layers and also pigments in acrylic medium were used to fill lacunas [39].
Muelle visited Galicia on several occasions in the second half of the 1980s. During these visits, he painted numerous pieces of graffiti writing in Vigo and two of his tags have survived: one in the Plaza de Compostela and another in the street San Lourenzo. Discovery of the latter was documented and presented in 2022 at the V International Congress on Visual Arts Research, organized by the Asociación Nacional de Investigadores en Artes Visuales (ANIAV, National Visual Arts Research Association) [40]. This led to the conservation and restoration of the pieces, organized by the Escuela Superior de Conservación y Restauración de Bienes Culturales de Galicia (Higher Institute for Conservation and Restoration of Cultural Heritage in Galicia) [22].
Signs of the presence of Muelle in Galicia have faded over time. Faint remains of one of his tags, written with black aerosol paint, can be seen on the granite facade of an abandoned building in Xinzo de Limia (Ourense, Galicia), on the road that connects the northwestern Iberia Peninsula and Madrid. This may indicate that Muelle made a stop on one of his long road journeys, usually by motorbike, during which he carried out mototagging [40], a sub-category of the road-writing style [41].
This type of tag, painted in a single colour (as in Xinzo de Limia and the Plaza de Compostela in Vigo), were the most abundant in Galicia and were probably painted on different occasions, as they differ slightly in style. Muelle also created outlined tags, such as the twin examples in the San Lourenzo street and the Talude street (in Vigo) and the tag painted on the city’s harbour walls, a red bevelled graffiti tag outlined in black.
The significant activity of Muelle in Vigo positions the city as a key site in the biography of the artist. Muelle represents a pivotal figure in the relationship between the cities of Vigo and Madrid in the 1980s regarding la Movida Madrileña, a countercultural movement that emerged in Spain in the late 1970s and early 1980s, after the end of Franco’s dictatorship. This was a period of artistic, musical, and social freedom, marked by experimentation, hedonism, and a break from traditional values, symbolizing Spain’s transition to democracy and modernity. The influence of this period on the development of local graffiti writing is obvious in the testimonies of urban art writers such as Let, Ace, Mason, Coke, and Miska.
This study examined a graffiti tag located in the city of Vigo and attributed to Juan Carlos Argüello Garzo, also known as Muelle, considered a pioneer of graffiti writing in Spain. The study used three different, complementary approaches: (1) artistic description of the tag and contextualization of its authorship and creation based on secondary sources of information about the author, (2) documentation of the tag by non-destructive techniques, and (3) characterization of the pictorial materials and the substrate by micro-invasive laboratory analysis and investigation of the deterioration factors affecting the conservation of the artwork. These different perspectives—descriptive and analytical—together contribute to highlighting the value of the artist and to furthering the understanding of the context of the creation of this specific signature, while also forming the basis of scientific knowledge required in order to develop a specific conservation protocol.

2. Materials and Methods

2.1. The Location of the Signature and the Environmental Context

The tag in question is located in an alley providing access to garages located between building numbers 4 and 6 in the Plaza de Compostela, in the city of Vigo (Galicia, Spain, Figure 1a–d). The part of the alley closest to the street is readily accessible to the public and is exposed to traffic moving along the square and vehicles entering the garages. This zone (about 3 metres in length) is separated from the rest of the alley by an automatic door that restricts further access (Figure 1b). The tag is located at the entrance to the alley, in the area accessible to the public, and is surrounded by numerous other tags by other authors (Figure 1c,d). Muelle’s tag is not affected by the other tags, although it is partly occluded by an electrical conduit pipe (Figure 1d).
The city of Vigo, which has approximately 300,000 inhabitants, is located on the southern coast of Galicia (an autonomous region in northwestern Spain) and extends from sea level to a maximum altitude of 31 m. It is an important port city in the Atlantic region, due to its industrial, commercial, fishing, and shipbuilding sectors. According to FAO agroecological zoning, Galicia’s climate is classified as temperate oceanic, with an average annual rainfall of around 1200 mm, mainly due to southwesterly low-pressure systems originating from the Atlantic Ocean [42,43]. According to historical climatological data facilitated by Meteogalicia, the regional agency reporting on climate and air quality [44], Vigo is characterized by an annual average rainfall of 1298 mm, a mean annual temperature of 15.5 °C (with maximum and minimum daily average temperatures of 20 °C and 11.8 °C, respectively), and relative humidity of 76%. According to the air quality data in Galicia for the last five years (2018–2023), provided by the Xunta de Galicia, the average daily SO2 concentration recorded in Vigo is 2.7 µg/m3, with an hourly maximum SO2 of 14 µg/m3 and an hourly minimum of 7.1 µg/m3. More than half of the SO2 found in bulk deposition along Galicia’s coastline originates from marine sources, with the proportion diminishing further inland [45]. Regarding NO2, Vigo is one of the cities with the highest values (average daily value of 22 µg/m3, with hourly maximum values of 159 µg/m3). The wind patterns in Vigo are predominantly governed by two prevailing directions, influenced, respectively, by low-pressure systems from the south–southwest (which comes from the Atlantic Sea) and high-pressure systems from the north–northeast. The wall where the signature that is under study is located faces west and is therefore exposed to rain associated with marine fronts from the southwest.

