# Effect of Ion Release on Color Stability of Zirconia: A Comparative Study
## 4 Discussion
In TT, thicker specimens (2.0 mm and 2.5 mm) showed marked increases in \(\Delta\)E following aging, accompanied by substantial elevations in L* values across all immersion times, suggesting that surface brightening and instability contributed to overall color degradation. These results are consistent with the findings of [PERSON] et al. [19], who reported that specimens processed using aged coloring solutions demonstrated increased brightness (L*) and chromatic shifts toward the red (a*) and yellow (b*) axes, indicating that prolonged use of such liquids alters their chemical composition and negatively impacts the final esthetic outcome. Conversely, DD specimens maintained low \(\Delta\)E values and stable L* ratios even after three months of continuous solution use, demonstrating superior resistance to chemical degradation. These findings reinforce the clinical importance of controlling not only the physical parameters, such as thickness, but also the chemical conditions associated with the aging of coloring solutions, to ensure the long-term optical performance of zirconia restorations [52; 53]. ICP-MS analysis confirmed substantial changes in ion concentrations in the coloring solutions over the three-month aging period. A notable increase in Fe ion levels and a concurrent decline in Er ion concentrations were observed. This shift in ion composition was accompanied by greater pH fluctuations and more pronounced color changes in the zirconia specimens. The interaction between pH variation, ion release, and optical degradation in zirconia is a multifactorial phenomenon rooted in the material's physicochemical properties. Acidic conditions can accelerate the dissolution of metallic oxides and rare-earth dopants from the zirconia matrix, destabilizing its crystalline structure [7; 54]. Critically, the interaction between pH variation and ion release plays a pivotal mechanistic role in optical degradation. Lower pH levels enhance the solubility of metallic oxides and destabilize the zirconia lattice, leading to increased Fe ion dissolution, which is closely associated with reddish discoloration [19]. In contrast, the depletion of rare-earth elements such as Er, essential for maintaining translucency and hue stability, contributes to optical degradation [34; 55]. These observations suggest a direct, pH-dependent ionic migration mechanism, where acidic environments exacerbate chromophore imbalance within the zirconia structure. The increase in Fe ion concentration correlates with elevated \(\Delta\)E values and higher L* ratios, indicating that surface brightening and color shifts are not merely superficial but stem from deeper chemical degradation. The \(\Delta\)E values obtained in this study were interpreted based on established clinical thresholds, with 1.2 considered the perceptibility threshold and 2.7 as the acceptability threshold [56]. Values exceeding these thresholds suggest clinically perceptible or unacceptable color changes, providing a more practical context for evaluating the material's long-term esthetic performance. In addition to the observed effects of ion release and coloring solution aging, the intrinsic translucency of zirconia must also be considered in the clinical context [57]. As translucency increases--particularly in restorations with reduced thickness--the optical influence of the underlying tooth structure and the color of the luting cement becomes more significant [49; 58]. This phenomenon can alter the perceived shade of the restoration despite material-level color stability. Accordingly, the cement shade should be cautiously selected in highly translucent zirconia systems, especially those applied in esthetically demanding regions, to prevent undesired shade mismatches [59; 60; 61]. Overall, the findings of this study highlight the importance of optimizing material selection and processing conditions to enhance the long-term performance of zirconia restorations. Ion release directly influences color stability and, consequently, clinical reliability.