Defects of High Silicious Borosilicate Glasses During Melting

It is well known that Pyrex glass, borosilicate glass with high silica-content, is more difficult to be melted homogeneously than other glasses, because striae, knots, and stones very often arise during melting. It is the purpose to confirm the cause of striae, knots, and stones arising during melting in commercial Pyrex glass wares. Experimental equipments used, are centrifuge, interferometer, and strain analyzer.Most of striae have lower refractive index. They arise from raw material difficult to melt, SiO2, at early stage. At later stage they mainly arise from surface volatilization. The striae with higher refractive index arise from Al2O3, ZrO2 exuded by refractory corrosion, and also stagnants which originate from refractory corrosion have higher refractive index.The knots occur mainly from segregation of silica sands in raw materials and the refractive index is lower. But the knots with higher refractive index can be seen occasionally. They arise in the melting process of stones from refractories.The stones almost arise from segregation of silica sands in raw materials as well as knots. The refractive index of the surrounding glass is lower than that of the parent glass. The stones with higher refractive index originate from refractories.