2.2. The Techniques Used to Document and Characterize the Materials

The first step in this study consisted of an initial phase of in situ observation in order to describe the materials from which the wall is constructed, the components of the pictorial materials, and the deterioration forms. The latter were described using the terms included in the following glossaries: ICOMOS-ISCS [46], EwaGlos [47], and CAPuS [14]. The tag and the areas around it were examined with a portable optical microscope (Dino-Lite AF4915ZTL, Dino-Lite Europe, IDCP B.V., Almere, The Netherlands) and images were captured with a digital camera (Canon SX 60 HS, Canon España S.A., Madrid, Spain).
Parallel to this phase and to complement the documentation, reflectance images of the wall were obtained by hyperspectral imaging (with a ClydeHSI Complete, Turn-key VNIR, 1936 × 1216 pixels, SC-PT Pan, and Tilt Hyperspectral Scanning System) (Clyde HIS, Clydebank, G81 1BF, UK). The system was operated in line-scan mode in the spectral range 400–1000 nm (the visible and near-infrared NIR region), with a spectral resolution of 3 nm FWHM. The VNIR objective used has a focal length of 50 mm. The images were acquired at night, and the light source was a halogen light bulb (Victory lamp 117 mm 24 V 300 W R7s 64243012). The camera was placed on a pan and tilt scanner at 165.4 cm from the wall. Three strips were obtained with an overlap of 20%. The images were obtained and processed with hyperspectral imaging software (ClydeHSI spectraSENS version 2025). To calibrate the reflectance images, an absolute-standard-white reference (Labsphere Spectralon®-Clyde HIS, Clydebank, G81 1BF, UK-, >95% total reflectance in the 250–2500 nm range) was placed in the field-of-view of the camera. The areas were mapped by principal component analysis (PCA) and a supervised classification algorithm, following Kruse et al. (1993) [48].
After the initial description and documentation stage, several samples of the building material, the paint used in the signature, and the alteration forms were collected. These samples were examined in the laboratory under a stereomicroscope (Nikon SMZ800N, Nikon Europe, Amstelveen, The Netherlands), and the colour, texture, and morphological characteristics were recorded. The samples were then analyzed using one or more of the following techniques, depending on the hypotheses established after the in situ observations:
  • A micromorphological analysis of the polished transverse sections of the samples with an optical microscope (OM) ZEISS Axioscope 5/7KMAT (Carl Zeiss Meditec Iberia S.A.U., Madrid, Spain). The specimens were first embedded in epoxy resin (EpoThin 2 Epoxy Resin and EpoThin 2 Epoxy Hardener) and then cut and polished to a mirror shine.
  • A micromorphological and chemical analysis of the polished transverse sections the of samples by scanning electron microscopy (SEM) (with a JEOL JSM 6010L, JEOL Ltd., Tokyo, Japan) with energy-dispersive X-ray spectroscopy (EDS) in both secondary (SE) and backscattered electron (BSE) detection modes. The observation conditions included a working distance of around 10 mm, an accelerating potential of 20 kV, and a specimen current of ~60 mA.
  • An X-ray diffraction (XRD) analysis (with a SIEMENS D5000 Siemens S.A., Madrid, Spain). The samples were ground to less than 50 µm and analyzed by the crystalline powder method. Analyses were conducted using Cu-Kα radiation, an Ni filter, a voltage of 40 kV, and a 40 mA intensity, applying the random powder method. The exploration range was 3° to 60° 2θ and the goniometer speed was 0.01° 2θ/s. The minerals were identified with Xpert Highscore 2.0 software (Malvern Panalytical B.V.).
  • Fourier transform infrared spectroscopy (FTIR) (with a Thermo Nicolet 6700 spectrometer, Thermo Fisher Scientific, Waltham, MA, USA). Spectra were recorded in the mid (400–4000 cm−1) infrared spectral region at a spectral resolution of 4 cm−1, for 32 sample scans and a 38.17 s collection time. Spectra were collected using an infrared microscope (Nicolet Continuum, Thermo Fisher Scientific, USA), first analyzing the sample surface using reflection mode and then on the bulk sample using attenuated total reflectance (ATR) mode (with the Smart Orbit Diamond II accessory).

