Preliminary visual examination of the Morse sculpture enabled conservators to identify the main ongoing conservation issues, which were mostly related to corrosion and to the presence of one or more degraded organic coatings. Over time, the statue’s surface has suffered from severe corrosive attack. Pitting and loss of sculptural surface details (Fig. 2) were caused by the exposure to atmospheric pollutants such as acid rain and chlorides from deicing salts and the marine environment, in addition to dry deposition of biogenic and anthropogenic pollutants such as NO2, SO2, and carboxylic acid. The exposed surfaces have remained stable since being regularly treated with cleaning interventions and protective coatings beginning in the 1990s. However, localized patches of virulent, irregularly shaped pustules of active corrosion were contributing to progressive surface loss (Fig. 3a) and associated coating failure in the deep recesses of the figure’s long cloak (Fig. 3b). The surfaces within the recesses are so well protected that they do not benefit from washing during typical rain events or associated run off. Morse’s sculptural geometry may explain the localization of corrosion phenomena in those recessed and protected locations where elevated chloride concentration accelerates surface loss [15]. The aged and degraded coatings had likely cross-linked and was otherwise admitting moisture and corrosive agents directly onto the bronze substrate, and was no longer providing adequate protection to the sculpture.
Analysis of Organic Coatings
The organic coating was investigated to determine its constituent materials and the possible presence of pigments. In order to do so, a sample (S3) was removed from an area of the sculpture underneath the telegraph, below the figure’s proper left thumb, and analyzed by Py-GC/MS. All chromatograms obtained with and without TMAH derivatization appeared to contain series of alkanes organized in a specific distribution that is characteristic of mineral waxes (Fig. 4). From a conservation perspective, mineral waxes are known for having a limited lifetime, ranging from 2 to 5 years, and for being reversible [16]. Besides, they are easily cleaned with water, which makes them suitable for a low cost and low maintenance application. However, the sample examined was found to contain also high relative amounts of acrylics, mostly methyl methacrylate and acrylic acid ethyl ester. The presence of these compounds is consistent with the use of Incralac® [17], which is, in fact, a product commonly used in the past and reported to have been applied to the Morse statue in the early 1990s according to the available conservation records. Acrylic coatings have a longer lifetime compared to mineral waxes, and they also show good reversibility properties [16]. Acrylic lacquers are coupled with a sacrificial wax topcoat that is renewed yearly in the case of Morse. Since 2003, two types of wax topcoats have been used by CPC’s conservators: 1) Boston Polish™, also known as “butchers” paste wax, a carnauba-based wax, in use from 2003 to 2006; 2) Renaissance microcrystalline wax™, a mixture of Cosmoloid H80 and BASF A wax [18–19], used from 2007 to date for protecting outdoor bronze sculptures in Central Park.
In terms of coloring, verbal accounts and visual observation suggested that the Morse sculpture might have been coated with pitch at some early stage, although there are no written records of this intervention. The color of organic coatings in bronze sculptures is also sometimes obtained through the addition of bitumen, which might lend the coating a dark or brown hue [20]. Accordingly, visual inspection indicated that, possibly, very low amounts of pitch or bitumen residues may have been present on the back of the bronze, although, in this case, no samples were removed to verify this hypothesis, given the small dimension of the areas in question and in order to preserve the sculptures’ integrity and appearance. However, an unexpectedly high amount of iron (Fig. 5) was detected by pXRF analysis on several locations of the surface still covered by the organic coatings, suggesting the use of iron oxide/hydroxide pigments as coloring agents in the acrylic layer. Comparisons with spectra collected on areas of bare metal confirmed the exclusive presence of iron, and the absence of other elements, in such organic layer.
Analysis of Corrosion Products
In areas of the Morse sculpture that were less exposed to the atmospheric agents and not covered by the protecting coatings, various shades of green corrosion products were observed and sampled for analysis. XRD detected several compounds mostly attributable to the following categories: sulfates, sulfides, chlorides, and oxides. The hydrous copper sulfate mineral antlerite (Cu3(SO4)(OH)4), found in all samples examined, appears to be the main sample constituent among those present, sometimes in association with minor amounts of copper trihydroxichlorides such as atacamite or clinoatacamite (Cu2(OH)3Cl) (as in the case of samples S1, S2, S4, S5). In addition, brochantite (Cu4SO4(OH)6) and cuprite (Cu2O) were identified in samples S5 and S7, the latter also containing copper sulfide and gypsum (Fig. 6). All these products are described in the literature as bronze disease typically related to copper alloys [21], either originating from the reaction of copper with water or as a result of its interaction with atmospheric pollutants (leading to the formation of chlorides and brochantite, respectively).
