*3.4. Characterization of the Altered Paint from the Mock-Ups and the Krater*

A greyed appearance on terracotta ceramics in-painted with acrylic paints has been observed before and was attributed to residual solvent retained in the terracotta substrate migrating to the surface carrying lighter pigments with it [24]. This was considered as a possibility, however, other terracotta ceramics treated in the same manner as the krater did not show the same alteration and not all of the mock-ups altered as might be expected if this was the case. The migration of pigments could also not be confirmed from SEM-EDS mapping of the paint films in cross-section, with the elements associated with the paint layers remaining relatively evenly distributed across the paint layer (see Figure 3).

A key observation in the alteration of the paint films on mock-ups 2 and 4 was the formation of dark needle-like structures which are visible only with high magnification in both the cadmium orange paint swatch and the cadmium orange/Titan Buff swatch, the latter shown in Figure 7a. Whilst these needle-like structures formed on both mock-ups, the formation was more extensive in the mock-up exposed to direct light (mock-up 2), suggesting that high light exposure acts as a catalyst for formation but is not required. The

analysis described here is focused on the cadmium orange/Titan Buff altered paint film on mock-up 4 as it most closely resembles the paint mixture and conditions under which alteration occurred on the krater.

**Figure 7.** Dark needle-like structures formed in (**a**) the cadmium orange/Titan Buff paint mixture on mock-up 4 (exposed to indirect light) and (**b**) the altered paint layer on the krater. (**c**,**d**) represent the same areas depicted in (**a**,**b**) taken with UV illumination (FITC filter cube). White arrows have been used to highlight some of the larger needle-like structures.

A sample of the altered paint from the krater was revisited and the same dark needlelike structures were observed (Figure 7b). These structures are a little more difficult to see in the visible and UV image (Figure 7b and d, with some structures highlighted by white arrows), due to their small size which is comparable to the dark inclusions present in the Titan Buff paint.

SEM-EDS imaging of the altered paint from mock-up 4 shows that the majority of the dark needle-like structures, which are in the order of 1/2 μm long, can be seen just underneath the surface of the paint film whilst a few protrude slightly from the surface. SEM-EDS mapping revealed that the structures are rich in Se (Figure 8a). None of the other elements associated with the pigments (Cd, S and Ti) or Cl appear to be associated with the structures. Raman spectra collected from the structures have a unique sharp Raman peak around 234 cm−<sup>1</sup> which is a match to a reference spectrum of Se available through the RRUFFTM project database (R050656, Figure 9) [25]. Comparison with the literature indicates that this Raman band corresponds to the trigonal, polymeric form of Se (*t*-Sen) [26]. Clusters of dark needle-like structures protruding from the paint surface, similar in size to the Se-rich structures observed in the mock-up paint, were identified in the altered paint taken from the krater with SEM-EDS mapping again revealing that the structures are rich in Se (Figure 8b) and the Raman spectrum matches the reference for Se already discussed (Figure 9).

**Figure 8.** (**a**) SEM backscattered electron image of surface of mock-up 4 with EDS maps showing Se, Cd, S, Ti and Cl channels. (**b**) SEM backscattered electron image of paint taken from altered region of the Krater, showing a branched structure located at the surface with EDS maps showing Se, Cd, S, Ti and Cl channels.

**Figure 9.** Raman spectra obtained from (**a**) a dark needle-like structure from the altered paint taken from the krater and (**b**) a dark needle-like structure within the altered cadmium orange/Titan Buff paint from mock-up 4 compared with (**c**) a reference spectrum of selenium available through the RRUFFTM database. The broader peaks visible in spectrum (**b**) at 446 and 610 cm−<sup>1</sup> are associated with the rutile in the Titan Buff paint in the paint matrix surrounding the needle-like structures.

The FTIR spectrum of the altered paint from both the mock-ups and krater remained relatively unchanged in comparison to the spectra of the fresh paint. Whilst the formation of different cadmium compounds (for example CdC2O4, CdCO3) may be identified in the FTIR spectrum [12,15], the strong signal from the acrylic binder prevented their identification if present. No apparent degradation of the acrylic binder was observed in the pyrolysis-GCMS chromatogram.