3. Results and Discussion

3.1. Description and Attribution

The tag under study, attributed to Juan Carlos Argüello (Muelle), is one of many elements that this pioneering graffiti artist from Madrid painted in diverse locations in Vigo at the end of the 1980s and which include signatures and outlined tags. The tag is located on a wall at the entrance of an alley leading to garages in the Plaza de Compostela in Vigo. It has been painted with black spray paint (Figure 1a–d), at an average height of 120 cm from the ground, on a granite wall, with two coats of paint comprising a grey coat covering a white coat, and occupies an area of 126 cm by 55 cm (Figure 1c,d). The tag is similar to another found in the Moratín street in Madrid (Figure 1f). In both sites, the author, who was around 1.8 m tall (information provided by Fernando Argüello, Muelle’s brother), would have had to crouch down to paint these signatures. Muelle’s tag has a series of characteristic features. The word “Muelle” (which means “spring”), formed by a continuous line with loops that imitate a spring, is underlined by an arrow pointing to the right, composed of three loops and three curves, ending in a long, slightly downward facing curved line (Figure 1d). This is accompanied by another characteristic element, a superscript registered trademark symbol (®), placed at the right of the final letter (“e”) in the name. However, rather than these features, it is the morphology and proportions of the letters that provide the most important information for authenticating and dating the tag. Muelle’s tag evolved after the graffiti artist first began to be active at the end of the 1970s. He originally wrote the name in angular capital letters (Figure 2a) but later used lowercase letters (except for the initial letter) and gradually incorporated some particular stylistic features thereafter (Figure 2). The changes would have enabled the artist to sign his name more quickly and thus lessen the chance of being caught in the act.
The tag in the Plaza de Compostela matches those painted by Muelle in the second half of the 1980s, in which the letter “M” is formed by a wide loop that rises above the other letters and is higher than the registered trademark symbol (Figure 2b). The lettering is more compact and rounder than in the previous period, in the mid-1980s, although some of the transitional features remain and the proportions typical of the final period are not yet apparent (Figure 2c). The proportions and size of both the name and the registered trademark symbol match those typical of the final period, although there is an anomaly in the way the letters “M” and “u” are joined, with an unusual inclination (Figure 1d). However, this could possibly be explained by the artist’s posture while painting the signature and by the sloping, uneven surface.
Inscriptions reading “SONY” and “$INDY” and the date “1991” can be seen on the concrete floor of the entrance where Muelle’s tag is located. “SONY” and “$INDY” appear on either side of a footprint and are enclosed by a line apparently forming a heart symbol (Figure 1e). Therefore, the inscriptions, made in wet concrete, probably indicate that they date from 1991. However, Muelle likely painted it before this date, as he is known to have been in the city in 1989 [40], as corroborated by extracts from his diaries, in which he recounts visiting Vigo at Easter with the musical group La Coartada, with whom he played the drums at that time. At the time when the tag was painted, the floor may have been different from its present state; it was probably lower and uneven, which could explain the previously mentioned anomaly in some of the lines joining the letters in the tag.
The choice of the location matches the modus operandi of Muelle, which has been described as “muellismo” [50], as it is known that the graffiti artist carefully chose the places where he painted. He was thus able to gain public favour by adopting an ethic based on not causing significant damage to the urban space, while also “being seen”. Thanks to this procedure, the tag at the Plaza de Compostela has survived to date, as the residents of the area have apparently not felt the need to remove the graffiti, given that it does not occupy a valued surface.
In personal communications, Fernando Argüello (Muelle’s brother) and Remebe, a contemporary of Muelle involved in the graffiti-writing scene in Madrid, informed us that Muelle usually used Novelty® and Duplicolor® spray paints. Remebe also stated that Novelty® was used to paint the tags in Vigo, as “the Dupli paints were more watery”.
Tags from different eras also appear around Muelle’s tag (Figure 1d). The oldest is a tag, below that of Muelle, by Spike, a graffiti writer from Vigo who painted this tag in 1992, as he himself has indicated. On top of this tag and to the left, on the plastic pipe, the words “Kaos” and “KVC crew” (a group to which the artist Kaos belonged), written in blue paint, date from the end of the 1990s. Some distance further along the alley, on the same wall, the symbol typically associated with Kaos, the number 2 with an arrow arising from the base and pointing towards the ground, also painted in blue, can be seen (Figure 1d). More recent additions include the number 103 painted in red spray by Sedon, and the words Gofre and Ooder painted in gold and red marker pens, also by Kaos (Figure 1c). These are considered tribute tags painted in recognition of Muelle, in what is considered a “homage collection” [40,50], indicating the influence that the artist had on the first generation of graffiti artists in Vigo.
On the upper part of Muelle’s tag, below that of Kaos and just touching the loop of the “M”, there is black spray paint tag by Wios (from Madrid). This tag was painted in 2022 during the O Marisquiño festival in Vigo, one of the largest urban culture and extreme sports festivals in Southern Europe, featuring skateboarding, BMX, breakdance, graffiti, and live music (Figure 1d). On being asked about the topic, Wios stated that the choice of the location was coincidental. Among the most recent signatures, those by Londoners Drax, Flash, and Jet97 (Figure 1c) are noteworthy, and whose authorship is pointed out by Lucas Mañas in personal communications. Drax denied that the signatures constituted a tribute, stating that the group simply left their marks in the alley as they thought that the site was suitable.
The growing attention given to the signature has led to an increase in the activity of graffiti writers in the alley, which may be intentional or coincidental. Likewise, the fact that the Plaza de Compostela has become one of the sites where the popular Christmas celebrations are held in Vigo has also led to an increase in the number of people visiting the site. Both of these situations represent a risk to the integrity of classic signatures. Indeed, the signature by Spike is the most deteriorated and at the time of writing was almost illegible.

