Introduction

One of the most common and effective methods for assessing the condition and safety of metal items of cultural heritage significance and artwork involves the investigation of production techniques, welding lines, casting defects, and previous repairs via visual observation, photography, optical examinations such as microscopy, as well as X-rays and gamma rays. In addition, to verify the types of metal damage and their causes, various scientific analysis methods are employed to identify corrosion byproducts.

Analytical methods such as X-ray diffraction (XRD), micro-Raman spectroscopy, and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM–EDS) are commonly used in indoor investigations to assess the causes and extent of damage to bronze; these methods typically involve destructive sampling. However, the scientific analysis and diagnosis of damage and surface corrosion products in indoor environments poses significant practical challenges. Furthermore, outdoor bronze sculptures are typically heavy and large, making them difficult to move. The limited sampling of artwork and cultural heritage items necessitate the application of non-destructive and non-invasive analytical methods that can be measured in outdoor environments. Because sampling is conducted in small quantities, results are limited to a partial understanding and interpretation of the analysis target, with further disadvantages of analysis time and cost.

Non-destructive analysis methods such as chromaticity and portable X-ray fluorescence (XRF) analysis have been utilized in the preservation of cultural heritage pieces and artwork owing to their rapid and easy measurement, as well as the ability to compare and analyze statistical data. Characterizing and analyzing the unique color and alloy composition of metal artifacts based on the type of bronze corrosion products could provide valuable information in the form of primary on-site data without requiring specimen sampling or moving of the target artwork. Chromaticity analysis has utilized in research on the color change and characterization of patinas [1,2,3,4,5,6,7,8,9,10]. Portable XRF has been utilized for the identification and quantification of bronze patinas and layers [11,12,13,14,15,16]. Research on corrosion products, previously centered around indoor laboratories, has since been applied to situ investigations using portable Raman spectroscopy [16,17,18,19,20,21,22,23,24].

Many studies have reported that sulfide and chloride are among the most common and important corrosive byproducts [5, 25,26,27,28,29,30,31,32,33]. Chloride patina can lead to pitting and bronze disease, making its early identification crucial [34]. In our previous studies, we conducted artificial patina corrosion experiments to identify various types of corrosion and their correlation among outdoor bronze sculptures and to obtain standard data on sulfide and chloride patinas, which are major air pollutants in urban-industrial and marine environments [34, 35].

This study aimed to develop a prediction model that can identify the type of corrosion and quantify the amount of corrosion on large, heavy bronze sculptures in outdoor environments using non-destructive analysis methods. The objective of the study was to quickly diagnose the corrosion type and preservation status of outdoor bronze sculptures, providing initial data for establishing preservation plans. Using the analysis results obtained from artificial patina corrosion experiments [34, 35], we examined the corrosion type and applicability of the patinas of 16 bronze sculptures exposed to outdoor environments for over 20 years. Specifically, data obtained from spectrocolorimeters, portable XRF, and portable Raman spectrometers, commonly used in the preservation of metal cultural heritage items and artwork, were compared and analyzed.

Research method

Research elements

We selected 16 bronze sculptures made from quaternary bronze (Cu–Zn-Sn–Pb) alloys from the National Museum of Modern and Contemporary Art (MMCA) collection. These sculptures were chosen as their small differences in alloy compositions and have been exposed to outdoor environments for an extended period of time (Table 1). Previous research on bronze corrosion mechanisms focused on binary (Cu-Sn) and ternary (Cu-Sn–Pb) bronzes. However, the more than 500 bronze sculptures produced in Korea and other countries since the twentieth century are predominantly made from quaternary alloys (Cu–Zn-Sn–Pb) [36,37,38,39,40,41,42].

Table 1 Bronze sculptures from the MMCA collection exposed to outdoor environments for over 20 years
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Analysis method

Microscopic observation

Portable microscopy (DG-3, Scalar, JP) and digital microscopy (RH-2000, Hirox, JP) were used to observe the surface morphology and condition of the outdoor bronze sculptures and artificial patinas.

Chromaticity and reflectivity analysis

Chromaticity and reflectivity analyses were conducted using a spectrocolorimeter (CR-400, Minolta, JP) in specular component excluded mode with a measurement diameter of 3 mm. The surface reflectivity was calculated as the average of five measurements taken in the visible light range of 350–750 nm. Chromaticity and reflectivity were measured at 2–4 points on the bronze sculptures by selecting areas with colors different from the base metal.

