Impact of Simulated Gastric Acid and Surface Treatment on the Color Stability and Roughness of Zirconia

Featured Application

The findings of this study can guide clinicians and dental technicians in selecting optimal sintering protocols and surface finishing treatments for monolithic zirconia restorations, particularly in patients experiencing high oral acidity due to gastroesophageal reflux disease (GERD). Specifically, fast sintering combined with glazing may provide enhanced aesthetic stability in anterior restorations, whereas slow sintering with polishing may be preferable for posterior restorations, ensuring durability and appropriate opacity.

Abstract

This in vitro study evaluated the impact of simulated gastric acid exposure on the optical (ΔE₀₀, translucency parameter TP, contrast ratio CR) and surface (roughness Ra) properties of monolithic zirconia ceramics under varying sintering rates and surface treatments. Forty-eight disc specimens (10 mm × 10 mm × 1.0 mm) were randomly allocated into four groups (n = 12): slow sintering + polishing; slow sintering + glazing; fast sintering + polishing; and fast sintering + glazing. Specimens were aged in 0.06 M of HCl (hydrochloric acid) for 96 h, and all measurement parameters were assessed against white and black backgrounds before and after aging. Statistical analyses (Shapiro–Wilk, Kruskal–Wallis, Wilcoxon tests; α = 0.05) revealed that acid aging caused a significant increase in ΔE₀₀ across all groups (p < 0.05), with the smallest change observed in the fast-sintering + glazing group and the largest in the slow-sintering + glazing group. Contrast ratios remained high in all groups (CR > 0.92), while only the slow-sintering + glazing group exhibited a significant reduction in TP (p < 0.05). Surface roughness decreased following aging in all groups, with the lowest Ra detected in the fast-sintering + glazing group. These results suggest that fast sintering combined with glazing enhances color stability and yields smoother surfaces under acidic conditions, recommending this protocol particularly for patients at elevated risk of increased oral acidity.

1. Introduction

In recent years, the growing demand in the field of prosthodontics for biologically compatible, aesthetically superior, and mechanically durable materials has significantly increased the use of zirconia ceramics [1].

Among the main factors negatively affecting the clinical success of zirconia-based restorations are issues such as chipping and delamination, which arise from bonding failures between the core and veneering materials. To minimize these issues, monolithic zirconia ceramics manufactured from a single block using computer-aided design/computer-aided manufacturing (CAD/CAM) systems have been developed [2,3]. These ceramics must undergo a sintering process to achieve the desired properties in their microstructure [4,5].

Finishing and polishing procedures play a crucial role in ensuring the longevity of restorations and the preservation of their aesthetic properties. In zirconia restorations where these procedures are not performed, long-term surface discoloration and increased plaque retention have been observed [6,7]. The glazing process applied after sintering smooths the surface of the material, while another approach involves mechanical polishing using specialized polishing rubbers.

Gastroesophageal reflux disease (GERD) is a common health problem, and approximately 50% of adult individuals experience reflux symptoms at least once in their lifetime [8]. GERD is characterized by the backflow of gastric contents (such as acid, pepsin, and bile) into the esophagus and oral cavity [9,10,11]. Typical symptoms include heartburn, regurgitation, and chest pain, while extraesophageal manifestations include chronic cough, laryngitis, asthma, and dental erosions [11]. GERD, especially when chronic, causes a decrease in the pH of the oral environment, which leads to the emergence of various oral symptoms. These symptoms include dry mouth (xerostomia), burning tongue (glossodynia), dental erosion, and inflammation of the oral mucosa [9].

After meals, gastric acid pH ranges between 3 and 5, while during fasting, it may drop to as low as 1. These low pH levels can damage not only the esophagus but also the oral cavity [12]. The long-term effects of GERD can also negatively impact the surface properties and water absorption behavior of dental restorative materials. In individuals exposed to frequent and severe reflux episodes, these materials are expected to be resistant to acidic environments. In a previous study, the color and translucency changes of various indirect restorative materials exposed to simulated gastric acid were examined; the highest color change was observed in the polyetheretherketone (PEEK) group, whereas the lowest color change was observed in the translucent zirconia group. Therefore, researchers recommend the use of zirconia restorations instead of lithium disilicate, hybrid ceramics, and PEEK in patients with conditions such as GERD or bulimia nervosa [10]. Recent studies have revealed that the prevalence of gastroesophageal reflux disease (GERD) exhibits regional variations across the globe. In North America, the prevalence ranges from 18.1% to 27.8%. In Europe, it ranges from 8.8% to 25.9%. East Asia shows a prevalence range of 2.5% to 7.8%. The Middle East exhibits a prevalence range of 8.7% to 33.1%. Australia reports a prevalence of 11.6%, while South America reports a prevalence of 23.0% [11].

