MIT’s Mediated Matter Group made a video showing off their first of its kind transparent glass printing process.
http://gizmodo.com/watching-mits-glass-3d-printer-is-absolutely-mesmerizin-1725433454
Almost perfect loop.
it look as a direct “pellet” 3d printer 
what is used in the mixture ?
Easy - extruding glass at a mere 1000°C…!
That should warm up my office very nicely 
It looks to be gravity feed only. No mechanism for stopping the glass flow. Only temperature control on the crucible to change flow rate. Nice output tough.
So no retraction? 
@Adam_Steinmark Mebbe if you pulled a vacuum on the crucible…
@Mark_Fuller I feel like that would just clog it.