Color Variation in Color-shade Polycrystalline Zirconia Ceramics by the Atmosphere Controlled Firing

Color shade variation was investigated in zirconia dental blocks, prepared using commercial powders. As a reference colorshade block we used the color indexes of A2, A3.5, A4 and B3, according to the VITA classical color scale. The zirconia powders for color shade blocks showed colors of white, yellow, pink and grey, respectively, after firing at 1530 o C. The zirconia powders were mixed according to the recipe of color shade blocks and shaped at lower pressure using a uniaxial hydrostatic press. The shaped sample was inserted into a vinyl pack and sealed in a vacuum form machine. The shaped block samples were reshaped at 450 bar using an isostatic cold press and fired at 1530 o C for three hours. In order to investigate the atmospheric color variation with firing temperature, the A2, A3.5, A4 and B3 sintered blocks were fired between 700 o C and 1300 o C under controlled atmosphere of pN 2 and pO 2 . The surface color picture was taken using a smart phone camera and compared with the results obtained using the VITA classical color scale. Quantitative color index value, CIELAB, was measured using a color-meter. Above 800 o C, the color darkness greatly increased with the increase of the reduction temperature and keeping time.