Methods: This study was performed in single blind experimental method. At first, a standard brass die with 7 mm length and 5 mm diameter was made and then it was prepared by an accurate machine in a classic chamfer form with 0.8 mm depth and 10 degrees of convergence. Copings were made on die with CAD/CAM method: One group of 10 samples with 0.5 mm thickness. Then the copings were placed on the die and were numbered randomly. The vertical gap of marginal zone was measured in microns in 10 different points by SEM microscopy. After firing, the porcelain restorations were placed on the die and vertical gap of the marginal zone in the previous points were measured again by SEM microscopy. After recording the data and determining the mean and standard deviation, ANOVA test was used for comparing the data before and after the firing process.
Results: The amount of marginal gap in porcelain with 0.5 mm thickness was 79.55±39.32 μ before porcelain firing and 82.50±22.40 μ after porcelain firing .Test showed that there is no significant difference between the marginal gap of this thickness before and after porcelain firing.
Conclusions: Porcelain firing process does not affect the amount of marginal gap in all-ceramic zirconia core with 0.5 mm thickness.