Art Casting in Portuguese 19th-Century Industrial Foundries: A Multi-analytical Study of an Emblematic Copper-Based Alloy Monument

The outdoor sculpture of the first Portuguese king, D. Afonso Henriques (~1109 – 1185 AD), placed in Guimarães (North Portugal) is one of the most emblematic national sculptures. Created in 1887 by António Soares dos Reis, it possesses a remarkable symbolic value in the presumed birthplace of the king. In addition to the artistic and heritage importance of the monument, it is one of the few sculptures cast by a Portuguese industrial foundry in the 19th-century. This study obtained data on the sculpture's elemental composition and corrosion products, gathering important historical and technical information. For this purpose, a multi-analytical approach consisting of X-ray fluorescence (XRF), X-ray diffraction (XRD), optical microscopy (OM) and scanning electron microscopy (SEM-EDS) was carried out to characterise the bulk metal and corrosion layers. The data revealed a ternary alloy of Cu, Sn, Zn with Pb, Fe, As, Bi and Mn as minor elements. The alloy matches that of other sculptures cast in that period. In terms of corrosion, it is characterised by the presence of oxides. These results represent the first step for applying an appropriate conservation strategy for bronze sculptures with similar characteristics.


Introduction
Outdoor metal sculptures in the 19 th -century were typically cast in iron or copperbased alloys, such as bronze. They were cast primarily at military and industrial foundries, but the specificity of these types of castings led, along the time, to the appearance of foundries specialised in sculpture.
Following the tradition of metal casting in the Italian Renaissance [1], art foundries experienced a great development during the 19 th -century, especially France [2][3][4], that soon became the European centre of the art foundry industry since the 19 th -to the early 20 th -century [2,3].
As for Portugal, the first monument cast was the statue of King José I (1775) that was produced in Lisbon in a military foundry [5]. This monument, 14 metres high, was the first large-scale piece of art to be cast in the country, thus constituting the beginning of the art foundry industry in Portugal.
Despite the large number of industrial foundries working in Portugal in the mid and late 19 th -century, artistic ones were non-existent, and the casting of pieces of art was delegated to either military foundries or industrial metalwork companies. The first Portuguese art foundry companies were only created later, in the late 19 th -century. In Porto, for example, the first exclusively art foundry was in the Malmerendas Street, having been established exclusively for the casting of the King Pedro V's sculpture (1863)(1864)(1865)(1866).
Two decades later this first experience, the monument of King D. Afonso Henriques was the second sculpture cast in Porto. This bronze sculpture was produced in 1887 by a local industrial foundry, known as Massarelos. Created in 1852, this company had over 400 employees in 1881, being one of the largest and most important industrial foundries of the 19 th -century, particularly known for producing large-scale structures and decorative cast iron [6].
The history of sculpture casting in Portugal is still to be done. Despite the significant amount of metal statuary, there are only a few limited studies on the history of industrial foundries, but not on art foundries [7,8]. Unfortunately, regarding this topic, historical archives have not survived to the present days. These were lost for unknown reasons, and information about production processes, materials, alloys, and patinas recipes are virtually unknown.
In order to fill these gaps, this paper adopts a multi-analytical approach based on insitu and laboratorial techniques: X-ray Fluorescence (XRF) was used for the chemical characterisation of the bulk metal; X-ray Diffraction (XRD) was performed for the identification of the corrosion products; optical (OM) and electron microscopy (SEM-EDS) were employed for the observation and characterisation of the microstructure of the alloy and corrosion layer.
Apart from the study of the corrosion of the 1775 equestrian statue of King José I [9,10], this paper highlights, for the first time, the results of a comprehensive and multianalytical study intended to characterise the bulk metal and corrosion of the Afonso Henriques's sculpture, located in Guimarães (North of Portugal). This work is part of a larger project that aims to study a group of 19 th -and 20 th -century metal sculptures with the final purpose to implement appropriate conservation strategies. The importance of studying the D. Afonso Henriques' sculpture is mainly determined by two different factors: a) the symbolic and heritage national relevance of this sculpture; and b) the effects of weather in the sculpture conservation, atmospheric pollution, and repeated vandalism acts, thus making urgent to acquire knowledge about the elemental composition of the sculpture, and the main corrosion products present on its surface. The data will allow the creation of appropriate conservation strategies that can apply to art pieces of similar characteristics.

