# Effect of Ion Release on Color Stability of Zirconia: A Comparative Study
## 1 Introduction  
Zirconia has emerged as one of the most promising materials for dental restorations, largely due to its outstanding mechanical strength [1, 2], improved translucency [3, 4], and versatile application [5]. Advancements in material formulations have increased zirconia's popularity in modern dentistry. However, despite its robust mechanical properties and corrosion resistance, maintaining its color stability in the oral environment remains challenging [6]. The material's exposure to staining agents, surface roughness, and mechanical wear can compromise its aesthetic appearance over time, which is critical to the clinical success of dental restorations [7, 8, 9]. Achieving optimal color stability is essential, as it directly influences restorative materials' longevity, functionality, and visual appealin clinical settings. Research has demonstrated that color stability is affected by various factors, including material composition [10,11] and environmental conditions [12,13]. One of the most critical aspects impacting the color stability of zirconia is the immersion in coloring liquids during manufacturing [14]. This immersion process not only alters the chemical composition of the material but also significantly affects its optical properties, resulting in changes in translucency and color stability [15,16]. Studies have identified that ion release from zirconia during immersion in these liquids is pivotal in causing color alterations [9,17]. [PERSON] and [PERSON] (2017) demonstrated that the mechanical performance of zirconia, particularly its hardness, can be significantly affected by the chosen coloring [18], suggesting that the manufacturing protocol has implications beyond esthetics alone. In a subsequent study, [PERSON] et al. (2019) reported that treatment with aged coloring solutions resulted in perceptible increases in L* and a* values, indicating surface brightening and reddish shifts [19]. While both studies underscore the sensitivity of zirconia to coloring processes, neither addressed the influence of immersion duration, specimen thickness, or ion release dynamics--factors crucial to understanding long-term optical stability. As ion release is closely linked to the chemical environment, particular attention must be given to the pH of coloring solutions, which directly affects ion solubility and color outcomes [20,21]. In particular, monitoring the pH of coloring solutions has emerged as a crucial factor, as pH directly affects the solubility and retention of metal ions, thereby impacting the final color of zirconia [22,23] and its characteristics after the sintering process [24]. However, a notable gap exists in the literature regarding the correlation between ion release, immersion duration, and the material's long-term color stability. Current studies lack comprehensive insights into how specimen thickness and soaking time impact ion release and subsequent color shifts, particularly in pH fluctuations over time. This study aims to address the existing knowledge gap by evaluating the long-term color stability of two multilayer zirconia materials, specifically focusing on the effects of ion release, specimen thickness, and immersion duration on optical outcomes. In addition, the study examines the aging behavior of coloring solutions by analyzing pH variations and ion concentration changes over time, to better understand their contribution to zirconia discoloration. By integrating optical and chemical analyses, this work provides mechanistic insights into the factors influencing color changes and contributes to improving the esthetic durability of zirconia-based restorations. This study aims to bridge this gap by investigating and comparing the long-term color stability of two zirconia materials, focusing specifically on how ion release, thickness variation, and immersion time affect optical outcomes.