Portable XRF analysis

Non-destructive chemical composition analyses were conducted using a portable XRF spectrometer (Alloy Plus, Olympus Delta, USA). The analysis was conducted under the following conditions: The collimator of the detector is 15 mm in diameter and Resolution ranges from 145 to 316 eV. 40 kV with Alumina filter for 10 s, and 8 kV without a filter for 30 s. The concentrations (ppm, wt%) of the main constituent elements of bronze, Cu, Zn, Sn, and Pb were determined and converted into percentages for the four elements.

Portable Raman spectroscopy

The Raman spectra were acquired using a portable Raman spectrometer (I-Raman Prime, B&W Tek, USA). 6mW diode laser at a wavelength of 532 nm was applied as an excitation source, and the output power of the laser at the exiting probe was 35mW, nominal. The spectral range was between 150 to 4200  Cm−1, with a resolution of 7.3 cm−1.

Atmospheric environment of bronze sculptures

Air pollutant concentration and rainwater acidity are factors that influence the corrosion of outdoor bronze sculptures. Thus, air pollution levels (1990–2022) and major ion concentrations of acid depositions (1999–2022) at the MMCA in Gwacheon, Gyeonggi-do, South Korea are shown in Fig. 1 [43]. This could indicate the correlation between the corrosion of outdoor bronze sculptures and atmospheric pollutants.

Fig. 1
figure 1

Air pollution status and major ionic component concentrations of acid depositions of the environments surrounding the outdoor bronze sculptures: ppm (SO2, NO2, O3, and CO), and mg/l (acid deposition)

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Regarding the air pollution status, SO2 levels have shown a consistent decreasing trend since 1990 with some levels exceeding the standard of 0.02 ppm in the early 1990s. However, the steady decline reached 0.003 ppm in 2022. Although NO2 levels have fluctuated around the standard value of 0.03 ppm, a decreasing trend has been observed since 2016. Over the past 30 years, O3 has consistently remained within the standard value of 0.06 ppm; however, an overall increasing trend has been observed. CO has exhibited a general decreasing trend within the standard range.

Regarding the major ionic concentrations in annual acid depositions, anions such as SO42−, NO3-, and Cl− become strongly acidic when combined with hydrogen ions. These ion concentrations have shown a continuous increasing trend since 1999 when measurements were first taken but started to decrease overall in the early to mid-2010s. Furthermore, the average pH of rainwater measured from 1990 to 2022 in Seoul, located approximately 10 km from Gwacheon, Gyeonggi-do, was 4.8, indicating acidification, with pH 5.6 representing neutrality [43, 44].

Results and discussion

Microscopic observation

The color of the patinas on the surfaces of the 16 bronze sculptures that had been exposed to outdoor environments for over 20 years were all shades of green and classified into three main groups (Fig. 2a).

Fig. 2
figure 2

Microscopic observations of outdoor bronze sculptures’ patinas and artificial patinas: a patina of bronze sculptures, b sulfide patina, and c chloride patina

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The type I dark green patina layers (sculptures SC1, SC3, SC4, SC6, SC8, SC9, S11, SC12, SC14, SC15) were observed to have mostly flat and dense surfaces. The type II patina layers were light green (sculptures SC5, SC7, SC10, SC13, SC16), and the type III patina layer was close to a light blue patina (sculpture SC2). In general, patina formed in urban atmosphere is known to have less porosity and higher adhesion compared to marine patina [45], and differences in the condition of the surface were also confirmed depending on the color of the patina [34, 35].

A comparison between the surface conditions of the outdoor bronze sculptures’ natural and artificial patinas revealed differences in density and other characteristics. The reason for these differences was determined to be the weak adhesion and porosity between the base layer and artificial patina generated over a short period of time. Furthermore, the outdoor bronze sculptures had been exposed to an acidic environment with an average pH of 4.8 and atmospheric pollutants such as SO2 and NO2 for over 20 years, as confirmed by the current levels of air pollution and acid depositions in the area. The corrosion type and patterns were anticipated to be similar owing to the similar alloy compositions and close proximity within a 300-m radius; however, differences in the color and surface conditions of the patinas were observed. This is likely owing to various parameters such as the surface condition at the time of production, solubility of the corrosion products, presence of protective measures (e.g., surrounding vegetation), different corrosion conditions (e.g., bird droppings, pollen, and pine pollen), preservation treatments (wax coating), and impurities within the alloy (segregation).

