The initial batch was melted and heat-treated in platinum and
glassy carbon crucibles in a muffle furnace SNOL 7.2/1300. The
temperature and melting time were optimized. Complete dissolution
of the components occurs at temperatures above 900 1C. The melt is
frequently stired at regular intervals to make it homogeneous. The
initial temperature of the glassy carbon crucible in which glass
casting was performed was varied to control the cooling rate of the
melt. The technique for making glass castings was developed. The
batch was first preheated at 800 1C in a muffle furnace (time
duration of this stage should be mentioned).