## 4 Discussion  
In this study, the color stainability of ceramic crowns after immersion in coffee followed by brushing was investigated. The crowns were constructed from three repeated heat-processed lithium disilicate ceramic ingots (E-max, Lisi, Celtra). It was evident that the three tested crowns were all susceptible to staining after immersion in coffee. However, with respect to the three axes of the CIE L * a * b * system, various behaviors were seen. In general, immersion in coffee followed by brushing decreased the staining of all tested ceramic crowns to the acceptability threshold of color change (\(\Delta\)E = 2.7); thus, the null hypothesis was accepted. The results of color measurements agreed with previous studies that indicated that exposure to common beverages such as coffee resulted in a reduction in the luminosity and a darker color in the specimens, with decreased L * values [18; 29; 31]. This pigmentation may be caused by chromogens in the coffee, which are substances that can stick to ceramic crown surfaces and cause discoloration [31]. Coffee contains various compounds that contribute to its characteristic color. The staining composition of beverages in general can vary depending on factors such as the brewing method, preparation, and additives. Coffee is typically made from roasted coffee beans and contains a variety of compounds such as caffeine, chlorogenic acid, and melanoidins. These compounds contribute to the dark color and staining properties of coffee. Additionally, coffee can contain significant amounts of oils and fats that can adhere to surfaces and contribute to staining. Since the baseline L, a, and b values of the various materials differ, it is essential to assess color changes in the specimens using the \(\Delta\)E values rather than the L, a, and b values obtained after aging/staining [19; 32]. Furthermore, the formula for the calculation of color changes (\(\Delta\)E) commonly used in previous studies is dependent on values of L, a, and b. This discussion therefore focuses on the results of color changes rather than discussing L, a, and b values one by one. Regarding the color measurements (L, a, and b) and differences (\(\Delta\)L, \(\Delta\)a, and \(\Delta\)b), a significant difference was observed between Celtra and the other two groups, E-max and Lisi. This may be due to the zirconia content of Celtra, which is not present in E-max or Lisi. Regarding the color changes (\(\Delta\)E), all groups showed a significant color change from baseline to post-saliva immersion. However, the reported \(\Delta\)E values (E-max = 1.02 \(\pm\) 0.24, Lisi = 0.82 \(\pm\) 0.22, and Celtra = 0.91 \(\pm\) 0.23) were smaller than the PT thresholds (\(\Delta\)E = 1.2). After brushing, the color changes (E-max = 0.91 \(\pm\) 0.28, Lisi = 0.71 \(\pm\) 0.21, and Celtra = 0.83 \(\pm\) 0.29) were not significant in any of the groups (\(p\) > 0.05). On the other hand, in the experimental groups, all groups showed a significant color change from baseline to post-coffee immersion, with values above the PT thresholds (\(\Delta\)E = 1.2). Lisi showed a smaller degree of color change (\(\Delta\)E = 2.84 \(\pm\) 0.95) that was significant (\(p\) < 0.05) compared to E-max (\(\Delta\)E = 3.81 \(\pm\) 0.75) and Celtra (\(\Delta\)E = 3.52 \(\pm\) 0.84). These results may be related to the possible highly smooth surface of Lisi, which contains equally dispersed small-size microcrystals (HDM) compared to the large-size lithium disilicate crystals in E-max and Celtra [20], making it difficult to be stained. However, this explanation cannot be confirmed, as we did not measure the surface roughness of the crowns. There is evidence in the literature that repressing lithium disilicate ceramic results in the growth of crystals due to the Ostwald ripening phenomenon [25]. Therefore, the large crystals of E-max and Celtra may become larger with repeated pressing, leading to greater color changes due to additional crystal boundaries that facilitate water and pigment penetration [18].