>>/6777/
Windows are walls, but they are not walls to Light. What is Light? The gods know.
The substance of a window includes all that passes through it. Includes all Light. Green, grey, white or black. Glaziery may thus be viewed as the craft of granting passage to Light. A lens is thus hermetic in its most fundamental essence. 
Also, if we subject sand to a crucible, it may become glass.
Also, nice digits.