Spectroscopy - Postlab

Question #5

This is the absorption spectrum of didynium glass. It is often used in glass blowing glasses to help the glass blower see the glass he is working on . The picture below left is what a rod of glass looks like when heated with a glassblowing torch. The picture below right is a similar picture taken through a piece of didynium glass.

Given that laboratory glass contains the oxides of silicon, sodium and a variety of other metals, what quality of didynium glass makes it useful for this purpose?