The D. Afonso Henriques' outdoor sculpture
The monument dedicated to D. Afonso Henriques has an important heritage value for the city of Guimarães, where it is considered a local symbol. The historical importance of the monument extends equally to the whole nation, where it is the best known and most recognisable sculpture of the first king of Portugal. The symbolism of this sculpture is so distinguished that life-size reproductions were made and are currently on display in Lisbon and Rio de Janeiro.
The idea of honouring the first Portuguese king, D. Afonso Henriques, by erecting a memorial came about in 1882. A local committee was set up to raise the necessary funds by public subscription and was later joined by a committee based in Rio de Janeiro, Brazil. The support of Portuguese and Brazilian citizens turned this donation-based project into an international cause.
In 1884, the monument, consisting of a sculpture and a marble base, was commissioned to sculptor António Soares dos Reis (1847-1889) and architect José António Gaspar (1842-1909), respectively. Soares dos Reis is considered one of the leading Portuguese sculptors of the 19 th -century. Due to personal circumstances, only two sculptures by the artist were cast during his lifetime, of which this is one.
In 1887, the sculpture was cast in the Massarelos Foundry with bronze donated by the Portuguese government and inaugurated in the same year. The sculpture, that is 2.65 meters high (Figure 1), has been produced by sand-casting technique. This casting was a great technical challenge at the time, as it has a shield attached to the hand and separated from the body, and a detachable sword. Over the time, the sculpture has suffered several acts of vandalism that have affected its integrity. The piece most affected by these acts is the sword, which has been broken and restored on more than one occasion. The sculpture has been produced in different pieces that were not initially welded. They were probably bolted on the inside, linking the parts. After that, the surface was smoothed by hammering metal burrs on the edge of each piece. These burrs were intentionally produced to be deliberately filed and joined to the other piece, giving the work of art a homogenous appearance. Despite this, in some lower areas of the sculpture, welding work can be noticed. This welding may have occurred due to removal or damage to some pieces during relocations of the statue within the city commemorations in 1911 and 1940 or when the sculpture was detached from its original column.

Experimental
Since the main objectives of this study was to gather data concerning the metal bulk, its elemental composition, and corrosion products, a multi-analytical strategy was devised. For this purpose, different techniques were used, namely in-situ X-ray fluorescence (XRF), X-ray diffraction (XRD), optical microscopy (OM), and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDS).

XRF
In-situ XRF analyses were performed using a Bruker Tracer III-SD handheld spectrometer equipped with an Rh anode tube and a Silicon Drift Detector (SDD). The operating conditions were set up at 40 kV, 3 µA current with an Al/Ti filter (304.8µm Al/25.4µm Ti), and 60 second acquisition.
Five points were analysed in total. The instrument was positioned by hand in front of the sculpture, benchtop configuration for the analysis procedure. The equipment was placed in direct contact with the sculpture surface, minimising any atmospheric interference ( Figure 2). The XRF analyses were performed without any previous removal of the corrosion layers covering the sculpture. Accordingly, in order to obtain the composition of the bulk metal underlying the corrosion layers, data were processed through a Monte Carlo (MC) simulation algorithm named X-ray Monte Carlo (XRMC) [11]. The XRF/MC protocol is a well-established non-destructive methodology that combines XRF for the spectrum acquisition and XRMC for data processing, which has proved to be fully reliable for the characterisation of both the structure and the elemental composition of each layer from a multi-layered artefact. The approach used in this paper has been described in detail [12][13][14][15], and it has already been applied successfully in the analysis of archaeological and historical metals [16][17][18].

Microscopy
The possibility to obtain a sample from a casting defect area in the front of the sculpture allowed performing scanning electron and optical microscopy.
Scanning electron microscopy was carried out using a HITACHI S3700N interfaced with a Quanta EDS microanalysis system equipped with a Bruker AXS Flash silicon drift detector (129 eV spectral resolution at FWHM/Mn Kα). The following operating conditions were set up: secondary electron mode (SE), accelerating voltage of 20 kV, working distance of ~10 mm and emission current of 90 µm.
Optical microscopy was performed with a Leica DM2500P, equipped with a digital camera LeicaMC170HD, coupled to a computer with the LAS V 4.4.0 software. The structure of the metal was revealed after the sample was mounted in the epoxy resin, and properly grounded, polished, and etched with FeCl3 and HCl in a solution of ethanol, following the methodology described by Scott [19].