Antlerite is frequently found as a main constituent of patinas in outdoor sculptures [22], while less common might appear the finding of chlorides. Due to the decrease of rainwater pH since the mid-20th century, antlerite also became a widespread component of corrosion crusts, mainly in partially exposed or sheltered surfaces [21]. Robbiola et al. [16] observed that brochantite often transforms into antlerite in outdoor sculptures, hypothesizing that the low pH rainwater might be the enhancing factor of this reaction. However, it has been demonstrated that degradation of bronze outdoor sculptures was more affected by dry deposition rather than acid rain in the 1980s [1]. In New York City, levels of SO2 have constantly decreased especially in the last decade [23], with the consequence that dry deposition should no longer have substantial effects, while the corrosion rates are expected to return to pre-industrial levels in the future [1].
Considering that Central Park is relatively close to the ocean and therefore subjected to its atmospheric corrosion environment, and, above all, taking into account the enormous amount of sodium chloride distributed on the roads during wintertime, the detection of copper chlorides on the Morse statue, should not surprise. The sculpture had originally been sited on the Mall in Central Park, a pedestrian promenade, where it remained for more than 100 years. In 1988, the statue was relocated to its current location at the East 72nd Street entrance to the Park on the South side of the Drive roadway and in proximity to Fifth Avenue, where the exposure to ice melting salts is substantial during the winter months. In fact, chloride ions that separate from sodium as the ice melts are extremely corrosive. It seems plausible that the water enriched with these ions could become aerosolized by the constant traffic whisking by and deposited onto the bronze. Thickett and coauthors [18–19] have demonstrated that water may be retained at the interface between the superficial layer of Renaissance wax and the coating underneath, further enhancing the corrosive attack. On ferrous metals, like steel, the prolonged exposure to this trapped moisture after the protective period of the coating might increase the rate of corrosion. A similar process cannot be ruled out for copper-based alloys.
Monitoring of Laser Cleaning
One of the main goals of the present study consisted in monitoring the laser cleaning intervention on the Morse statue by qualitatively analyzing the surface elemental composition before (pXRF campaign 1) and during (pXRF campaigns 2 and 3) the process. To achieve this goal, analysis was repeatedly carried out on eighteen select locations prior to cleaning and throughout a few subsequent phases of treatment to monitor changes on the target areas. Following a comparison between measurements recorded on the sculpture’s external surface and reference ones collected on areas of bare metal on the pedestal, sulfur, chlorine, and iron were selected as the ideal candidate elements to monitor the progress of the laser cleaning. These elements, in fact, are considered respectively indicative of the presence of sulfates and sulfides (e.g. antlerite, brochantite, copper sulfides), chlorides (e.g. atacamite, clinoatacamite, paratacamite), which are the main corrosion products constituting the patina, as well as the coating, whose brown color, as explained above, is likely due to iron-containing pigments.