The prevalence of GERD varies by region. Epidemiological data in Turkey are similar to those in Western countries [13,14]. In Turkey, the prevalence of reflux is higher in women, and regurgitation symptoms are observed more frequently [14]. Additionally, factors such as obesity, advanced age, the use of nonsteroidal anti-inflammatory drugs, and low socioeconomic status are significant risk factors for GERD [11,12,13,14,15]. For all these reasons, the resistance of dental restorative materials to acidic conditions in such patients is of great clinical importance both in the short and long term.

Monolithic zirconia ceramics are widely used, particularly in the restoration of posterior teeth. However, the effects of gastric acid on these materials—especially in terms of color change and surface roughness—have not been sufficiently investigated [16,17].

A previous study investigating the effects of different acids on ceramic surfaces in the literature demonstrated that immersion of ceramics in acidic substances increased surface roughness [18]. It is known that increased surface roughness values affect the mechanical and optical properties of ceramics [17].

The aim of this in vitro study is to evaluate the effects of simulated gastric acid on the optical properties and surface roughness of monolithic zirconia specimens subjected to different sintering and surface finishing protocols. H₀ (null hypothesis): simulated gastric acid aging has no statistically significant effect on the color change (ΔE₀₀) and surface roughness (Ra) of monolithic zirconia ceramics subjected to different sintering protocols and surface treatments.

2. Materials and Methods

2.1. Sample Preparation

A total of 48 discs were sectioned under water cooling and vacuum conditions using a low-speed precision cutting device (Metkon Micracut 201, Bursa, Turkey). Specimens were 10 mm × 10 mm × 1.0 mm in dimension and prepared from high-translucency monolithic zirconia blocks in shade A2 (Copran Zr-i Monolith Zirconia Blank, White Peak Dental Solutions, Essen, Germany). According to the manufacturer’s technical data, the zirconia blocks used in this study contain approximately 94–95 wt% ZrO₂ and ~5 wt% Y₂O₃. Additionally, to control grain growth during sintering, the composition includes 0.03–0.07 wt% Al₂O₃. Trace amounts of iron oxide are also incorporated for A2 Vita shade pigmentation. This composition corresponds to partially stabilized tetragonal zirconia containing 3 mol% yttria, providing the material with high mechanical strength and structural stability. One surface of all specimens was ground under running water for 20 s using silicon carbide abrasive papers with grit sizes of 600, 800, and 1200, respectively. Subsequently, all specimens were cleaned in an ultrasonic cleaner (CD 4820, İstanbul, Turkey) using distilled water for 10 min. The samples were randomly divided into four groups (n = 12):

Group 1: slow sintering + polishing;

Group 2: slow sintering + glazing;

Group 3: fast sintering + polishing;

Group 4: fast sintering + glazing.

2.2. Sintering Protocols

Sintering was performed in a sintering furnace (Ceramill Therm 3, Ceramill Therm DRS, Amann Girrbach, Koblach, Austria) following the manufacturer’s instructions under both fast and slow-sintering protocols.

Fast sintering: heating at 50 °C/min up to 1100 °C and then at 20 °C/min up to 1500 °C; held for 30 min.

Slow sintering: heating at 5 °C/min up to 950 °C and then at 2 °C/min up to 1500 °C; held for 120 min.

All specimens were cooled in a controlled manner after sintering [4,5,6].

2.3. Surface Finishing Procedures

All samples were ground under running water using a silicon carbide abrasive paper with a grit size of 1000 on a grinding machine (Metkon; Bursa, Turkey) to ensure standardization of specimen surfaces. The dimensions of the specimens were verified using a digital caliper. Fast and slow-sintering groups were subdivided based on the surface finishing procedure to be applied, with 12 specimens in each subgroup (n = 12). As surface finishing procedures, mechanical polishing and glazing were applied to the measurement surfaces of the specimens.

Polishing:

In the mechanical polishing procedure, Dura-White Stone (0243, Shofu Inc., Kyoto, Japan) was initially applied for 60 s. The specimen surfaces were then cleaned with pressurized water. Subsequently, Standard Ceramiste, Ultra Ceramiste, and Ultra II Ceramiste polishers (Ceramiste, Shofu Inc., Kyoto, Japan) were applied sequentially for 60 s each. The mechanical polishing was performed at a speed of 10,000 rpm in accordance with the manufacturer’s instructions [19].