XRD
The corrosion products were characterised with a µ-XRD BRUKER D8 Discover System with the DAVINCI design with a Cu Kα source operating at 40 kV and 40 mA and a LINXEYE™ 1-dimensional detector was used. The sample scraped from the oxidized corrosion patina of 16 points of the sculpture was deposited onto a flat zero-background sample holder and irradiated through a 0.6 mm slit. The micro beam was achieved using a Göbel mirror and a 1mm collimator. The angular range (2θ) was scanned from 3° to 70° at a step size of 0.02° with a counting time of 3 s/step. Evaluation of X-ray diffractograms was made by using the routines of the Diffrac. EVA software package and the PDF-2 database files. These areas sampled were selected according to the distinct surface colours, which indicates the likely variability in terms of corrosion compounds (Figure 3).

Metal alloy
Data on the elemental composition are reported in Table 1. The bulk metal of the D. Afonso Henriques' sculpture corresponds to a ternary alloy (Cu+Sn+Zn), in which Cu was alloyed with Sn and Zn. In addition to major elements, Pb, As, and Bi also occur as impurities, not being able to affect the mechanical properties of the alloy significantly. According to the data, Cu ranges from 67.5 to 90.75 wt%. Although copper content in sculptural bronze is generally higher than 80%, some recipes claim that its concentration may vary between 60% and 90% [20][21][22][23][24][25], as also confirmed by the results of analyses carried out on sculptures cast in the 19 th -and early 20 th -centuries [2,[24][25][26][27][28][29][30].
As for Sn and Zn, both show a great variability, ranging respectively from 6.3 to 17.5 wt%, and 0.7 to 12.2 wt%. In each of the points, Sn shows a higher concentration than Zn. Regarding Zn, based on the analyses of a large group of 20 th -century bronzes from Parisian foundries, it has been recently speculated that its concentration in the final alloy could likely depend on the casting process technique. In fact, it has been reported that sandcasting technique would lead to Zn values lower than 5%, while lost-wax casting would allow attaining higher Zn levels [2]. Even though reasons for that are still unknown (i.e., tradition vs. practical casting limitations), the variability of Zn (and in a different extent of Sn as well) in the Afonso Henriques' sculpture could be related to two possible hypotheses.
On one hand, the variability of the main elements could be related to the technique employed to produce this sculpture. In fact, sand-casting technique has been commonly used for large sculptures since it allows to produce the different pieces of the sculpture separately, to be later attached. This means that the Afonso Henriques's sculpture might have been cast on different days, thus explaining the variability in terms of its composition. This variation in the composition of the bronze in diverse areas is not unusual in large outdoor sculptures [24][25].
On the other hand, however, bearing in mind that the Massarelos Industrial foundry was specialised in casting iron, the lack of experience in producing copper-based alloy sculptures cannot be discarded as well. Therefore, the variability of Zn and Sn could be explained as an indicator of the little control over the production process by the workers of the foundry. Figure 4 shows experimental and MC simulated spectra of the point H1. MC detected a film of organic coating likely due to a protective agent. As shown in Figure 4, in the upper layer, barium was also identified. The presence of Ba, along with Fe, on the surface may be related to the chemical composition of the mixture used in the patination of the sculpture, as the original artificial patina is dark brown [31][32]. Iron, in particular, has been detected both in the corrosion layer and in the bulk, where it appears to do not distributed in high concentrations (<0,1%). In comparative terms, the kind of alloy used to produce the Afonso Henriques' sculpture has been frequent since the 18 th -century sculptures, although with differences in the abundance of Zn, Sn, and Pb. In Ganio et al. [30], for example, have recently analysed a group of 23 copper-based sculptures from the Smart Museum of Art at the University of Chicago by Antoine Bourdelle, Félix Charpentier, Édgar Degas, André Derain, Jacques Lipchitz, Aristide Maillol, Henri Matisse, Mahmoud Makhtar, and August Rodin, cast in art foundry active in the first half of the 20 th century in the USA and France. Although cast more recently with respect to the Afonso Henriques' monument, XRF analyses performed on these sculptures also show a great variability of Zn (from 1.2 to 29.2 wt%) and Sn (from 0.6 to 10.0 wt%) that authors ascribe to the different art foundry where they were produced. Similar conclusions were drawn in Pouyet et al. 2019 [2], commenting the XRF analysis carried out on a vast group of 20 th -century bronzes from Parisian foundries.
Finally, it should also be stressed that XRF data are consistent with the analysis performed by EDS in the cleaned sample ( Figure 5), containing 90.75 wt% Cu, 5.2 wt% Zn, 2.9 wt% sn, and 1.15 wt% Pb. Optical and scanning electron microscopy allowed to observe the microstructure of the sample. OM shows a dendritic structure typical of as-cast metal ( Figure 6). The SEM image presents a homogeneous alloy in which Pb stands out in the microstructure of the alloy as globular bright spots scattered across the sample. These spots are interdendritic micro-porosities that remain in the metal when the high melting point elements of the alloy have solidified. These micro-porosities are filled by lead, which is in a liquid state within the solid metal but still at elevated temperatures [33][34]. EDS detected Si, Zn, S and Al due to the occurrence of grain of quartz placed close to the outside of the metal and patina. Due to their composition, it may correspond to particles of the sand used as a mould for casting the different pieces of the sculpture (Figure 7).  Table 2 shows the main corrosion products identified by XRD in the samples analysed. The XRD results reveal that the most abundant phase in the patina samples was brochantite (Cu4(SO4) (OH)6) in 14 of the 16 samples analysed, followed by cuprite (Cu2O), quartz, posnjakite (Cu4SO4(OH)6·H2O), tenorite (CuO), and cassiterite (SnO2). These results coincide with an environment far from the sea without highly polluting industrial activity in the area, which benefits the sculpture with passivating corrosion products [26] These elements are also present in similar sculptures, being their formation usually ascribed to the environment in which they are located [25,34,35]. The presence of cuprite and brochantite in most samples is expected given the rainy environment of Guimarães and the favourable reaction of these compounds to humid environments [36].