3.2. Characterization and Diagnosis of the Conservation Status

Muelle’s tag is painted on a masonry wall built from a two-mica granite of medium grain size (Figure 3a); the texture of the stone is typical of that of local granites which emerged during the final stages of the Hercynian orogeny and which have been classified as granites of alkaline affinity [51]. The ashlar blocks appear to be dry-laid, although in some areas there is evidence of hard, strongly adhesive dark grey joint mortar, possibly made from cement. The surface of the wall has been prepared by applying a layer of fine-grained white mortar covered by a grey paint (Figure 3a). Most of the tags, including Muelle’s tag, are painted on this coating (consisting of white and grey layers), although this is missing in some parts of the wall (Figure 1d and Figure 3).
Muelle’s tag is painted in black paint (Figure 3b–d). Examination of the tag under an optical microscope (DinoLite) revealed that the edges of the line forming the signature are diffuse, which suggests that it was painted using spray paint. However, at some points, the line has sharp edges (Figure 3b), possibly indicating that the signature has been outlined by someone else. It is also possible that some of the paint has run downwards due to the effect of gravity, thus accumulating and creating the straight edges. Indeed, the sharp edges always appear on the lower edges of the signature (Figure 3b).
Regarding the alteration forms affecting the tag, two forms commonly observed in urban art in Galicia [6] were identified, as follows:
  • Scaling: Detachment of the pictorial layer along with the substrate material. This is the most severe type of alteration and has strongly affected the integrity of the tag, causing the loss of large portions of the paintwork (in the order of centimetres) (Figure 3b–d). Scaling has affected the underlying substrate (the white mortar and grey paint) as well as the pictorial layer of the signature. In the most severely affected areas, the granite surface of the wall is exposed.
  • Peeling: Loss of the pictorial layer. The black paint layer has peeled at some points, leaving the underlying substrate exposed (Figure 3d). This type of alteration affects much smaller areas than scaling, in the order of millimetres.
In addition to these two types of alteration, there are some whitish areas on the grey paint, suggesting that a third layer of lighter grey paint has been applied at some stage. However, it is difficult to establish by visual examination whether the light grey paint was applied before or after the tag was painted (Figure 3d).
As indicated in Materials and Methods, the hyperspectral camera data were processed using two different statistical methods: PCA and a supervised classification algorithm following Kruse et al. (1993) [48].
Principal component analysis (PCA) is an unsupervised technique used for data exploration without prior knowledge of the dataset. Hyperspectral imaging files typically contain a huge number of images, each corresponding to a specific wavelength, and PCA is therefore particularly valuable for reducing the number of spectral bands, which are often highly correlated. PCA captures the maximum variance across bands by applying second-order statistics (covariance), thus reducing redundancy [52]. In this study, PCA was computed for the entire region of the camera and the output dimensions were limited to five. The results of the PCA enabled identification of the different types of material in the substrate (wall) on which the tag is painted (Figure 4a,b). The camera captured the granite material from which the wall is constructed (Figure 4c), which was clearly observed in the areas where the coating layers have been eroded. The images also enabled differentiation of the tags by Muelle, Wios, and Spike, all of which are black, and the Kaos tag, painted in blue (Figure 4d,e). The images also revealed that the colour of Muelle’s tag was particularly intense (not perceived by visual examination), suggesting that it was painted using a different type of paint than used in the other tags. Finally, the analysis confirmed an interesting observation, i.e., that the spectral reflectance of the coating underneath the tag by Muelle is different from that of the coating underneath the tags surrounding the signature (the Kaos and Wios tags above the signature and the Spike tag below). This observation may be related to the lighter grey area visible around Muelle’s tag. On the other hand, the hyperspectral imaging confirmed that the signature was painted on top of the (supposed) lighter grey paint (Figure 4f).
Data treatment with the supervised classification algorithm, following Kruse et al. (1993) [48], classifies pixels based on the similarity of the spectral signature to the reference spectra (considered an endmember), thus generating the Spectral Angle Mapper (SAM) classification algorithm (Figure 4g). In this case study, the reference spectra were selected on the basis of prior knowledge of the different tags and the materials from which the wall was constructed. The accuracy of the classification is reported in Table 1. A threshold angle is imposed to recognize similar and dissimilar spectra; in this case, the angle was set at 0.15 rad for all classes. The SAM algorithm calculates the spectral angle between the reference vector and each pixel vector. If the angle is below the threshold, the pixel is classified as belonging to that reference class.
When working within the visible range, differences between tags are related to colour. By previously determining the threshold angle, it was possible to identify differences between the black tags, specifically between Muelle’s tag and Wios’ tag (Figure 4g). Although some pixels associated with the Muelle’s tag were also found within Wios’ tag, it was still possible to establish differences between the types of black. Nevertheless, some discrepancies were observed regarding the classification of endmembers according to their colour features. In particular, discrepancies were observed when classifying the conduit pipe and the white tile (used for image correction), which were mistakenly categorized as grey paint underneath the tag. Despite this minor misclassification, the supervised classification method also confirmed that the spectral reflectance of the grey layer underneath Muelle’s tag is different from that of the layer on which the other surrounding tags are painted, particularly the tags by Kaos and Wios.
Considering the observations of the materials and the forms of deterioration and considering the results obtained with the hyperspectral camera, the following samples were collected for different purposes (Table 2):
(1)
Two samples of the wall coating (which, in turn, consist of a white layer of mortar and an overlying layer of grey paint): sample S1 is from the coating on the wall in the area with the graffiti tags (Figure 5a) and sample S2 (Figure 5b) is from the coating on another part of the wall (further along the alley, on the other side of the metal door, see Figure 3b) where there are no tags or other graffiti. These samples were examined to determine whether the signatures were painted directly on the wall, without prior preparation of the surface. Given that the samples were micro-samples, it was necessary to optimize the analytical protocol. To this end, an aliquot of each sample was first analyzed by FTIR (reflection mode), followed by XRD (random powder), and subsequently by FTIR (ATR mode). Another aliquot was studied under an OM and by SEM after being prepared as a cross-section.
(2)
A third sample of the coating that forms the substrate for Muelle’s tag, which is of a lighter grey colour than the rest of the wall (S3, Figure 3d and Figure 7). This sample was obtained with the aim of determining whether there was a layer of lighter grey paint under Muelle’s tag, as suggested by visual observations made in situ. The whole sample was prepared as cross-section and analyzed by an OM and SEM-EDS and the observations were compared with those of sample S1.
(3)
Two samples of black paint from the lines forming the signature: one in an apparently good state of conservation (PN1) and another affected by peeling (PN2); an aliquot of each sample was first analyzed by FTIR (reflection mode) and subsequently by FTIR (ATR mode). Another aliquot was studied under an OM and by SEM after being prepared as a cross-section.
The XRD analysis detected the presence of gypsum (CaSO4.2H2O) and calcite (CaCO3) in samples S1 and S2; gypsum was more abundant than calcite in S1, while both phases were present in similar amounts, together with barite (BaSO4), in S2. These findings were confirmed by FTIR (Figure 6): in both samples, bands were detected at 3509 cm−1 and 3400 cm−1, assigned to the stretching vibration of O-H groups, at 1619 cm−1, assigned to O–H bending vibration, and at 1088 cm−1 and 659 cm−1 and 597 cm−1, typical of the functional S-O group in sulphates; the presence of barite was confirmed by the detection of the characteristic peak at 985 cm−1 [53,54,55]. A weak band detected at 1321 cm−1 may correspond to the stretching vibration of the C-H group and of the O-H functional group. In S2, two weak bands were detected at 1408 cm−1 and at 871 cm−1, indicating the present of the C-O group of carbonates [54]. FTIR analysis did not identify the organic base of the grey paint.