Following the initial phase of the cleaning, pXRF measurements showed relatively high levels of iron in all the spots analyzed, with the exception of a reference reading collected from an area of bare metal. Iron tends to decrease from measurements acquired during pXRF campaign 1 to those recorded in campaign 2, while it is found in trace amounts on the uncoated bronze (Fig. 5). This likely indicates that the occurrence of this element might be limited to the superficial organic coating, and, therefore, associated with the presence of iron oxide/hydroxide pigments in such coating, as mentioned earlier. Minor amounts of lead were also detected on locations yet to be cleaned, with a noticeable decrease in spectra collected upon treatment during campaign 2. This observation, along with the absence of lead in the reference measurements acquired from the bare metal, suggests that this element, whose presence was limited to the uppermost portion of the sculpture’s stratigraphy and thus attributable to external causes, may be interpreted as a residue of the atmospheric pollution deposition. Interestingly, the amount of greenish corrosion products, namely chlorine-based and sulfur-based compounds, was found to decrease upon initial lasering, while an increase followed by a small decrease was noted following the second and third phases of treatment. The increased concentrations of chlorine and sulfur after cleaning phase 2 indicate that the laser cleaning performed up until that point might have exposed a deeper portion of the corroded layer (Fig. 5). Another possible explanation for the observed increase of these elements’ peak intensity might lie in the practice of misting the sculpture with distilled water after each lasering session. This process activates entrained chlorides and precipitates them onto the surface, from which they are ablated in subsequent cleaning sequences. As initial pXRF analyses were carried out after misting but before the following cleaning, it is plausible that re-precipitated chlorides were measured. A final phase of overall laser cleaning was applied to add variance of the laser’s angle of attack. This was especially important for eradicating the deeply pitted areas where minute undercuts partially obscured direct access to those surfaces. The goal of this additional cleaning step was to further reduce the presence of powdery corrosion products and tenaciously adhered coating residue that may negatively affect proper adhesion of the new coating.
To evaluate the distribution and thickness of the various corrosion layers within the sculpture’s stratigraphy, and to determine the exact composition of the metal alloy, a small sample removed from the rear of the Morse statue was mounted as a cross section and analyzed by SEM/EDS (Fig. 7). Results on the composition of the sculpture’s constituting copper alloy (layer 1), calculated by means of EDS, are reported in Table 2. In detail, the relative amounts of copper (Cu) and tin (Sn) detected, alongside the observed zinc (Zn) content, are consistent with the composition of a ternary alloy and are very similar to gunmetal bronze (also known in the United States as red brass). This alloy was first identified in a 13th -century English memorial sculpture and later used for casting cannon. Its utilitarian characteristics include ease of casting in detail, toughness, resistance to corrosion, and machinability. The frequent use of this alloy to cast statuary in the United States in the mid-19th century may be attributed to the prevalence of the English emigrant group and its craft traditions, or even possibly to an expedient coincidence. For instance, the Ames Manufacturing Company of Chicopee, Massachusetts, is a prominent example of an American foundry concurrently producing military ordnance and “bronze” statuary during that period.
Table 2
Compositions of the Morse sculpture’s corrosion layers and zinc sulfides in the main copper alloy obtained from SEM/EDS analysis of a cross section and expressed as normalized wt%. Numbers in brackets refer to layers in Fig. 7.
| | S | Fe | Cu | Sn | Zn | SiO2 | Al2O3 | K2O | P2O5 | SO3 | Cl | FeO | CuO | SnO2 |
| Copper alloy (1) | | | 91.0 | 6.4 | 2.6 | | | | | | | | | |
| Red corrosion layer (2) | | | | | | | | | | 0.8 | 1.0 | | 85.5 | 12.5 |
| Green corrosion layer (3) | | | | | | 0.2 | | | 0.3 | 18.1 | 0.1 | 0.5 | 55.8 | 25.1 |
| Very thin green corrosion layer (4) | | | | | | 1.6 | 0.9 | 0.2 | 0.3 | 23.5 | 2.2 | 0.5 | 70.3 | 0.6 |
| Copper sulfide | 21.4 | 0.2 | 78.4 | | | | | | | | | | | |
| Zinc sulfide | 33.1 | 0.3 | 4.4 | | 62.2 | | | | | | | | | |
| Zinc sulfide | 32.9 | 0.6 | 5.7 | 0.3 | 60.5 | | | | | | | | | |
| Zinc sulfide | 33.1 | 0.4 | 4.2 | 0.2 | 62.1 | | | | | | | | | |
| Zinc sulfide | 34.1 | 0.5 | 4.9 | | 60.6 | | | | | | | | | |
| Zinc sulfide | 33.6 | 0.4 | 4.0 | | 62.0 | | | | | | | | | |