Chromaticity and reflectivity analysis

The chromaticity and reflectivity analyses revealed that the patinas on the surfaces of the 16 bronze sculptures were all in the green color range and could be classified into three main groups. The change in the b* value was relatively small compared to the significant changes observed in the L* and a* values. Based on the analyses of actual sculptures, the chromaticity and reflectivity as well as the applicability of sulfide and chloride artificial patinas were evaluated through artificial patina corrosion experiments (Figs. 3 and 4).

Fig. 3
figure 3

Chromaticity comparison between outdoor bronze sculptures and artificial patinas: The chromaticity values of the bronze sculptures’ patinas are in the chromaticity range of the sulfide patina

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Fig. 4
figure 4

Reflectivity comparison between outdoor bronze sculptures and artificial patinas: The reflectivity values of the bronze sculptures’ patinas are in the reflectivity range of the sulfide patina (green)

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The sulfide artificial patina closely resembled the surface patina color of bronze sculptures exposed to outdoor corrosion (Fig. 3). The L* value gradually increased from 53.50 to 58.86, indicating a brightening effect. The analysis of the b* values representing the yellow–blue color showed no significant changes after the initial stage (S5), but gradually shifted toward the blue color. Specifically, the analysis of the a* values representing the red–green color revealed a color shift similar to that of outdoor bronze sculptures (-8 to 0), and the L* values were also within the brightness range of the bronze sculptures. The correlation between the growth of patina and chromaticity of the outdoor bronze sculptures was confirmed to progress from type I to type II to type III by the chromaticity and corrosion behaviors of the artificial sulfide patina.

The chloride artificial patina showed a significant difference in L* values of 20–40 compared to the artwork, with a* and b* values concentrated in the ranges of -18 to -15 and -3.6 to 6.4, respectively, indicating differences from the actual patina colors of the sculptures.

The reflectivity analysis yielded similar results (Fig. 4). The reflectivity of the outdoor bronze sculpture revealed low reflectivity across all wavelength ranges, gradually increasing in the blue (380–500 nm) and green (500–565 nm) spectra. However, a decrease in reflectivity was observed as the transition was made from the yellow (565–625 nm) to the red (625–750 nm) spectrum. The patina of alkaline sulfide also exhibited a similar spectrum to that of the sculpture. The blue–green spectrum increased, whereas the yellow–red spectrum decreased consistent with the corrosive behavior. The artificial chloride patina exhibited higher reflectivity than the artificial sulfide patina in all wavelength ranges for the outdoor bronze sculptures. Particularly, a reflectivity difference of 40–50% or higher was observed in the blue–green spectrum.

The above findings confirm the possibility of identifying corrosion type for bronze sculptures using the chromaticity and reflectivity of artificial patinas. Chromaticity was found to be particularly suitable for the quantitative comparison with other colors, while reflectivity was useful for identifying the corrosion type of specific colors. However, the wide distribution values of brightness and reflectivity of outdoor bronze sculptures also showed discrepancies (L* was approximately 12–17) for the artificial sulfide patina. The differences in surface conditions of natural and artificial patinas on outdoor bronze sculptures are influenced by factors such as dust, pollen, and atmospheric pollutants, which affect the brightness and reflectivity of the sculptures. In addition, the optical characteristics of the patina surface can be altered by light diffraction and absorption, as well as interference effects such as the patina layer thickness, refractive index, and incident light.

One of the crucial factors in distinguishing bronze patinas is known to be the L* value [10]. According to the optical properties of materials, an object's reflectivity generally influences its color and brightness. When an object exhibits higher reflectivity across all wavelengths compared to the reference, it appears brighter [46]. The artificial chloride patina exhibited a brightness (L*) value more than 20 higher than that of the outdoor bronze sculpture and artificial sulfide patina, with high reflectivity observed across all wavelength ranges. In particular, a difference of 40–50% or higher in reflectance was observed in the blue–green spectrum. This suggests that brightness (L*) can be used in conjunction with reflectivity to identify the type of corrosion of a bronze patina.

Portable XRF analysis

The chemical compositions of the patina surfaces of the 16 bronze sculptures were classified into three main groups. Based on the analysis of actual sculptures, the results of the corrosion experiment components of sulfide and chloride artificial patinas were compared, and the applicability was assessed using a Zn-Sn–Pb ternary diagram (Fig. 5).

Fig. 5
figure 5

Comparison of the chemical composition of outdoor bronze sculptures and artificial patinas

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The artificial sulfide patina exhibited a tendency similar to the alloy composition of bronze sculpture type I. When compared and analyzed based on bronze sculpture type I, the standard deviations of the elements were found to be 2 wt% for Cu, -1 wt% for Zn, -0.5 wt% for Sn, and -0.5 wt% for Pb. We attempted to observe changes in the alloy elements resulting from artificial patina corrosion behavior in a ternary system; however, no significant changes were observed in the ternary diagram. This suggests that significant compositional changes are unlikely in a short period of time (2400 h).