Analysis by OM and SEM-EDS confirmed the differences between the grey coating in the area where the tags are located (S1) and the coating on the rest of the wall (S2), in regard to the texture, structure, and composition of the layers (Figure 5c–f). The grey coating in the area where the tags are located (sample S1, Figure 5c) is formed by the following:
  • A white base layer (bl in Figure 5c) of variable thickness (90–200 µm) formed, in turn, by different sublayers (bl-1, bl-2, and bl-3). The deepest sublayer is rich in S and Ca (forming gypsum, according to the XRD analysis) and the other two layers are richer in Ca, C, and O (forming calcium carbonate, according to the XRD analysis). This deep layer may correspond to plaster made from air lime and gypsum.
  • Covering the aforementioned white base layer, there is a layer of thickness 270–110 µm (cm, Figure 5c), formed by angular-shape grains of sizes between 90 and 10 µm agglomerated by a cementing matrix. SEM-EDS (Figure 5e) confirmed that the grains are calcium carbonate and that the matrix is formed by two phases: one of very fine grains rich in C, Ca, and O (forming calcium carbonate) and another formed by grains of mixed composition, formed by Si- and Ca-rich phases intermixed with Al- and Ca-rich phases; these grains correspond to the typical calcium silicates and calcium aluminate hydrated mineral phases of early Portland cement (manufactured between 1820 and 1890; [56]). The mixed composition (cemented aggregates) suggest that this layer is a mortar made from lime and Portland cement.
  • This layer of mortar is covered by a thin coat (25–150 µm) of grey paint (grey paint, gp, Figure 5c,e). This grey paint is composed of a C-rich matrix (of low contrast under BSE detector, Figure 5e) that agglomerates subangular fragments of size 2–12 µm, rich in Ca, C, and O (probably calcium carbonate), S and Ca (probably gypsum), and also bright microparticles (<2 µm) rich in S and Ba (present in an atomic ratio equivalent to that of barium sulphate) (Figure 5e). Calcium carbonate, gypsum, and barite, which were detected by XDR, are typical mineral phases used as fillers in modern paints [57,58].
Sample S2, taken from a part of the wall where there is no graffiti, also has a base layer (bl, Figure 5d,f), formed of successive sublayers rich in Ca, C, and O and, to a lesser extent, S (forming calcium carbonate and gypsum, following XRD analysis), probably corresponding to the application of several coats of lime slurry. The base layer (bl) is covered by another layer (gp layer, Figure 5d,f) of a thickness between 30 and 150 µm and with a similar texture and composition to the grey paint layer of sample S1: a C-rich matrix that agglomerates subangular grains rich in C, Ca, and O and numerous microparticles rich in S and Ba (Figure 5f). However, this grey paint layer is not the outermost layer in the S2 sample, as it is covered by another superficial layer of a thickness of 100 µm (gp-2, Figure 5d,f) formed by a C-rich matrix that agglomerates angular fragments rich in Mg and Ca (possibly forming dolomite, another mineral phase commonly used as a filler in modern paint; [57,58]; mixed with the dolomite grains, microfragments rich in Ti (probably forming titanium oxide) (Figure 5f) are identified.
So, the grey paint from the area where Muelle’s tag and the other tags are located (S1) is, therefore, different from the grey paint on the rest of the wall (S2): (1) the grey paint from the area with graffiti tags contains barite, gypsum, and calcite as fillers; on the rest of the wall, this grey paint is covered by a coat of another paint of the same colour but with a different composition (containing dolomite and titanium oxide as fillers); (2) in the zone with the graffiti tags, there is an intermediate layer of mortar made from Portland cement, that is not present on the rest of the wall.
These results indicate that the coating on the part of the wall where the signatures are located is different from that on the rest of the wall, in regards to the type of paint and the presence of the intermediate layer of cement mortar. The area where the tags are located is readily accessible and covered by unwanted paintings; therefore, the most likely explanation for the differences is that this part of the wall has been repainted on various occasions, with paint of a similar colour but of a different composition, in order to cover the unwanted paintings.
The analyses also confirmed that the presumed layer of lighter grey paint, which appears to have been applied beneath Muelle’s tag (sample S3, Figure 3d and Figure 7), actually corresponds to degradation of the grey paint layer of the coating. This degradation was confirmed by examination of the sample by stereomicroscopy: in the lightest coloured areas, the paint is slightly disaggregated (Figure 7a,b). Examination under OM and with SEM of these deteriorated areas revealed that the paint layer is thinner, more porous, and disaggregated and also contains fewer filler particles, particularly of barite. Enrichment of S- and Ca-rich particles, possibly from gypsum, was also confirmed in this area (Figure 7d). In addition, a very thin layer (2 µm) of grains rich in Si, Al, K, and Na, which may correspond to dust and dirt, was also detected (Figure 7d). Surface degradation was detected using hyperspectral imaging, which revealed that the spectral reflectance differed in the whitish areas beneath Muelle’s tag compared to the rest of the grey paint on the painted area of the wall (Figure 4f). Therefore, it is confirmed that the grey paint on the painted area of the wall is deteriorated only beneath Muelle’s tag and that this degradation was already present before Muelle’s tag was painted. It is unlikely that this degradation can be attributed to external environmental factors, such as ultraviolet radiation, rainwater, or salt crystallization processes (which the presence of gypsum in the deteriorated paint might indicate); these factors would have acted on the entire area, but the degradation is restricted to the area beneath Muelle’s tag. This suggests that the surface of this part of the wall was cleaned at some point before the signature was applied, resulting in the surface degradation of the grey paint layer.
Regarding the characterization of the paint used to form the signature, examination of sample PN1 with OM and SEM showed that the layer of black paint forming the tag is rather thin (20–50 µm) (Figure 8a). SEM (Figure 8b) revealed that the paint is formed by a low contrast matrix rich in C and numerous macroparticles rich in S and Ba, probably forming barite, which show strong contrast under a BSE detector. This composition was confirmed by FTIR (Figure 6), which detected the bands typical of the S-O group (1086 cm−1, 659 cm−1, and 597 cm−1) and a peak at 987 cm−1 assigned to barium sulphate. The black paint was also shown to contain calcite as a filler, indicated by detection of the bands typical of the C-O group of the carbonate, at 1408 cm−1 and 871 cm−1. Using FTIR, it was not possible to identify either the nature of the paint polymer or the colourant; regarding the latter, although the black colour in modern aerosol paints can be produced using various pigments, dyes, and mixed metal-oxides, a study that analyzed a wide range of formulations used in urban art [59] identified carbon black (PBk7) as the main colourant. Confirmation of the presence of this colourant in the present case would have required the application of other spectroscopic techniques, such as Raman spectroscopy.
The black paint forming Muelle’s tag lays on top of on the grey paint (gp, Figure 8). A clear boundary can be observed between the two layers (black and grey paints) both in areas where the grey paint is intact (Figure 8b) and in areas where the grey paint is deteriorated; thus, in Figure 8c, corresponding to an area where the black paint layer overlies the grey paint (gp) with a whitish coloration, the deterioration features of this gp layer can be observed, including the absence of barite filler particles and an enrichment in S- and Ca-rich particles. This observation confirms that the signature was painted by applying the black spray paint over the already deteriorated grey paint layer.
In the areas where the black paint is affected by peeling (sample PN2, Figure 8d), numerous pores of unequal size were detected in the grey paint layer, some also affecting the black paint layer (Figure 8d,e). This may indicate that the peeling of the black paint may be related to the deterioration process affecting the grey paint.
This artwork is on a wall highly exposed to rain, which in Galicia is often associated with storm fronts coming from the sea; also, the marine influence on the aerosol’s composition is notable in this city. It is also known that marine aerosols can cause salt weathering in building materials and in urban environments they can also contribute to scaling [60,61]. However, chloride, which would indicate contamination by a marine aerosol, was not detected (by EDS) in the samples, which could indicate that the deterioration process (scaling and peeling) is due to another factor different than the crystallization–dissolution process of sodium chloride. Gypsum, which was detected, is known to be involved in superficial detachment processes in granite rocks and paints [60,62,63,64]; however, in this case, it does not appear to have been derived from liquid sources or from an aerosol as saline efflorescence was not detected on the wall and the pores in the pictorial and substrate layers were empty (if the salts were mobilized, they would precipitate within the pores of the materials).

