Porcelain shade stability after repeated firing.

There were no color changes detected in the first five firings for both air- and vacuum-fired porcelain. Slight changes were noticed in subsequent firings. These color changes were more noticeable in the air-fired samples than in the vacuum-fired samples. However, changes in the porcelain color after repeated firing were not sufficiently significant to cause rejection of the samples. The porcelain shade remains stable with repeated firing if the firing procedure is carefully carried out.