Crystals in porcelain — A correction

The writer recently presented a series of articles in the Journal in which the composition of porcelain was described, together with the pyrochemical reactions which take place during the firing. It was shown that the firing brought about a combination of certain parts of the silica and alumina first in the amorphous and crypto-crystalline form and later, due to the effect of time and temperature, needle like crystals wereformed. These crystals have been generally understood to be of the composition Al2 O3 Si O2 and hence have been called sillimanite. Sillimanite is a definite natural mineral of the composition Al2 O3. SiO2 and petrographers have repeatedly pronounced the crystals found in porcelain to have the same optical properties as the natural mineral and hence have called it sillimanite.