According to the manufacturer’s instructions, silicone polishers are recommended to be used intermittently at a rotational speed of 10,000–15,000 rpm (minimum) and up to a maximum of 20,000 rpm. In line with these recommendations, Ceramaster Coarse Assorted was applied to the abraded surfaces of the specimens using a micromotor and contra-angle handpiece (Kavo, Warthausen, Germany) at 10,000 rpm in a back-and-forth sweeping motion in the same direction for 30 s. During the second 30-second interval, polishing was repeated in a direction perpendicular to the initial one. Subsequently, Ceramaster Assorted was applied to the specimen surfaces in the same manner, completing a total polishing time of 2 min.

The glazing procedure was performed using a porcelain furnace (Programat X1, Ivoclar Vivadent, Schaan, Liechtenstein) in accordance with the manufacturer’s instructions. The specimens to which glaze material (Ivoclar Vivadent) had been applied were initially dried at a starting temperature of 403 °C for 2 min and then heated to 710 °C at a rate of 60 °C/min. The specimens were held at 710 °C for 2 min, followed by a cooling period of 2 min.

2.4. Aging Procedure

The samples were subjected to an aging process by immersion in a simulated gastric acid solution containing 0.06 M of HCl at 37 °C for 96 h. The pH was monitored every 3 h using a pH meter (Seven2Go S2 pH/mV, Mettler-Toledo, Greifensee, Switzerland). Although the pH was not recorded numerically, it was observed to start at approximately 1.2 and gradually increase to around 1.3–1.35 before each solution refresh. To maintain a consistent acidic environment, simulating gastric conditions, and to minimize deviations in pH due to potential neutralization effects from the specimen surfaces, the solution was refreshed every 12 h. This cycle was repeated eight times. This procedure simulates more than 10 years of clinical exposure [10,17].

2.5. Color Measurement and ΔE₀₀ Calculation

Color measurements were performed in a color assessment box equipped with a Master TL-D Super 80 18W/865 1SL fluorescent lamp (Philips, Amsterdam, The Netherlands), which has a color temperature of 6500 K and a color rendering index (CRI) of 85 [20]. Measurements were conducted over both black and white backgrounds [21]. The baseline color coordinates (and the derived TP and CR values) were measured for each specimen prior to aging, and the measurements were repeated after 96 h of acid exposure to evaluate the changes. All measurements were performed by the same investigator using a VITA Easyshade Advance 4.0 spectrophotometer (VITA Zahnfabrik, Bad Säckingen, Germany) and repeated three times; an average value was calculated separately for each measurement. The obtained mean values were recorded according to the CIELab color system. The CIELab color coordinates were calculated from spectral reflectance data based on the CIE D65 standard [20] illuminant and the 2° CIE 1931 Standard Observer [20]. The device was calibrated after every 12 measurements.

Color change before and after aging was calculated using the CIEDE2000 color difference formula [21]. This formula is as follows:

ΔE₀₀(kL:kC:kH) = √[(ΔL′/kL·SL)² + (ΔC′/kC·SC)² + (ΔH′/kH·SH)² + RT·(ΔC′/kC·SC)·(ΔH′/kH·SH)]

In this formula, ΔL′, ΔC′, and ΔH′ represent differences in lightness, chroma, and hue, respectively. RT is the rotation function that accounts for the interaction between chroma and hue differences in the blue region [21]. SL, SC, and SH are weighting functions, while kL, kC, and kH are parametric factors determined based on experimental conditions. In this study, the parametric factors were set to 1 [22].

Translucency measurements were performed using the same spectrophotometer based on the translucency parameter (TP) method over white (w) and black (b) backgrounds. The TP value was calculated using the following formula [21,22,23,24,25]:

TP = √[(Lw − Lb)² + (aw − ab)² + (bw − bb)²]

The L* values were also used to calculate the spectral reflectance, Y (luminance from tristimulus color space/XYZ), as follows [24,25]:

Y = Y_n × ((L* + 16)/116)³

For simulated object colors, the specified white stimulus normally chosen is one that has the appearance of a perfect reflecting diffuser, normalized by a common factor so that Yn is equal to 100.19. Y values of the specimens recorded on white (Y_W) and black (Y_B) backgrounds were used to calculate the contrast ratio (CR) as follows [21,24,25]:

CR = Y_B/Y_W

2.6. Surface Roughness Measurement (Ra)

Surface roughness was measured before and after aging using a stylus-type profilometer (Surftest SJ-210, Mitutoyo, Kawasaki City, Japan). The device performed scans with a 5 μm diameter, 90° diamond tip at a speed of 0.5 mm/s, a force of 4 mN, and a cut-off length of 0.8 mm [18,19]. Three parallel measurements were obtained from each specimen, and the average Ra value was calculated and used for analysis. The device was calibrated with a standard of 2.94 μm Ra after every 12 measurements.

