Fusing borosilicate glass in a silica crucible - using an outer platinum crucible filled with glass mixt. to prevent fracture on cooling

摘要

Making a glass esp. a borosilicate glass consisting of melting a mixt. of powdered minerals in a silica crucible and cooling he mixt. is claimed in which, to avoid formation of fractures in the glass during cooling, the heating of the mixt. (I) also affects a second mixt. (II) of minerals melting it (II) of minerals melting it, (II) being in a second crucible surrounding the first, and in which both melts are cooled simultaneously. The second crucible is metallic, esp. of Pt. The borosilicate in the inner crucible does not contact Pt as in prior art and hence does not pick up impurities such as Fe prevent in the Pt. The Melt in the outer crucible permits the melt in the inner crucible to cool more slowly and hence the fracturing due to the difference of thermal expansion coefft. does not occur. Since the SiO2 cools at the same rate as the glass. It is thus not necessary to remove the glass from the SiO2 crucible while still hot.