EFFECT OF SIZE AND SHAPE OF TITANIUM OXIDE CRYSTALS ON SPECTROPHOTOMETRIC PROPERTIES OF TITANIUM‐BEARING PORCELAIN ENAMELS*

Abstract

A bstract Color changes of three titanium enamels given varied firing treatment were investigated. The size and shape of titanium oxide particles were studied with the electron microscope, the relative amounts of anatase and rutile were determined from X‐ray analyses, and spectrophotometric curves of the fired panels were made. Electron micrographs showed that rutile particles recrystallized as needles or “sticks,” whereas anatase appeared as ill‐defined, irregularly shaped, rounded particles. With increasing firing temperature or firing time (1) rutile particles showed a greater increase in size than anatase particles, (2) the amount (by weight or volume) of anatase crystals decreased and the amount of rutile crystals increased, and (3) a color change from blue‐white to cream‐white was observed. The change in color seemed to be related to the scattering of blue light by small particles at lower temperatures or shorter periods of firing. As the size of opacifying particles increased, the scattering of blue light decreased, and the characteristic absorption of visible light in the shorter wave lengths was more apparent. In enamels containing both anatase and rutile, the smaller anatase crystals were predominant at lower and the larger rutile crystals at higher temperatures, so that sharp changes of color were observed as the firing temperature was increased.