2.7. Statistical Analysis

The statistical adequacy of the sample size was assessed via an a priori power analysis using G*Power version 3.1.9.4 (Heinrich Heine University, Düsseldorf, Germany). Specifically, we selected the “F-test: ANOVA—fixed effects, one-way” module, inputting a medium effect size (Cohen’s f = 0.50), α = 0.05, and desired power (1 – β) = 0.80 [17], following standard conventions. The analysis indicated that 12 specimens per group were required, and accordingly, we used 48 specimens in total (n = 12 per group × 4 groups).

The data were analyzed using SPSS 22.0 software (IBM Corp., Armonk, NY, USA). The normality of the data was assessed using the Shapiro–Wilk test. Since the data were not normally distributed, the Kruskal–Wallis test was used for comparisons between groups, and the Wilcoxon signed-rank test was applied for analyzing paired data before and after aging. When the Kruskal-Wallis tests indicated significant differences among groups, post hoc pairwise comparisons were performed using Bonferroni-corrected Mann–Whitney U tests. In addition, a Spearman correlation analysis was conducted to assess the relationship between surface roughness and optical properties (ΔE₀₀, TP, and CR).

A p-value < 0.05 was considered significant.

3. Results

3.1. ΔE₀₀ (Color Change)

Groups subjected to fast sintering exhibited significantly lower ΔE₀₀ values, indicating better color stability compared to those subjected to slow sintering. The lowest color change was observed in Group 4 (fast sintering + glazing), with a mean ΔE₀₀ of 0.947 ± 0.655, whereas the highest was found in Group 2 (slow sintering + glazing), with 3.497 ± 3.936. Since the data were not normally distributed (Shapiro–Wilk, p < 0.001), the Kruskal–Wallis test revealed significant differences among groups (p = 0.007). The Wilcoxon test showed that color change before and after aging was significant within each group (p < 0.001). Post hoc pairwise comparisons identified a statistically significant difference between Group 2 and Group 4 (p < 0.005), while no significant was observed between the polished groups (

Table 1. Statistical evaluations for ΔE₀₀.

Test	Test Statistics	p-Value
Shapiro–Wilk (normality)	0.579	<0.001
Kruskal–Wallis (group comparison)	12.047	0.007
Wilcoxon test (aging effect within each group)	-	<0.001

Statistically significant difference at the p < 0.05 level.

and

Table 2. Average ΔE00 and standard deviations by group.

Group	Mean ΔE00	SD ΔE00
Group 1	2.092	1.636
Group 2	3.497	3.936
Group 3	1.767	0.839
Group 4	0.947	0.655

SD: Standard deviation.

). In dentistry, ΔE₀₀ values around 0.8–1.2 are considered the perceptibility threshold, while values exceeding approximately 2.25 are generally regarded as the 50:50 acceptability threshold for color mismatch. Changes beyond these thresholds may be considered clinically unacceptable [22]. Therefore, the ΔE₀₀ value observed in Group 2 (3.497 ± 3.936) indicates an unacceptable level of color change, whereas Group 4 remained well within acceptable limits, demonstrating superior color stability after aging.

3.2. Translucency Parameter (TP)

In the translucency parameter (TP) analysis, a statistically significant increase was observed only in Group 2 (slow sintering + glazing) after aging, with TP values increasing from 4.26 ± 1.71 to 6.39 ± 2.66 (p = 0.009), indicating a notable increase in translucency. In contrast, the TP changes in the other groups were not statistically significant (p > 0.05). The highest pre-aging TP value was recorded in Group 4 (fast sintering + glazing), with 7.43 ± 0.92, which slightly decreased to 6.67 ± 0.92 after aging, though this change did not reach statistical significance (p = 0.092). Group 1 (slow sintering + polishing) and Group 3 (fast sintering + polishing) showed minimal variation in TP values before and after aging (Group 1: 3.88 ± 1.77 to 3.59 ± 1.31; Group 3: 3.37 ± 1.22 to 3.66 ± 1.28; p > 0.05), demonstrating the highest optical stability among all groups (

Table 3. Comparison of intergroup translucency parameters.