Raman spectra of the paint surrounding the Se-rich structures are shown for the altered paint from the krater, the altered cadmium orange/Titan Buff swatch from mockup 4 and the altered cadmium orange swatch from mock-up 2 in Figure 10. The only recognizable feature from the fresh paint is the presence of rutile in the spectra from the krater and mock-up 4 (Figure 10a,b, respectively). The peaks associated with the cadmium orange pigment (shown in Figure 4a, with peak locations shaded in orange in Figure 10) are absent in all three spectra. Instead, the presence of Se remains visible (highlighted by the red shaded area in Figure 10) and a new peak has formed in the mock-ups at 251 cm<sup>−</sup>1, noted in Figure 10 by the area shaded blue. This peak remains unidentified, seemingly not associated with any of the previously identified degradation products of cadmium pigments (CdSO4·xH2O, CdCO3, CdC2O4 or CdO).

**Figure 10.** Detailed region from 200–700 cm−<sup>1</sup> of the Raman spectra obtained from the altered paint surrounding the Se-rich structures from (**a**) the krater, (**b**) the cadmium orange/Titan Buff paint from mock-up 4 and (**c**) the cadmium orange paint from mock-up 2. The orange shaded areas indicate where the peaks associated with the cadmium orange pigment were located in this region (296 and 595 cm<sup>−</sup>1, now absent). The red shaded area indicates the peak associated with selenium at 234 cm−<sup>1</sup> and the blue shaded area highlights the new peak observed at 251 cm<sup>−</sup>1. The broad peaks centered at 446 and 610 cm−<sup>1</sup> are associated with rutile from the Titan Buff paint.

#### **4. Conclusions**

Using complementary analytical techniques it was determined that the alteration observed for the restoration paint applied to the krater is the result of degradation of the cadmium orange (CdSSe) pigment. This conclusion is supported by the identification of selenium-rich needle-like structures of the trigonal Sen form within the paint film after alteration occurs, as observed in both the altered paint from the krater and in the mock-ups created to study the cause of the alteration. Based on the behavior of the restoration paint on the ancient Greek krater and on the mock-ups, alteration occurs only in areas of acidified ceramic and in the presence of light. It is hypothesized that the residual chloride ions in the terracotta, resulting from either burial or incomplete treatment with HCl to remove burial accretions, and light exposure act as catalysts for the observed alteration. This is consistent with observations in the literature in which mobile chloride ions have been associated with promoting the oxidation of cadmium sulfide [12] as well as making the pigment more sensitive to photo-oxidation [5,27].

Based on previous research it was anticipated that cadmium would form white (e.g., CdSO4, CdCO3) or brown (CdO) compounds, however, no cadmium-containing compounds were identifiable during this preliminary study. This research will hopefully be extended in the future to fully deduce the cadmium–containing degradation products and further investigate the role of acidity/pH in the alteration. Together, this will allow for a better understanding of the full mechanistic process in which the degradation of cadmium orange occurs.

For the re-treatment of the krater, the altered paint film was removed and noncadmium containing pigments (Golden Fluid Acrylics cadmium red and yellow) used for in-painting after mock-ups determined them to be stable under the conditions tested during this study [4]. This same treatment will be used in the future for other ceramics known, or suspected, to have been treated with hydrochloric acid.

**Author Contributions:** Conceptualization, S.D.C., K.E. and G.R.; methodology, S.D.C., K.E. and G.R.; formal analysis, G.R., K.E., A.M. and A.A.; investigation, G.R., K.E., A.M. and A.A.; resources, S.D.C.; writing—original draft preparation, G.R.; writing—review and editing, G.R., S.D.C., K.E., A.M. and A.A.; visualization, G.R.; supervision, K.E. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was performed in part at the Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Coordinated Infrastructure Network (NNCI), which is supported by the National Science Foundation under NSF award no. ECCS-2025158. CNS is part of Harvard University.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study is available on request from the corresponding author. Publicly available datasets were used for reference material in this study. This data can be found here: http://www.irug.org/search-spectral-database, accessed on 9 June 2021; https://rruff.info, accessed on 9 June 2021.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

#### **References**

