反復焼成による歯科用陶材の色調変化

Abstract

From the viewpoint of dental prosthodontic purpose, it is important to restore function and esthetics. Harmony in color is one of the important factors in dental esthetic requirement. Dental porcelain have been widely used as a restorative material because of its ability to maintain its surface texture and color for an extended period without losing its naturalness and esthetic qualities in matching the adjacenting tooth structure in translucence, color and intensity. In the past, however, it was rather difficult to effect porcelain restoration which harmonized in color with the adjacent natural teeth. There are many factors which affect color stability during porcelain firing. With a realization that the number of firing affects color, an experiment was conducted to determine the relations of the number of firing and of the materials to color. For the experiment, vacuum fired porcelain test pieces, containing either opaque, body, or enamel, were subjected to repeated firing in vacuum for a different number of times and to natural glazing after firing. The color and color difference were then determined with a color meter. The integrating sphere system of JIS Z8722-II was used for measuring the color and color was represented by Y.X.Z. of tristimulus values of CIE system. A computer program with the numerical value of Hunter's and CIE color systems was prepared to obtain Hunter's numerical representation (L.a.b) of color. Representation of color and color difference were calculated with L.a.b. numerical values for every test piece in each group, which had been subjected to repeated firings and natural glazing. From the value of Hunter's and Munsell's color systems, the color and its variations were described with popularly used 'Hue, Chroma, and Value'. The ⊿E of color difference in repeated firings and glazing was compared with that of the first firng for each test piece. The results were as follows : 1. The ⊿E of color difference in repeated firings of the opaque test pieces was almost below 1.5 NBS, which varied from 'trace' to 'slight' macroscopically. The changes were caused by an increase in chroma or in value. 2. The ⊿E of color difference in repated firings of the body test pieces was between 0.5 and 1.5 NBS. The change in the range of 'slight' were observed from the 3rd to 6th firings. The changes ranged from 'noticeable' to 'much' after the 7th to 10th firings (After the 2nd to 6th firings in the enamel test pieces). 3. Over 1.5 NBS were detected macroscopically following the 7th or 8th firing of the body test pieces (After the 4th to 7th firings in the enamel test pieces). 4. The degree of color difference increased in proportion to the number of firing in both the body and the enamel test pieces. The changes were caused in most cases by an increase in value and particularly by an increase in chroma. 5. Better surface glossiness was seen following the 1st to 3rd firings but the glossiness decreased thereafter in proportion to the number of firing. 6. Glossiness was lost in natural glazing following the 8th firing in the body test pieces (After the 5th to 7th firings in the enamel test pieces). 7. The test pieces with glossiness after glazing decreased in value.