4. Conclusions

Considering the location and the morphology of the graffiti and taking into account its characteristic elements and proportions, it was possible to attribute the tag under study to Juan Carlos Argüello, “Muelle”, from an undetermined date in the second half of the 1980s. The presence of Muelle in Vigo has been confirmed by the existence of this and other documented tags and outlined tags, as well as by oral testimonies and extracts from the artist’s diaries. The anomaly between the joining of the letters “M” and “u” can possibly be explained by the height at which it was painted and the uneven surface of the substrate.
The area of the granite wall where the Muelle’s tag is located has a coating consisting of a lime- and gypsum-based mortar and a superficial layer of grey paint. This coating is the same colour and texture as that of the rest of the wall, but the composition differs both in the grey paint composition and in the presence, in the area where the tag is painted, of an intermediate layer of mortar containing Portland cement. The difference in the composition of the coating may be explained by building maintenance actions during which the grey coating may have been renewed at some point before the tags were painted on the wall, possibly to cover unwanted paintings.
The tag consists of a very thin layer of black paint, typical of spray paints. Another noteworthy finding is the severe state of deterioration of the coating below the tag, which was not detected in the coating below the other nearby signatures on the same wall. The analyses conducted ruled out physical degradation processes associated with salt crystallization. It is therefore possible that the surface was previously cleaned with an aqueous solution, which may have caused deterioration of the surface layer of the grey paint coating the wall before the tag was applied. If this were the case, Muelle’s tag would not have been spontaneous, showing a certain deliberate and planned intention.
This research showcases Muelle as a pioneering figure in Spanish graffiti-writing, by studying the state of conservation of one of his tags located in Vigo (Galicia, Spain). The study identified the alteration forms and their possible causes, thus providing a basis for the development of preventive conservation strategies by the relevant authorities. Regarding conservation efforts, the results rule out deterioration phenomena associated with the crystallization and dissolution of soluble salts. The factors contributing to the signature’s deterioration are more closely linked to the wall’s maintenance and the interventions carried out within a context of insufficient awareness of the artistic and social significance of what it contains: the signatures of Muelle and other artists representing this artistic movement. Raising awareness of Muelle’s importance and informing the neighbourhood about the existence of such a significant artist’s signature would constitute the first step towards ensuring that the space containing it is protected, at least from the local community, who could then take action to protect it through more conservative maintenance measures for the wall.

Author Contributions

Conceptualization, T.R., A.S.-H., and J.S.P.-A.; methodology, T.R., A.S.-H., and J.S.P.-A.; software, T.R. and L.A.-H.; validation, T.R., A.S.-H., and J.S.P.-A.; formal analysis, T.R., L.A.-H., and J.S.P.-A.; investigation, T.R., A.S.-H., and J.S.P.-A.; resources, T.R. and J.S.P.-A.; data curation, T.R., A.S.-H., and L.A.-H.; writing—original draft preparation, T.R., A.S.-H., L.A.-H., and J.S.P.-A.; writing—review and editing, T.R., A.S.-H., and J.S.P.-A.; visualization, T.R., L.A.-H., and J.S.P.-A.; supervision, T.R. and J.S.P.-A.; project administration, T.R. and J.S.P.-A.; funding acquisition, T.R. and J.S.P.-A. All authors have read and agreed to the published version of the manuscript.

