*3.4. Color Tests*

Figure 6 shows the color coordinates L\*, a\* and b\* for the mixes studied in the different media tested, in the test phases after 28 and 120 days. Each pigment displays the luminosity values L\* on the left and the chromaticity diagram a\*-b\* on the right. In all cases, the black circles represent the coordinates of the pure pigment, the blue circles the binders, the white circles the pastes after 28 days and the grey circles the pastes after 120 days.

With regard to the binders, they all feature very small tonal amounts (a\*-b\*) of red and blue, except the water glass, whose tones are blue and green. The luminosities (L\*) are extremely high in all cases, with values which are very close to 100%, generating a visual impression that is very similar to white. Meanwhile, the pigments used in all of the mixes in proportions of 20% feature shades which are characteristic of the color palette designed, and they are obviously affected by their interactions with the different binders on forming the pastes.

In terms of shade, the CY pigment is composed of a significant proportion of yellow and also features a considerable amount of green (a\*-b\*). The luminosity of this chromium pigment is very close to 100%. The luminosity of the pastes hardened for 28 or 120 days tended to differ, darkening in some way by approximately 5%; the color saturation decreased more markedly—by up to 40%.

The MO pigment is largely composed of yellow and red tones (a\*-b\*), and the luminosity is low: slightly higher than 50%. The changes induced by the different binders increased the luminosity in all cases, this being greater after 120 days, with values of close to 13%. In the chromaticity diagram the saturation of the pastes drastically reduces to levels which are very close to those of the binders (≈80%). Equally noteworthy is the reduction induced by the binders in relation to the yellow shades, at the expense of the associated red shade.

**Figure 6.** CIELab1976 colour space representation of binders and their corresponding pigmentcoloured pastes (PPS, PALS, PHLS and PWGS). Plot L\*, indicates brightness values of the different samples. Plot a\*-b\*, indicates chromaticity values of the different samples. Blue circles are the binder samples without pigment; black circles are the pigment samples; white circles are the paste samples at 28 days; grey circles are the paste samples at 120 days.

The NS pigment has high luminosity (L\*) values, of close to 90%, and where chromaticity (a\*-b\*) is concerned, yellow stands out as the principal shade alongside small amounts of red. The pastes obtained provoke subsaturation, with values which are close to those of the binders, although the relative reduction was ≈10%. The luminosity increased ≈ 14%. It is notable how there were significant reductions in the yellow and red shades, and the red was replaced by green in the PWGS paste, after 120 days.

The GE pigment has average luminosity (≈60%) and chromaticity values (a\*-b\*), along with comparatively small amounts of yellow and green. The pastes featured increased luminosity (≈12%), particularly those measured after 120 days, contrary to what happens in the chromaticity diagram, where the pastes display relative subsaturation of ≈8% after 28 and 120 days.

The CG pigment has low luminosity (<50%) and chromaticity components (a\*-b\*) based on green and yellow shades. The variations in luminosity were significant, particularly the increase in white, which occurred in the pastes after 28 and 120 days (≈15%). The most significant variations in terms of chromaticity stemmed from subsaturation, which arose in the mixes after 28 and 120 days; these have a clear tendency towards the binder values (≈3%).

The O pigment reached luminosity values slightly higher than 50% and tonal proportions (a\*-b\*) of yellow and red. The changes were similar to those experienced by the rest of the pigments and their pastes: a clear increase in luminosity (≈6%) and a substantial loss of saturation (≈14%). The PWGS sample after 120 days provided a slight green tone compared to the reddish tones of the other samples (PPS, PALS and PHLS).

The ZY pigment has very high luminosity values (≈90%), whilst the chromaticity includes tonal components based on yellow and green. The variations experienced by the pastes on hardening with the binders were very similar to those described in all cases: increased luminosity (≈3%) and loss of saturation (≈30%).

The UB pigment has low L\* values, making it dark (≈25%). The changes induced by the inclusion of the binders studied increased luminosity by up to (≈11%) after 120 days. The chromaticity values include a blue tonal component, which dominates in relation to the red tone, which represents half the amount of the blue. The pastes provoked a loss of saturation which left it close to the binders' values (≈33%).

Table 8 shows that all of the paste samples studied experienced total color variations (ΔE) clearly detectable by the human eye ΔE ≥ 3 [76] after 28 or 120 days. These changes are determined by the chroma variation (ΔC), which entails variations in saturation (subsaturation). This doubled after 120 days in the majority of cases, compared to the amount after 28 days. Likewise, the increased luminosity (ΔL) induced by the binders used to prepare the pastes is a decisive factor in the total color variation (ΔE). The only exception of note occurred in the mixes containing pigment O, whose variations just exceeded the limit of detection by the human eye.

**Table 8.** CIELab\* 1976 values for the pigments and pastes used in this study and total color variations (ΔE), chromaticity variations (ΔC) and luminosity variations (ΔL) of the pigments used in the preparation of the pastes after 28 and 120 days.


**Table 8.** *Cont.*


All of these changes are represented in the form of a color chart in Figure 7, taking into account the average values for a\*, b\* and L\* for each group of samples. For each pigment and paste type, the results are shown for 28 days and 120 days according to their color coordinates (CIELab 1976).

Щ

Щ

**Figure 7.** Color chart for the different mixes and inorganic pigments used, with an indication of their color code for both of the test phases (28 days and 120 days).

## **4. Conclusions**


**Author Contributions:** Conceptualization, M.P.S.-P. and J.A.D.-S.; methodology, M.P.S.-P. and J.A.D.-S.; validation, M.P.S.-P. and J.A.D.-S.; formal analysis, M.P.S.-P. and J.A.D.-S.; investigation, M.P.S.-P. and J.A.D.-S.; resources, M.P.S.-P., J.A.D.-S., A.V.-V. and T.G.-L.; data curation, M.P.S.-P. and J.A.D.-S.; writing—original draft preparation, M.P.S.-P. and J.A.D.-S.; writing—review and editing, M.P.S.-P., J.A.D.-S., A.V.-V. and T.G.-L.; supervision, M.P.S.-P. and J.A.D.-S., A.V.-V. and T.G.-L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** This work was supported by the REMINE Programme for Research and Innovation Horizon 2020 Marie Skłodowska–Curie Actions and WARMEST H2020-MSCA-RISE-2020 (Marie Skłodowska–Curie Research and Innovation Staff Exchange and was carried out under the auspices of Research Groups RNM 0179 and HUM 629 of the Junta de Andalucía.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