The chemical compositions of artificial chloride patinas were broadly classified into three groups: type A (Cl1–Cl3) for early corrosion, type B (Cl4–Cl25) for intermediate corrosion, and type C (Cl26–Cl50) where Zn was not detected. Specifically, type A was distributed along the boundary of the bronze sculptures type I and type II, whereas the other groups exhibited differed significantly from the sculpture composition.

The chemical composition of the outdoor bronze sculptures showed an overall decrease in Cu and Zn contents, whereas the Sn content exhibited an increasing trend. However, a consistent trend was difficult to demonstrate owing to factors such as alloy composition, corrosion degree, surface condition, and various environmental parameters. Furthermore, the XRF analysis from the surface, depending on the chemical element considered due to the Beer-Lambert law the measurement does not concern the same depth. In general, measurement of the elements Cu and Zn is done over a depth of approximately 200 to 300 µm while that of Sn and Pb is done over more than 1 mm [45]. However, in this study, we were not able to measure the thickness of the patina layer. Notwithstanding, Cu exhibited distinct differences in composition from the other elements according to the corrosion behaviors of the sulfide patina (87.47 → 89.04 wt%) and chloride patina (88.8 → 78.2 wt%). Hence, the compositional differences according to the corrosion behavior of Cu are likely highly applicable in identifying the corrosion type of sulfide and chloride patinas.

The identification and quantification of patina layers are known to be achievable in the range of 20–100 μm using p-XRF, depending on factors such as the alloy of bronze and the degree of corrosion [11, 14]. In particular, owing to environmental influences, the patina layer on bronze artifacts exposed to outdoor conditions is expected to be thin and less complex in terms of corrosion mechanisms than artifacts with thick patina layers. Therefore, portable XRF analysis is expected to be applicable.

Portable Raman spectroscopy

Before analyzing the corrosion products on the patina surfaces of the outdoor bronze sculptures in the field, previous studies have already conducted preliminary analyses on the corrosion products of sulfide and chloride artificial patinas [34, 35]. The potential applicability of portable Raman spectroscopy was confirmed by comparing the analysis results of the corrosion products on the surfaces with artificial patinas using XRD, bench-top Raman microspectroscopy, and portable Raman spectroscopy.

An analysis of the portable Raman spectroscopy on the patina surfaces of the 16 outdoor bronze sculptures revealed the presence of cuprite, brochantite, and antlerite (Fig. 6). Raman shifts at 214, 216, and 218 cm−1 were detected in eight sculptures (SC1, SC4, SC7, SC8, SC10, SC12, SC13, SC14), closely matching those of cuprite (Cu2O). The similarity in Raman shifts and peak shapes confirmed the presence of cuprite.

Fig. 6
figure 6

Portable Raman spectroscopy graphs of outdoor bronze sculptures

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Raman shifts of 384, 480, and 973 cm−1 were detected in 12 sculptures (SC1, SC2, SC4, SC6, SC7, SC9, SC10, SC11, SC12, SC14, SC15, SC16). These were confirmed to be brochantite (Cu4SO4(OH)6), as their Raman shift closely matched that of alkaline sulfide, and their peak shapes were also similar. In particular, cuprite and brochantite were detected together in 6 sculptures (SC1, SC4, SC7, SC10, SC12, SC14).

In two sculptures (SC11, SC15), Raman shifts of 420, 973, and 1047 cm−1 were detected, closely matching the Raman shift of the basic sulfide antlerite (Cu3SO4(OH)4). The similarity in peak shape further confirmed the presence of antlerite. In particular, the characteristic peaks (971 and 973 cm−1) of antlerite and broochantite are similar. Therefore, the two compounds were classified by Raman shift of 1044 and 1047 cm−1.

Additional verification and research are deemed necessary for two sculptures, SC3 and SC5, owing to difficulty in accurately identifying the compounds present in these sculptures.

These results are consistent with previous visual and microscopic observations, chromaticity and reflectivity, and portable XRF analysis results. Thus, we confirmed that the identification of corrosion type and the corrosion products of a bronze sculpture is possible using portable Raman spectroscopy.