Group	TP Before (Mean ± SD)	TP After (Mean ± SD)	Wilcoxon p-Value
Group 1	3.88 ± 1.77	3.59 ± 1.31	0.73340
Group 2	4.26 ± 1.71	6.39 ± 2.66	0.00928
Group 3	3.37 ± 1.22	3.66 ± 1.28	0.12939
Group 4	7.43 ± 0.92	6.67 ± 0.92	0.09229

TP: translucency parameter; SD: standard deviation.

).

These results suggest that the combination of slow sintering and glazing may enhance light transmission, possibly due to microstructural or glaze-layer-related alterations during acid exposure. However, the fast-sintered, glazed samples (Group 4) still maintained the highest absolute TP values both before and after aging, highlighting the role of sintering kinetics in determining baseline translucency.

3.3. Contrast Ratio (CR)

All groups exhibited contrast ratio (CR) values above 0.92, indicating high opacity in the materials after aging. The most opaque group was Group 1 (slow sintering + polishing), with a CR of 0.942 ± 0.032, followed by Group 2 (0.938 ± 0.209) and Group 3 (0.926 ± 0.031). The lowest CR value was observed in Group 4 (fast sintering + glazing), with 0.920 ± 0.013 (

Table 4. Contrast ratio values for all groups after aging with Mann–Whitney U tests applying Bonferroni correction.

Group	CR Average	CR Standard Deviation (SD)
Group 1	0.942	0.032
Group 2	0.938	0.209
Group 3	0.926	0.031
Group 4	0.920	0.013

SD: standard deviation; CR: contrast ratio.

).

A decrease in CR values was particularly noted in the glazed groups (Groups 2 and 4), suggesting that the glassy glaze layer may promote greater light transmission, thereby reducing opacity. In contrast, the polished groups (Groups 1 and 3) maintained higher CR values, which may be attributed to the smoother and density surface structure created by mechanical polishing, enhancing light reflectance.

Although statistical analysis using the Bonferroni-corrected Mann–Whitney U test revealed differences between groups, these were not all statistically significant after correction. Nevertheless, the trend indicates that surface finishing procedures play a notable role in influencing the optical behavior of zirconia ceramics following acidic aging.

3.4. Surface Roughness (Ra)

After aging, surface roughness (Ra) values significantly decreased in Group 2 (slow sintering + glazing), from 0.269 ± 0.206 µm to 0.197 ± 0.060 µm (p = 0.0215), and in Group 4 (fast sintering + glazing), from 0.179 ± 0.108 µm to 0.149 ± 0.049 µm (p = 0.0417), indicating a smoothing effect possibly induced by the glaze layer or surface restructuring due to acid exposure.

In contrast, Group 1 (slow sintering + polishing) showed a slight increase in surface roughness, from 0.399 ± 0.135 µm to 0.406 ± 0.102 µm, although this change was not statistically significant (p = 0.7539). Group 3 (fast sintering + polishing) exhibited a decreasing trend (0.454 ± 0.129 µm to 0.404 ± 0.085 µm), but it did not reach statistical significance (p = 0.0642) (

Table 5. Surface roughness before and after aging: descriptive statistics and Wilcoxon signed-rank test results.

Group	Before Ra (Mean ± SD)	After Ra (Mean ± SD)	Wilcoxon Test p-Value
Group 1	0.399 ± 0.135	0.406 ± 0.102	0.7539
Group 2	0.269 ± 0.206	0.197 ± 0.060	0.0215 *
Group 3	0.454 ± 0.129	0.404 ± 0.085	0.0642
Group 4	0.179 ± 0.108	0.149 ± 0.049	0.0417 *

* Statistically significant difference at the p < 0.05 level; SD: standard deviation; Ra: surface roughness.

, Figure 1).

Pairwise comparisons using the Bonferroni-corrected Mann–Whitney U test revealed statistically significant differences between the following groups: Group 1 and Group 2, Group 1 and Group 4, Group 2 and Group 3, and Group 3 and Group 4 (corrected p < 0.05) (

Table 6. Results of the Bonferroni-corrected Mann–Whitney U test for surface roughness (Ra) values after aging.

Group 1	Group 2	Raw p-Value	Bonferroni-Corrected p
Group 1	Group 2	0.0001	0.0006 *
Group 1	Group 3	0.8852	1.0000
Group 1	Group 4	0.0000	0.0002 *
Group 2	Group 3	0.0002	0.0012 *
Group 2	Group 4	0.0102	0.0610
Group 3	Group 4	0.0000	0.0002 *

* Statistically significant difference at the p < 0.05 level.

). These results indicate that glazed surfaces, particularly in fast-sintered specimens, were less affected by acid-induced roughening, potentially due to the formation of a protective glassy layer that reduces micro-porosity and fills surface irregularities.