EDS analysis showed the additional presence, in the sculpture’s ternary alloy, of small particles of less than 10 µm in size, with irregular shapes, scattered throughout the metal matrix, which were identified as zinc sulfides. These compounds appear to be sometimes associated with copper sulfides. Exact compositions of both zinc and copper sulfides are reported in Table 2. Regarding the pitting phenomenon also observed in the Morse sculpture, one must consider that the presence of pits in copper-based alloys is often related to the dezincification of the alloy [21]. Here, however, this hypothesis is unlikely, as red brass displays less of a proclivity for this type of deterioration than other alloys, in particular when protected by a thin layer of copper oxides, such as cuprite [24]. In the present case, analysis of the corrosion layers shows a clear transition from an underlying cuprite-based layer to a more superficial region that is mostly composed of copper sulfates, as indicated in Fig. 7. The first corrosion layer observed (layer 2, Fig. 7), showing red in the optical microscopy image, was found to be mainly composed of cuprite and enriched in tin. A second corrosion layer (layer 3, Fig. 7) above that, appearing green under polarized light, is characterized by the presence of copper sulfates and is richer in tin than layer 2. Traces of an additional, very thin, green corrosion layer (layer 4, Fig. 7), which contains a mixture of copper chlorides and dirt, are still visible on the surface. These data suggest that the corrosion process might have induced a preferential dissolution of copper and zinc linked to the presence of oxygen within the whole thickness of the corroded layer, as shown by the X-ray elemental maps of copper, chlorine, tin, and sulfur within the corrosion layer (Fig. 7). In outdoor conditions, the initial formation of a reddish-brown layer of cuprite is typically followed by the formation of different additional compounds, as also confirmed in the present study: copper basic sulfates, such as antlerite and brochantite, can be found on bronzes exposed in urban environments, while copper hydroxychlorides, such as atacamite and paratacamite, are usually formed in chloride-rich environments [9 and references therein].
Additional Conservation Treatment
As some of the Morse statue’s surfaces were not easily accessible for laser cleaning, an additional treatment using propane-fueled torches to heat the surface and volatilize the remnant coating and corrosion products was performed in the deepest pits and recesses of the sculpture. Following the technical guidelines of the U.S. General Services Administration [25], two applications of benzotriazole (BTA), an effective corrosion inhibitor, were selectively applied on the undercuts and recesses where pustules had formed in an attempt to stabilize the surface from further loss by arresting or eradicating propagation of the condition. BTA reacts with corrosion products such as cuprite and copper trihydroxychlorides to form Cu(I)BTA and Cu(II)BTA copolymers, resulting in an optimal and stable coating that ensures durability of the artwork [26]. The double application of BTA, guaranteed thorough coverage in recessed areas, ensured complete reaction of this chemical with all the remaining available unstable compounds that were not complexed during the first application. After application of BTA, the treated area was cleaned with soft bronze bristle brushes and allowed to dry. The resulting precipitate matter was then flushed clear and the surfaces thoroughly rinsed with distilled water. After that, the sculpture was carefully steam pressure washed to remove any remaining residue. On the undersides and recesses of the long coat, i.e. the location most affected by bronze disease, the patina had been removed completely by laser ablation, exposing the bare bronze. Apart from this area, complete removal of the corrosion layers was outside the scope of the present project.
In terms of coloration, a selective application of cupric nitrate and cupric chloride patina solutions to achieve a range of green, and of ammonium sulfate for darker brown in the recesses of the long coat, were used to blend and unify tone and to enhance legibility of the sculptural forms. Finally, a translucent, warm brown coating that established a contiguous protective film of appropriate dry film thickness and unveiled nuances of the underlying greenish patina was applied as the new finish (Fig. 8). The identification of iron-containing pigments to color the lacquer finish guided the decision of applying a new coating containing the same type of coloring materials. Such lacquer, selected from Nikolas Coatings, is a low volatile organic compounds (VOC) lacquer (#14135 Clear NY Coat RFU) that was specifically formulated and re-named for this project (#15794 Morse brown toner RFU). The new toned lacquer achieved the desired visual nuance for the finish and the iron contained in it may provide sacrificial anodic protection to the bronze. In fact, it has been demonstrated that lamellar micaceous iron oxide, used as inert, electrochemically non-reactive pigment for anticorrosive coatings, reduces the reactivity of the material it is in contact with, acting as a protective barrier [27–28]. However, the substance used in this case is in fact an iron-based pigment composed of spherical particles. The solvent-borne coating was thinned and brush applied twice overall to assure penetration into the pitted texture of the Morse sculpture and to obtain complete coverage in deep recesses and undercuts. Three subsequent high volume - low pressure spray coats completed the coating application.