It is said that under the equilibrium condition, potash feldspar decomposes into leucite and melt at about 1150°C, which is within the temperature range for the firing of porcelain. Therefore, it is an important problem to know whether the leucite is actually formed in porcelain bodies containing potash feldspar during the firing process.
Potash feldspar specimens fired at 1150°-1450°C, or soaked at 1260°C for 4hrs and kept at 1300°C for 10.5hrs, were examined by means of polarization microscope and X-ray diffraction (Fig. 1.), but with a result that no leucite was detected in the fired specimens. Furthermore, specimens consisting of potash feldspar and quartz and -or kaolin were examined by the same way without being able detect in their fired specimens either.
From the results obtained it was concluded that the potach feldspar does not decompose leucite and melt, but melts completely into glass, so far as it is fired with the schedule used in the porcelain bodies.
The fact can be explained as follows: If the potash feldspar begins to melt at higher temperature than T2 (Fig. 3A), it does not decompose into leucite and melt which are coexsting in the most stable condition, but usually melts into a glass, the only one which is the next stable. These relations between the phases formed with temperature and time are infered as Fig. 3B.
The compositions of porcelain bodies containing potash feldspar are usually in the shadowing range of K2O-Al2O3-SiO2 diagram (Fig. 4.). From the diagram it is apparent that the leucite is formed only in the very high potash feldspar bodies which are higher in K2O than that of N-Z line in Fig. 4. Therefore, in the porcelain bodies, leucite is hardly formed even if they are fired under the equilibrium conditions.