Conversely, mechanically polished groups demonstrated more variability, suggesting that polishing may not offer the same resistance to chemical degradation as glazing. These findings align with previous studies suggesting that glazing reduces surface energy and acid permeability, contributing to smoother and more stable ceramic surfaces under acidic conditions.

3.5. Correlation Analysis

Spearman correlation analysis revealed no statistically significant relationships between surface roughness (Ra) and optical parameters, including ΔE₀₀, TP, or CR (p > 0.05). Specifically, weak correlations were found between Ra and ΔE₀₀ (ρ = 0.20, p = 0.80), Ra and TP (ρ = –0.20, p = 0.80), and Ra and CR (ρ = 0.40, p = 0.60) (

Table 7. Spearman’s correlation coefficients for the correlation assessments.

Metric	Correlation Coefficient (ρ)	p-Value
Roughness vs. ΔE00	0.20	0.80
Roughness vs. TP	−0.20	0.80
Roughness vs. CR	0.40	0.60
TP vs. CR	−0.80	0.20

Statistically significant difference at the p < 0.05 level.

). These findings suggest that surface topography alone may not be a dominant factor in determining the optical behavior of monolithic zirconia.

Interestingly, a strong negative trend was observed between the translucency parameter (TP) and contrast ratio (CR) (ρ = –0.80), although it did not reach statistical significance (p = 0.20). This inverse relationship is consistent with theoretical models of light transmission in ceramics, where increased translucency is typically accompanied by decreased opacity. While TP reflects the depth of light penetration and internal scattering, CR is more affected by surface reflectance. Their interplay highlights the multifactorial nature of optical performance in zirconia.

The lack of significant correlations between Ra and the optical parameters may also be explained by the relatively small variations in Ra among the tested groups and the presence of glaze layers, which can obscure underlying surface differences. In zirconia, a dense polycrystalline ceramic, optical properties are largely governed by bulk microstructure—including grain size, crystal phase distribution, and porosity—rather than minor surface irregularities. Although rougher surfaces may theoretically scatter light more diffusely, this effect appears limited when the material’s internal optical behavior is dominant.

To better understand these interactions, future studies should incorporate advanced imaging techniques such as scanning electron microscopy (SEM), atomic force microscopy (AFM), or confocal microscopy. When combined with spectrophotometric surface mapping, such approaches could offer deeper insight into how surface morphology and internal microstructure together influence the optical properties of zirconia-based ceramics.

4. Discussion

In this study, monolithic zirconia ceramic specimens subjected to different sintering protocols (fast and slow) and surface treatments (glazing and polishing) were aged using gastric acid. The effects of these conditions on the specimens’ ΔE₀₀, translucency, contrast ratio, and surface roughness values were evaluated.

The results demonstrated that the slow-sintering group exhibited higher ΔE₀₀ values compared to the fast-sintering group, indicating more pronounced color differences. Therefore, our null hypothesis was rejected. This outcome may be attributed to microstructural alterations induced by slow sintering in the material. In Group 2, the ΔE₀₀ values exceeded clinically acceptable thresholds, suggesting that glazing under slow-sintering conditions may have an amplifying effect on color change. In contrast, Group 4 showed the lowest color change values and yielded the most clinically stable outcomes. The observed improvements in surface and optical properties in the fast-sintering + glazing group suggest that both thermal and chemical mechanisms may contribute to these effects.

These findings are consistent with previous studies by Erarslan et al., who reported that different sintering protocols influence the color change of monolithic zirconia restorations and that shortening the sintering duration leads to a reduction in ΔE₀₀ values [26].

Similarly, Miura et al. [27] reported that the color changes resulting from conventional and fast-sintering protocols vary depending on the type of ceramic used. Notably, in multilayered zirconia specimens containing 3Y-TZP and 5Y-PSZ (Multi2), fast sintering was associated with pronounced color changes [27].

Translucency is one of the most critical determinants of aesthetic success in dental ceramics and is directly related to the material’s light transmission capability. This property plays a vital role in the selection of restorative materials, particularly in the anterior region, as it helps achieve a natural tooth-like appearance. According to the literature, sintering temperature and holding time influence the density, grain size, and microstructural characteristics of ceramics, thereby directly affecting the translucency parameter [28,29].

