**1. Introduction**

The Harvard Art Museums Bell Krater: Torch Race (1960.344, dated c. 430-420 BCE, Figure 1a) was one of several Greek terracotta artefacts which received treatment in preparation for display in 2014. After one year of display the cadmium orange restoration paint on the krater had altered in color in some areas from orange to grey (Figure 1b,c). In comparison, other objects displayed in the same case also in-painted with the same paint showed no sign of alteration. The appearance was deemed unacceptable and re-treatment was required. To appropriately re-treat the krater an understanding of the cause of the alteration was essential and began with understanding the treatment history of the object.

There is no record of any conservation work performed on the krater before its bequest to the museum, however, the object had been re-assembled in the past and this treatment resulted in misplaced joins, abraded surfaces and noticeable over-paint. During a 2004 loan, an efflorescence was observed on the object. The efflorescence was analyzed by SEM-EDS and found to contain calcium and chlorine. FTIR analysis was also performed but the mineral could not be identified with the available spectral databases at that time.

When the spectrum was revisited in the early stages of this investigation, the expansion of the Infrared and Raman Users Group (IRUG) spectral database [1] led to the identification of calclacite (Ca(CH3COO)Cl.5H2O), a calcium chlorine acetate salt (see Figure 5 later). The formation of calclacite has previously been attributed to archaeological ceramics that have residual chlorides from burial or treatment with hydrochloric acid and have been stored in wooden cases [2]. Hydrochloric acid, along with other acids such as acetic or nitric, were used in the past to remove burial accretions [3]. For hydrochloric acid, if not completely removed from the object by rinsing, residual chlorine salts can remain and can react with the acid vapors from the wood resulting in the efflorescence. Based on the

**Citation:** Rayner, G.; Costello, S.D.; McClelland, A.; Akey, A.; Eremin, K. Preliminary Investigations into the Alteration of Cadmium Orange Restoration Paint on an Ancient Greek Terracotta Krater. *Heritage* **2021**, *4*, 1497–1510. https://doi.org/ 10.3390/heritage4030082

Academic Editor: João Pedro Veiga

Received: 28 June 2021 Accepted: 28 July 2021 Published: 29 July 2021

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identification of the efflorescence, the assumption was made that the object underwent incomplete treatment with hydrochloric acid to remove burial accretions at some point in the past.

**Figure 1.** (**a**) Bell Krater: Torch Race, in the Manner of the Peleus Painter (c. 430-420 BCE, 1960.344) after conservation treatment in 2014. Harvard Art Museums/Straus Center for Conservation and Technical studies, bequest of David Moore Robinson. (**b**) Inset of male figure after conservation treatment in 2014. (**c**) Inset of male figure showing alteration of the restoration paint (highlighted by the white arrow) after one year of display.

Desalination was considered during the 2014 treatment of the object but was eventually deemed an unnecessarily risky treatment as restoration joins were considered stable and storage in a controlled environment had prevented the formation of any further efflorescence [4]. Instead, treatment of the object focused on aesthetic work. Disfiguring paint

was removed, restoration fills levelled, and break lines were filled and sealed with acrylic materials, followed by in-painting. Cadmium orange acrylic paint (Golden Heavy Body Artist Acrylics in C.P. Cadmium Orange), containing CdSSe, was chosen for in-painting due to its ability to lighten the darkened, abraded areas of the terracotta. Titan Buff acrylic paint (Golden Heavy Body Artist Acrylics in Titan Buff), containing titanium white (TiO2), was added as required to adjust the tone. Full details of the materials used for the treatment may be found in a separate publication [4].

Cadmium pigments offer a broad range of colors from light yellow to deep red. Cadmium yellow is composed of cadmium sulfide (CdS) and the incorporation of Se in increasing amounts changes the color from yellow through orange to red. The alteration of cadmium yellow has been reported in paintings by Henri Matisse [5–9], Edvard Munch [9–12], Vincent Van Gogh [13,14], Pablo Picasso [13,15] and others [13,16–18], all dating from the late 19th-early 20th century, before the stability of pigment was improved [16]. Alteration in areas identified as cadmium sulfide presents itself as either lightening or darkening of the once vibrant pigment.

Researchers have utilized a multitude of analytical tools, and most recently, synchrotronbased radiation techniques, to probe the degradation of the pigment. The observed alteration has been determined to be the result of degradation of the cadmium sulfide pigment through a photo-oxidation process. In the initial stages of the process cadmium sulfate is formed (CdSO4·xH2O) which then converts to cadmium carbonate (CdCO3) and cadmium oxalate (formed from the breakdown of organic components such as binding media [14], CdC2O4). The formation of these white compounds contributes to the observed lightening of the pigment. The formation of cadmium oxide (CdO), a brown compound, has been associated with darkening [13].

The alteration of cadmium orange (CdSSe) in an artist's work has not previously been identified, however, in the field of semiconductor chemistry, the photo-oxidation of cadmium selenide (CdSe), has been observed. Studies determined that during irradiation of CdSe in the presence of oxygen, photo-oxidation occurs producing cadmium oxide (CdO) and selenium dioxide (SeO2) [19,20].

Whilst the degradation of artist materials is not unexpected, in any museum, restoration work performed by conservators is expected to last many decades. Increasing access to technical analysis within museum communities has made it easier for appropriate materials to be chosen and those found not to be suitable, such as fugitive colors, can be avoided. In the unlikely event that restoration materials fail quickly, such as in the case presented here, unusual circumstances are likely involved. The theory developed that the krater had received incomplete treatment with hydrochloric acid in the past warranted further investigation in order to determine if this was the cause of the paint alteration and to inform how to proceed with re-treatment.
