In this work we chose B

2

O

3

-La

2

O

3

-Y

2

O

3

-ZrO

2

system as the base glass composition. The glass composition of LaK 747/509 which we obtained at last is free of Thorium. Because this glass has a series of good properties such as narrow devitrification range. Slow devitrification speed and high viscosity etc

so it can be melted successfully in 12-30L platinum pot and glass blocks about φ200×25mm with good quality were obtained.