In this study, no statistically significant changes were observed in the translucency parameter after aging in Group 1 (slow sintering + polishing), Group 3 (fast sintering + polishing), and Group 4 (fast sintering + glazing) (p > 0.05). However, the significant effect of aging on translucency observed in Group 2 suggests that the sintering rate and surface finishing procedures may play a decisive role in light transmission. On the other hand, the significant change in translucency observed in Group 2 (slow sintering + glazing) is noteworthy. This finding suggests that glazing applied in combination with slow sintering may not be sufficient on its own to enhance the material’s light transmittance. This finding suggests that the glazing procedure applied under the slow-sintering protocol had a strong effect on translucency. The increase in TP values may be attributed to the glassy nature of the glaze layer, which is more likely to reflect light rather than absorb it. Nonetheless, whether this effect is limited to the surface or also involves internal structural changes should be clarified through advanced microstructural and microscopic analyses.

Factors such as the thickness of the glaze layer, the application method, or its response to aging conditions should also be considered, as they may negatively affect translucency. These results indicate that sintering strategies should be customized not only based on temperature and time parameters but also according to the type of surface treatment used. The outcomes obtained in Group 2 may be interpreted as evidence that the glazing procedure did not improve aesthetic performance and failed to ensure translucency stability. Therefore, the clinical aesthetic effectiveness of the protocol used in this group may need to be re-evaluated. Additionally, not only the sintering time but also polishing and glazing procedures also affected the TP values when comparing Group 1 to Group 2. Both groups were slow-sintered; however, polishing had positive effects more than glazing. In cases where slow sintering is applied, polishing before glazing may be recommended clinically to achieve better esthetics and translucency.

The findings of this study demonstrate that sintering protocols and surface treatment strategies significantly influence the translucency and opacity of zirconia materials. In particular, fast sintering combined with appropriate surface finishing methods may offer clear advantages in achieving superior aesthetic outcomes. Mai et al. reported that polishing enhances microstructural homogeneity on the zirconia surface, resulting in increased light reflection from the surface [30], a finding that is in agreement with the results of the present study.

Contrast ratio (CR) is a parameter used to assess the opacity of a material; as the CR value approaches 1, opacity increases, whereas values closer to 0 indicate greater transparency [4]. In ceramics with a highly crystalline structure, such as zirconia, translucency (TP) and CR values typically exhibit an inverse relationship. Although a negative correlation was also observed in our study, it was not found to be statistically significant.

In this study, all groups exhibited contrast ratio (CR) values above 0.92, indicating that the monolithic zirconia materials used possessed high opacity. This suggests that zirconia has limited light transmittance, which may reduce the depth effect in aesthetic restorations. The highest CR value was observed in Group 1 (0.942 ± 0.032). This may indicate that the prolonged sintering duration followed by mechanical polishing resulted in a more compact grain structure, increasing light reflection on the surface and thereby contributing to greater opacity.

Numerous studies in the literature support the influence of sintering protocols on the optical properties of zirconia. Jiang et al. (2023) reported that monolithic zirconia specimens subjected to fast sintering exhibited lower CR and higher TP values, suggesting that fast sintering may allow for more aesthetic and translucent restorations [31]. Similarly, Alshali et al. (2025) [32] stated that fast sintering reduces grain size, which, in turn, decreases light scattering and enhances translucency. Notably, an increased cubic phase content in zirconia materials containing 5Y-PSZ has been shown to significantly improve translucency [32].

The Copran Zr-i Monolith A2 material used in our study exhibits optical properties suitable for full-contour aesthetic restorations. Based on the findings of our study, it can be determined that the fast-sintering protocol is more suitable for aesthetic restorations due to its enhancing effect on translucency. Polishing increases CR values, thereby reinforcing opacity, which may offer an advantage for restorations in posterior regions. While glazing has a limited effect on translucency, it can contribute to a more translucent appearance by reducing CR values.

On the other hand, a study by Kim et al. [23] reported that staining and surface treatments applied to monolithic zirconia significantly affected surface roughness, with the smoothest surfaces achieved through glazing. However, these treatments were found to have minimal impact on translucency. The same study also reported that glazing caused a slight reduction in CR values [21]. The combination of sintering protocol and surface treatment is a critical factor that directly influences the aesthetic and optical performance of monolithic zirconia. Therefore, material selection and application protocols should be carefully tailored according to clinical requirements.

One of the key parameters evaluated in this study was the effect of the aging process on the surface roughness (Ra) of monolithic zirconia ceramics. The 96-hour aging protocol using gastric acid, as widely accepted in the literature, simulates approximately 10 years of clinical exposure [16,33].

