Wow, thanks very much to all who responded! All the responses have been most enlightening.
So it seems that I was thinking generally along the right lines (which is always encouraging!) -- but with much more complexity than I had imagined. But essentially, the answer is that those...
I'm looking at a glass "recipe" that heats the raw ingredients to melt at 1550C. This seems to present a problem in that the raw ingredients include:
Al2O3: melting point = 2072 C
MgO: melting point = 2852 C
And even SiO2 (the primary ingredient) has a melting point (range) of...