Effects of Refiring on The Flexural Strength of Low-fusing Porcelain

Objective: To investigate the effect of refiring on the flexural strength and colour changes of low fusing porcelain. Methods: Four groups of ten porcelain bars each were prepared from enamel powder of low fusing Finesse Porcelain. The specimens were fired under vacuum and ground to a final dimension of 4.0x1.5x25.0mm using abrasive wheel, coated with 30µm grit diamond particles on a Buehler, Metasserv Grinder. Group A served as a control group with no refiring treatment. Group B, C and D were subjected to one, two and three refiring cycles, respectively. The flexural strength was determined using a three point bend test, on a universal testing machine (Instron) at a crosshead speed of 0.5mm/min. A spectrophotometer was then used to measure the colour changes DE) of refired specimens. Results: Statistical analysis using ANOVA and Tukey's post hoc test revealed that the mean flexural strength significantly increased after three refiring cycles at p<0.05. There was a 7% significant increase in flexural strength observed between Group A (80.01±3.93MPa) which received no refiring treatment and Group D (85.58±3.61MPa) which was refired 3 times (p<0.05). No significant increase occurred in Groups B (82.82±3.03MPa) and C (83.89±2.46MPa). There was no decrease in strength observed after refiring procedure. However, the specimens became significantly lighter (p<0.001) in groups, which were refired twice (C) and three times (D). The mean DE values for groups of specimens which were refired once, twice and three times were 2.37±1.54, 7.26±2.00 and 8.97±1.47, respectively. Conclusions: Clinically, when there is a need for refiring of low fusing porcelain restorations, such procedure of refiring up to three cycles is regarded not detrimental to strength as long as the manufacturer's firing recommendations is strictly followed. This study was supported by Vot F 0105/2002B (University of Malaya).