The findings revealed that the aging process generally reduced the surface roughness of zirconia specimens. The most notable decrease in roughness was observed in Group 2 (slow sintering + glazing). This may be attributed to the glaze layer filling microstructural irregularities and producing a smoother surface. Similarly, a reduction in roughness was also observed in Group 4 (fast sintering + glazing) after aging. The fact that this group already exhibited the lowest Ra value before aging (0.170) suggests that fast sintering may result in a more compact and smoother surface structure. The further reduction in this value to 0.140 after aging indicates that this protocol could offer clinical advantages. Additionally, the heat treatment during glazing might contribute to recrystallization or densification of the surface layer, enhancing resistance to acidic degradation. Previous studies have also reported that glaze layers can reduce surface energy and act as a physical barrier against acid penetration, thereby preserving the integrity of the surface under low pH conditions [16,33]. These findings are consistent with the study conducted by Sulaiman et al., who reported that gastric acid resulted in a smoother surface on monolithic zirconia; however, this effect had limited impact on the material’s optical properties [16]. In addition, Theocharidou et al. [34] studied acid effects on monolithic zirconia and lithium disilicate in terms of TP and ΔE₀₀ after acidic storage and aging. They found that monolithic zirconia was advantageous over lithium disilicate. In contrast to the present study, which observed no significant correlation between surface roughness and optical properties, Li et al. (2022) [35] reported highly significant positive relationships between ΔE₀₀ and translucency parameters (r ≈ 0.88, p < 0.001), particularly in specimens with distinct shade differences. This discrepancy may stem from differences in ceramic composition, pre-treatment surface roughness, shade A3 vs. A1, or different spectrophotometric evaluation methods [35].

Statistical analyses revealed significant differences between the groups after aging (Kruskal–Wallis, p < 0.001). Pairwise comparisons using the Mann–Whitney U test showed that Group 1 (slow sintering + polishing) differed significantly from the other groups. The increase in surface roughness observed in this group after aging suggests that mechanical polishing may offer less resistance to aging effects.

On the other hand, when examining the relationship between surface roughness and optical parameters (ΔE₀₀, TP, CR), no statistically significant correlations were found. In particular, the relationship between roughness and contrast ratio (CR) did not reach statistical significance (p = 0.914). In accordance with our study, Kulkarni et al. [17] studied gastric acid on different porcelains (feldspathic porcelain, lithium disilicate glass-ceramic, and monolithic zirconium oxide) in terms of optical and mechanical factors such as color and surface roughness, and they concluded that there were no statistically significant changes between porcelains. However, they also revealed that zirconia ceramic showed resistance to surface treatments.

A major limitation of this study is that it was conducted in vitro. Since biological and mechanical interactions in the actual oral environment may influence material behavior, the direct generalization of these findings to clinical applications should be approached with caution. Additionally, the study was limited to a single commercially available monolithic zirconia material. However, zirconia materials from different manufacturers may exhibit varying optical and surface properties due to differences in composition and manufacturing technologies. Therefore, future studies are recommended to include zirconia materials from various brands and compositions. In addition, fracture resistance or bond strength should also be examined after different sintering and acidification.

From a clinical perspective, smoother and more chemically stable surfaces—as observed in fast-sintered glazed specimens—may offer advantages in terms of long-term plaque resistance, color maintenance, and patient satisfaction. However, the actual performance in the oral environment is subject to additional variables such as occlusal forces, pH fluctuations, and oral hygiene habits.

5. Conclusions

Within the limitations of this in vitro study, the following key findings were obtained: Groups subjected to slow sintering demonstrated higher ΔE₀₀ values compared to those subjected to fast sintering, indicating that slow sintering may result in greater color change. Aging with simulated gastric acid significantly affected color change across all groups. No statistically significant relationship was found between surface roughness (Ra) and the optical properties (ΔE₀₀, TP, CR). Although a moderate negative correlation trend was observed between contrast ratio (CR) and translucency parameter (TP), this relationship was not statistically significant.

From a clinical perspective, the selection of sintering speed and surface finishing protocols should not rely solely on material parameters. Patient-specific factors, oral environmental conditions, and the aesthetic demands of the restoration site must also be carefully considered. A multifaceted approach will contribute to achieving both aesthetically pleasing and functionally successful outcomes.

Nevertheless, several limitations of this study should be acknowledged. First, the in vitro nature of the experimental design does not fully replicate intraoral conditions, including thermal cycling, mechanical loading, and salivary enzymatic activity. Second, the study did not assess mechanical properties such as flexural strength or fracture resistance, which are crucial for clinical longevity. Therefore, while fast sintering combined with glazing appears to enhance esthetic properties, further in vivo studies and mechanical evaluations are required before definitive clinical recommendations can be made.