# Effect of Ion Release on Color Stability of Zirconia: A Comparative Study
## 4 Discussion  
This study investigated the intricate relationship between ion release, immersion conditions, and the resultant color stability of zirconia materials, focusing on two prominent commercial brands, TT and DD. By analyzing variations in specimen thickness and immersion durations, the research provides critical insights into the physicochemical mechanisms that govern the optical performance of zirconia [28; 33]. The findings not only underscore the dynamic interaction between material properties and environmental factors but also align with prior studies highlighting the importance of ion leaching, particularly highlighting the influence of Fe and Er ion leaching on long-term color accuracy. Despite extensive research on the optical properties of zirconia--particularly the influence of crystal structure, material purity, and processing techniques [34; 35; 36]--few studies have comprehensively examined the combined effects of ion release and immersion  
Figure 5: pH variation of coloring solutions used for TT and DD zirconia over a 90-day period of continuous use. TT: TT ONE Multilayer; DD: cubeX\({}^{2}\) ML. Figure 6: (**a**) Fe and Er (**b**) ion concentrations in coloring solutions after 1, 2, and 3 months of use, measured by ICP-MS. TT: TT ONE Multilayer; DD: cubeX\({}^{2}\) ML. conditions on color stability across different commercial brands. Most of the existing literature has primarily focused on surface treatments, sintering protocols, and aging processes [37; 38; 39; 40], with limited attention given to ion migration under prolonged immersion environments. This study addresses this gap by demonstrating how variations in material thickness and exposure duration influence ion dissolution and subsequent discoloration, while illustrating the interdependent role of the physicochemical environment. A significant correlation was observed between specimen thickness and color parameters, particularly lightness (L*), chromatic coordinates (a*, b*), and chroma (C*). Thickness substantially impacted color stability more than immersion time across all experimental conditions. In this study, zirconia specimens with thicknesses ranging from 1.0 mm to 2.5 mm were evaluated, revealing that thicker specimens (2.0 mm and 2.5 mm) generally exhibited higher L* and C* values and demonstrated greater resistance to color change (lower \(\Delta\)E values), even after prolonged immersion and aging. This behavior may be explained by the reduced penetration depth of coloring liquids and limited diffusion pathways for ionic migration in thicker zirconia specimens, in accordance with [PERSON]'s second law of diffusion [41] and the Beer-Lambert law [42], which stabilizes the optical performance of zirconia. Conversely, thinner specimens (1.0 mm and 1.5 mm) exhibited greater color deviations and more pronounced reddish (a*) and yellowish (b*) shifts, indicating higher susceptibility to environmental influences and ion absorption. These findings are consistent with the studies by [PERSON] et al. [43] and [PERSON] et al. [44], which demonstrated that zirconia thickness significantly modulates translucency and color perception. Thicker zirconia layers exhibit reduced translucency due to increased light scattering, while simultaneously enhancing color masking capability, thus improving the ability to conceal underlying tooth structures [45; 46; 47; 48]. However, it is also recognized that excessive thickness, while improving color masking, may limit light transmission, affecting the desired esthetic outcomes [49; 50]. Additionally, variations in surface finishing and polishing procedures further influence translucency and color stability depending on the material thickness [51]. In the present study, \(\Delta\)E values increased notably, particularly in specimens immersed in aged solutions, with TT specimens exhibiting more significant optical alterations than DD.