*3.1. Analysis of Cross-Sections from the Krater*

The discoloration of the paint was exclusively observed to occur in areas where the restoration paint was in contact with abraded areas around break lines in the ceramic. No alteration was observed when the paint was applied over the acrylic fills from the 2014 treatment. In Figure 2a, the terracotta of the krater is present in a cross-section prepared from an area where the paint altered, observable as a very uniform substrate layer. In Figure 2b, a cross-section prepared from an area where the paint did not alter, the paint is applied to a mixture of restoration materials instead of terracotta. Clearly different from the original terracotta (Figure 2a), this substrate is consistent with the presence of fill materials applied during the 2014 treatment and from the earlier, undated treatment.

The paint layer, measuring between 3–5 μm, appears almost completely white in the altered cross-section with only a few remaining colored particles. The paint layer is difficult to see in the unaltered cross-section, however with ultraviolet (UV) illumination a distinct green fluorescence from the paint layer is observable.

Analysis using SEM-EDS (Figure 3) confirmed the presence of chlorine (Cl) in the altered paint layer and in the terracotta (not shown in Figure 3). No Cl was observed in any of the layers in cross-sections prepared from the unaltered paint. This observation, combined with the identification of calclacite from the 2004 efflorescence, suggested that the alteration may be caused by an interaction between the cadmium orange restoration paint and residual chloride ions in the terracotta.

**Figure 2.** Microscope images of cross-section samples taken from the krater. (**a**) altered paint sampled from the male figure (pictured in Figure 1c) in which the restoration paint sits directly on top of the terracotta surface. (**b**) unaltered paint sampled from an area in which the paint was not in contact with the terracotta due to the presence of fill materials. (**c**,**d**) represent the same areas depicted in (**a**,**b**) taken with UV illumination (FITC filter cube).

**Figure 3.** (**a**) Microscope image of cross-section prepared from a sample taken from an area of alteration on the krater. (**b**) SEM backscattered electron image of the altered paint film which has been highlighted in (**a**) by the red rectangle with EDS maps below showing Cd, S, Ti and Cl channels.
