Any PORT in a ...

Myself and a couple of others are in the initial process of fabricating a cupola furnace.  I am wondering about the glass for the tuyeres and remembered yours.  So I wondering what what diameter and thickness you used for your tuyere?  I assume it was high temperature glass.

Air piping - here fitted to a ceramic tube tuyere. The end fitting to the upper left is sealed with the clear lens under discussion here.

We had looked into special filter / high temperature glass at the beginning.
Getting a disk of filter glass the right size was going to be horribly expensive.

What we ended up with is a piece of simple, 1/8 thick plexi (plastic)
What we totally forgot - there is no heat at that point in the system!
With our rig, we have the input air coming in via a T or Y joint, located 'down stream' towards the furnace. This means the view port end is being washed with cold (ambient temperature) air.  The plastic was cheap, and we figured easy enough to replace if needed. That end port you saw has been through something plus 50 smelts at this date!

With the use of a copper tuyere - there is some heat transmitted back along that metal (which is how the copper keeps from melting btw). This effect is not significant with the ceramic tube tuyeres we use as out other standard.
I have one Y tube actually made of ABS plastic with a leather cover (used in historic demonstrations). So far that unit has gone through about a dozen full smelts! 

ABS plastic Y, leather covered - here fitted to a steel pipe tuyere

So - I would suggest you consider the tuyere material (may be steel pipe, set flush to the refractory interior lining on the furnace side?) Conduction down the tuyere itself is more likely your consideration.

There would be some value to using a dark colour plexi (would avoid the use of the welding lenses I have). I know you can purchase replacement plastic filters for welding helmets, something like $5 for enough to easily cut two end caps.