Funding

This study is part of the CNS2022-135645 research project funded by MICIU/AEI/10.13039/501100011033 and by “European Union NextGenerationEU/PRTR”. Laura Andrés-Herguedas is grateful for the receipt of grant PRE2022-105106 from MICIU and ESF+. For more information: https://sosmurals.webs.uvigo.es/.

Data Availability Statement

Data are available upon request. Please contact the corresponding author.

Acknowledgments

The authors would like to thank the Research Support Services of the University of Vigo (CACTI) for the SEM-EDS analysis.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Map of Spain showing the location of Vigo (within the red rectangle); (b) entrance to the garage where Muelle’s tag is located; (c) view of the wall where the tag is located; (d) close-up view of the tag; (e) concrete floor with the inscriptions “SONY” and “$INDY” beside a footprint and enclosed by what appears to a heart symbol. The number (date) “1991” can also been seen; (f) another of Muelle’s tags, in the street Moratín in Madrid.
Figure 1. (a) Map of Spain showing the location of Vigo (within the red rectangle); (b) entrance to the garage where Muelle’s tag is located; (c) view of the wall where the tag is located; (d) close-up view of the tag; (e) concrete floor with the inscriptions “SONY” and “$INDY” beside a footprint and enclosed by what appears to a heart symbol. The number (date) “1991” can also been seen; (f) another of Muelle’s tags, in the street Moratín in Madrid.
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Figure 2. The evolution of Muelle’s tag, shown by different examples from (a) the end of the 1970s; (b) the first half of the 1980s; and (c) the second half of the 1980s. Adapted with permission from [49].
Figure 2. The evolution of Muelle’s tag, shown by different examples from (a) the end of the 1970s; (b) the first half of the 1980s; and (c) the second half of the 1980s. Adapted with permission from [49].
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Figure 3. (a) Detail of the ashlar masonry and surface coating formed by a layer of white mortar and a layer of grey paint; (b) detail of the tag and the underlying substrate (i.e., the coating); diffuse edges of the line forming the signature can be seen in some areas (indicated by red arrows) and sharp edges in others; exposed areas of granite, caused by scaling of the coating, can also be observed; (c) area affected by scaling; (d) area affected by scaling and peeling (black paint), with some light grey areas (indicated by red arrows) possibly due to application of lighter coloured paint.
Figure 3. (a) Detail of the ashlar masonry and surface coating formed by a layer of white mortar and a layer of grey paint; (b) detail of the tag and the underlying substrate (i.e., the coating); diffuse edges of the line forming the signature can be seen in some areas (indicated by red arrows) and sharp edges in others; exposed areas of granite, caused by scaling of the coating, can also be observed; (c) area affected by scaling; (d) area affected by scaling and peeling (black paint), with some light grey areas (indicated by red arrows) possibly due to application of lighter coloured paint.
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Figure 4. (a) The area analyzed by hyperspectral imaging. (b) A map of the areas of the wall with similar spectral reflectance properties, derived by PCA in October 2025. The differentiated areas correspond to (c) the substrate (granite); (d) black tags; (e) blue tags; (f) grey paint underneath the tags. (g) A classification map of the studied area constructed using the Spectral Angle Mapper (SAM) algorithm, which enabled differentiation of the different black tags (the tags by Muelle and Wios).
Figure 4. (a) The area analyzed by hyperspectral imaging. (b) A map of the areas of the wall with similar spectral reflectance properties, derived by PCA in October 2025. The differentiated areas correspond to (c) the substrate (granite); (d) black tags; (e) blue tags; (f) grey paint underneath the tags. (g) A classification map of the studied area constructed using the Spectral Angle Mapper (SAM) algorithm, which enabled differentiation of the different black tags (the tags by Muelle and Wios).
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Figure 5. (a,b) The points where samples S1 and S2, respectively, were taken. (c–f) Micrographs of a cross-section of samples S1 (c,e) and S2 (d,f) taken with OM (c,d) and using SEM (e,f); in OM images, (e,f) areas are indicated by a red rectangle). The different layers are indicated as follows: base layer (bl), cement mortar (cm), grey paint (gp) on Muelle’s tag area, and grey paint (gp-2) on an area without graffiti tags. The aggregates of calcium silicates and aluminates in the hydrated cement are indicated by yellow arrows. The figure also includes EDS spectra of the grains of the paint fillers from the grey paint (gp) under Muelle’s tag: calcium carbonate (EDS 1), gypsum (EDS 2), and barium sulphate (EDS 3), and of the paint fillers from the outer coat of grey paint, sample S2, rich in Ca, Mg, O, and C (EDS 4).
Figure 5. (a,b) The points where samples S1 and S2, respectively, were taken. (c–f) Micrographs of a cross-section of samples S1 (c,e) and S2 (d,f) taken with OM (c,d) and using SEM (e,f); in OM images, (e,f) areas are indicated by a red rectangle). The different layers are indicated as follows: base layer (bl), cement mortar (cm), grey paint (gp) on Muelle’s tag area, and grey paint (gp-2) on an area without graffiti tags. The aggregates of calcium silicates and aluminates in the hydrated cement are indicated by yellow arrows. The figure also includes EDS spectra of the grains of the paint fillers from the grey paint (gp) under Muelle’s tag: calcium carbonate (EDS 1), gypsum (EDS 2), and barium sulphate (EDS 3), and of the paint fillers from the outer coat of grey paint, sample S2, rich in Ca, Mg, O, and C (EDS 4).
