# Deterioration of White Tempera Mock-Ups Paints in a SO\({}_{2}\)-Rich Atmosphere
## 3 Results
### Pigment Characterization
The most affected colorimetric parameter after the SO\({}_{2}\) test was \(b^{*}\) for most of the samples, except for the rabbit-glue-based paints, G-EF-R and WL-R, and the egg-yolk-based paint with BSG-C. The \(b^{*}\) parameter in the egg yolk paints typically exhibited a decrease (except for paints BSG-C-E and WL-E) suggesting the loss of the characteristic yellow coloration of the egg yolk paints, as was stated in [13]. Conversely, in the rabbit glue paints, \(b^{*}\) suffered an increase (except G-M-R) resulting in a yellowing effect. In BSG-C-E, G-EF-R and WL-R, the most affected parameter was \(L^{*}\), showing a decreasing trend. The \(L^{*}\) parameter increased in the egg-yolk-based paints, with the exception of the BSG paints, demonstrating a whitening effect on their color. Conversely, \(L^{*}\) diminished in all rabbit-glue-based paints, except for CA-EF-R. The color difference \(\Delta E^{*}{}_{ab}\) was higher in the egg-yolk-based paints comparatively to those made with rabbit glue, with the exception of paints made with G-F. Therefore, egg-yolk-based paints experienced the most intense colorimetric change in comparison to the rabbit-glue-based paints. Gloss variations (\(\Delta\)G) after the SO\({}_{2}\) exposure test were not important except for two samples: the egg-y yolk-BSG ST paint and the rabbit-glue-G EF paint. It was not possible to find out a relation between the gloss variation and the type of binder (Figure 3). The data  
Figure 1: Photomicrographs taken with stereomicroscopy of the references (Ref, on the (**right**)) and the aged paint mock-ups (SO\({}_{2}\), on the (**left**)). Check Table 1 for the identification code of samples. Figure 2: Differences on colorimetric parameters \(\Delta L^{\star}\), \(\Delta a^{\star}\) and \(\Delta b^{\star}\) and color difference \(\Delta E^{\star}_{ab}\) suffered by the aged paints considering the original color of the paints before aging as reference. (**A–C**) calcite-, (**D–F**) gypsum- and (**G**) lead white-based paints. Check Table 1 for the identification code of samples. for this property showed no relationship between the magnitude of the change and the type of binder or pigment grain size. The reflectance spectra of the reference and the aged paint mock-ups are depicted in Figure 4. In agreement with [27], we found that, before the aging test, the morphology of the spectral curve of the reference mock-ups prepared with the same pigment was similar regardless of the binder used. However, differences were found in the reflectance intensity between the samples made with the same pigment though with a different binder. Thus, the reference paints made with rabbit glue (Figure 4B,D) showed in the lower region of the spectrum (450-500 nm) higher reflectance values than those prepared with egg yolk (Figure 4A,C), except for the paints made with G-M and G-F (Figure 4C,D). However, and contrary to what has been found in previous studies [28], the reflectance intensity did not show a relation with the grain size of the pigment. After the SO\({}_{2}\) aging test, the intensity of the reflectance increased in all the mock-ups, except in the paints prepared with pigments BSG-ST and BSG-C mixed with egg yolk (Figure 4A). Figure 5 shows the roughness variation after the aging test. In general, egg-yolk-based paints suffered roughness increases and rabbit-glue-based paints experienced roughness decreases, except for the BSG-C-R paint mock-up (Figure 5C) and, to a lesser extent, the LW-R paint mock-up (Figure 5G). Moreover, although there were not statistical differences between the paints made with egg yolk or rabbit glue, those prepared with egg yolk showed higher Rz variations than those made with rabbit glue, except for the BSG-C and G-F paints. The mineralogical analyses carried out on the paints before being subjected to the test yielded interesting results when comparing the X-ray diffractograms of the paints with those of the original pigments before mixing them with the binders (Table 1).