When bronze is exposed for decades to the outdoor environment contaminated with SO2, the SO2 dissolves in the surface moisture and reacts with the bronze to form alkaline sulfides. The most commonly observed alkaline sulfide in outdoor bronze sculptures is green brochantite. Brochantite is typically stable under acidic conditions (pH 3.5–6), while antlerite is commonly found in regions with low acidity (pH 3.5 or lower) and high concentrations of sulfates.

In actual outdoor environments, acidity from rain, snow, and fog can dissolve copper corrosion products on the outermost layer and promote the alteration of minerals. Two examples are brochantite and antlerite. When the acidity of the water membrane where corrosion occurs increases (pH 3.5 or lower), brochantite dissolves and antlerite is formed. However, the formation of antlerite requires the prolonged maintenance of acidic conditions in the water membrane layer over a significant period. A short duration of acidic conditions can prevent the growth of antlerite and allow the surface recrystallization of brochantite. Thus, antlerite has been reported to form first in the lower parts or relatively less exposed areas of outdoor bronze sculptures where the moisture evaporation rate is lower [25, 29, 47]. Therefore, the presence of antlerite indicates a higher level of contamination, requiring increased caution regarding potential damage.

Conclusion

This study aimed to develop a prediction model that can identify corrosion type and quantify the amount of corrosion on large, heavy bronze sculptures in outdoor environments using non-destructive analysis methods. The corrosion types and preservation status of outdoor bronze sculptures were quickly diagnosed to establish a preservation plan. We investigated the corrosion patterns of artificial patinas through experimentation and evaluated the identified corrosion types and applicability for 16 bronze sculptures exposed to outdoor environments for over 20 years. Specifically, data obtained from spectrocolorimeters, portable XRF, and portable Raman spectrometry, commonly used for the preservation of metal cultural heritage artifacts and artwork, were compared and analyzed. The results are summarized as follows.

The 16 bronze sculptures exposed to outdoor environments for over 20 years were classified into three groups based on patina chromaticity, reflectivity, and chemical composition. The corrosion characteristics were identified as copper oxide and sulfide patinas, such as cuprite, brochantite, and antlerite, commonly formed in urban-industrial atmospheric environments.

According to the evaluation of the chromaticity, reflectivity, and applicability of the outdoor bronze sculptures’ patinas and artificial chloride patina, the colors of the bronze sculptures’ patinas differed significantly. In contrast, the artificial sulfide patina was found to closely resemble the patina color of outdoor bronze sculptures owing to its corrosion behavior. Specifically, the a* value representing red–green showed a similar color variation to that of outdoor bronze sculptures, and the L* value also fell within the brightness range of bronze sculpture. Furthermore, the density of the patina surface decreased, while its porosity increased according to the corrosive behavior. We also confirmed the relationship between the patina growth on outdoor bronze sculptures and its chromaticity. Specifically, a turning point existed where 20% of the reflectivity in the yellow–red (565–750 nm) range could identify cuprite and brochantite. Furthermore, the chromaticity L* value was found to be an important factor in identifying corrosion type along with reflectivity. Some discrepancies in brightness and reflectivity values were observed depending on the surface condition and level of contamination of the patina. However, the chemical composition changes of the patina corresponded to the formation of corrosion products, indicating a high potential for identifying corrosion type.

The result of using portable XRF spectroscopy to assess the applicability of chemical composition revealed that, some of the bronze sculptures (Type I) showed a similar tendency in alloy composition to sulfide artificial patina. However, no significant changes were observed in the ternary diagram. This suggests that significant compositional changes are unlikely in a short period of time (2400 h). In addition, a consistent trend was difficult to demonstrate owing to factors such as alloy composition, corrosion degree, surface condition, and various environmental parameters. Nevertheless, the element Cu exhibited lower error rates than other elements, indicating that differences in composition owing to the corrosion behavior of Cu could be highly applicable in identifying the type of corrosion of sulfide and chloride patinas.

Upon evaluating the on-site applicability of portable Raman spectroscopy, basic sulfides such as brochantite, antlerite, and cuprite were detected. The XRD and bench-top Raman spectroscopy results of the previous artificial patina corrosion experiments confirmed the presence of brochantite and cuprite in the sulfide artificial patina, indicating a high potential for the application of portable Raman spectroscopy.

This study confirmed the feasibility of using portable non-destructive analytical equipment in outdoor environments to identify corrosion types and quantify corrosion amounts for large, heavy bronze sculptures. The findings of this study can provide important information as primary data in outdoor locations and are expected to be utilized for promptly diagnosing the types of corrosion affecting bronze sculptures in outdoor environments and their conservation status to establish conservation treatment plans.