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Figure 6. FTIR-ATR spectra of the samples S1, S2, and PN1.
Figure 6. FTIR-ATR spectra of the samples S1, S2, and PN1.
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Figure 7. (a,b) Micrographs of sample S3 (the surface of the lighter grey paint) taken with a stereomicroscope; (b) area where loss of the outermost layer of paint is evident; (c) micrograph of a cross-section of a discoloured area, taken with OM; (d) SEM micrograph of the area marked by a red rectangle in OM micrograph (c). In OM and SEM images, the layer of grey paint (gp), the layer of cement mortar (cm) and the underlying white paint (bl-3) can be seen. The figure also includes EDS-SEM spectra of gypsum crystals (upper) and silicate particles of depositional origin (lower).
Figure 7. (a,b) Micrographs of sample S3 (the surface of the lighter grey paint) taken with a stereomicroscope; (b) area where loss of the outermost layer of paint is evident; (c) micrograph of a cross-section of a discoloured area, taken with OM; (d) SEM micrograph of the area marked by a red rectangle in OM micrograph (c). In OM and SEM images, the layer of grey paint (gp), the layer of cement mortar (cm) and the underlying white paint (bl-3) can be seen. The figure also includes EDS-SEM spectra of gypsum crystals (upper) and silicate particles of depositional origin (lower).
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Figure 8. (a) An OM micrograph of a cross-section of the sample PN1 (black paint) showing the thickness of the layer of black paint and the underlaying layers (grey paint, gp, and cement mortar, in cm) are shown; (b) an SEM micrograph of the area marked by a red rectangle in the OM micrograph, in which a black paint layer (of variable thickness) is observed covering the grey paint layer (gp), which is affected by large pores; (c) the detail of the boundary between the black paint layer and the deteriorated grey paint (gp), which is affected by a barite depletion and a gypsum enrichment (see red double arrow); (d) an OM micrograph of a cross-section of sample PN2 (black paint affected by peeling), in which numerous pores affecting the grey layer and lacunae in the black paint layer can be observed; (e) an SEM micrograph of the area marked by a red rectangle in (d) the OM image, confirming the deterioration of the grey layer and the lacunae of the black paint layer associated with pores in the underlying layer.
Figure 8. (a) An OM micrograph of a cross-section of the sample PN1 (black paint) showing the thickness of the layer of black paint and the underlaying layers (grey paint, gp, and cement mortar, in cm) are shown; (b) an SEM micrograph of the area marked by a red rectangle in the OM micrograph, in which a black paint layer (of variable thickness) is observed covering the grey paint layer (gp), which is affected by large pores; (c) the detail of the boundary between the black paint layer and the deteriorated grey paint (gp), which is affected by a barite depletion and a gypsum enrichment (see red double arrow); (d) an OM micrograph of a cross-section of sample PN2 (black paint affected by peeling), in which numerous pores affecting the grey layer and lacunae in the black paint layer can be observed; (e) an SEM micrograph of the area marked by a red rectangle in (d) the OM image, confirming the deterioration of the grey layer and the lacunae of the black paint layer associated with pores in the underlying layer.
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Table 1. A description of the different elements considered in this study. The area of each element and the classification accuracy, for a threshold angle of 0.15 rad, are also shown.
Table 1. A description of the different elements considered in this study. The area of each element and the classification accuracy, for a threshold angle of 0.15 rad, are also shown.
NameArea (%)Classification Accuracy (%)
Muelle’s tag4.0193.3
Granite substrate53.4695.4
Grey paintwork underneath the tag33.7995.17
Blue tag1.6994.07
Wios’ tag2.3292.55
Unclassified4.73100
Table 2. A list of micro-samples collected, their description and location and the analyses performed on them.
Table 2. A list of micro-samples collected, their description and location and the analyses performed on them.
NameDescriptionLocationAnalyses
S1Wall coatingGraffiti painting areaFTIR, XRD (bulk sample)
OM and SEM (cross-section)
S2Wall coatingInaccessible area of the alleyFTIR, XRD (bulk sample)
OM and SEM (cross-section)
S3Wall coatingSubstrate of Muelle’s tagOM and SEM (cross section)
PN1Black paint in good conditionMuelle’s tagFTIR (bulk sample)
OM and SEM (cross-section)
PN2Black paint affected by peelingMuelle’s tagFTIR (bulk sample)
OM and SEM (cross-section)
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MDPI and ACS Style

Rivas, T.; Santos-Hermo, A.; Andrés-Herguedas, L.; Pozo-Antonio, J.S. Documenting a Graffiti Tag by Muelle, a Pioneer of Graffiti Art in Spain. Heritage 2026, 9, 23. https://doi.org/10.3390/heritage9010023

AMA Style

Rivas T, Santos-Hermo A, Andrés-Herguedas L, Pozo-Antonio JS. Documenting a Graffiti Tag by Muelle, a Pioneer of Graffiti Art in Spain. Heritage. 2026; 9(1):23. https://doi.org/10.3390/heritage9010023

Chicago/Turabian Style

Rivas, Teresa, Alberto Santos-Hermo, Laura Andrés-Herguedas, and Jose Santiago Pozo-Antonio. 2026. "Documenting a Graffiti Tag by Muelle, a Pioneer of Graffiti Art in Spain" Heritage 9, no. 1: 23. https://doi.org/10.3390/heritage9010023

APA Style

Rivas, T., Santos-Hermo, A., Andrés-Herguedas, L., & Pozo-Antonio, J. S. (2026). Documenting a Graffiti Tag by Muelle, a Pioneer of Graffiti Art in Spain. Heritage, 9(1), 23. https://doi.org/10.3390/heritage9010023

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