Corrosion products
Cuprite and tenorite, primary corrosion products in the bronze passivation process, are located above the surface of the alloy, giving a dark-brown, brown-red or orange tone to the metal [25,37]. In the sculpture of D. Afonso Henriques, while cuprite has been detected in most of the areas analysed, tenorite was only found in the sword. This sculpture element has been the object of various interventions due to episodes of vandalism, which meant that it was subject to abrasion, welding and possibly other undocumented actions.
Posnjakite and brochantite are produced by the reaction of cuprite patina with sulphates and humidity present in the environment. The posnajkite evolves into brochantite losing the water of its composition [25,38]. Brochantite settles as a second layer of the patina, giving the metal a bluish-green hue [39,40] (Figure 8). Cassiterite, a natural product of tin corrosion was only found in two samples collected from the front of the sculpture. An unidentified Sn and Cu compound was also detected. Like cuprite and tenorite, cassiterite is placed on the alloy surface, sometimes covered by these two compounds [25][26]. The presence of quartz in the patina is generally connected to airborne particles. This mineral can increase the volume of the patina and reduce the adhesion of this to the metal surface [26].
Finally, SEM reveals other types of corrosion on the sample (Figura 9A). Pitting, intergranular and stress-crack corrosion are evident, both in the patina and in the bulk metal. These forms of deterioration are typical in urban and marine bronzes and are predominantly studied in archaeological objects [41][42][43][44] (Figure 9B).

Conclusions
The metal of the sculpture of D. Afonso Henriques (1887) was analysed for the first time. It is known that it was produced by sand-casting technique, and it has now been possible to conclude that it was made of an alloy consisting of Cu, Sn, and Zn, with Pb, Fe, As, and Bi as impurities. This type of alloy resembles other pieces cast in Europe from the 18 th to the early 20 th century. The sculpture, which has been in the open air since 1887, has a patina befitting an atmosphere with no significant polluting industries or marine environment in the vicinity.
Brochantite, a sulphate mineral, represents a stable top layer patina that occupies the surface and provides the sculpture with a bluish-green hue.
It is worth noting the increase of Sn and reduction of Cu that gradually occurs from the interior of the bulk metal towards the exterior. This alteration process has been likely produced by creating tin and copper oxides on the surface and the subsequent leaching of the latter element due to rainfall events characteristic of northern Portugal.
Although the techniques and materials used to develop the artificial patina are unknown, the presence of barium and iron on the surface may suggest the sculpture was covered with a preparation containing these elements.
The information obtained in this study is essential for determining the state of conservation of the monument and providing preliminary data on its manufacture. It additionally provides a first insight into the initial casting techniques used in northern Portugal at the time, as this was the only bronze sculpture made by an industrial foundry in this part of the country.  Data Availability Statement: Data generated and analysed during this study is available from the corresponding author upon reasonable request.