https://vimeo.com/131786000?from=outro-embed
It’s already possible to use tiny granules of glass in a powder bed with conventional 3-D printing techniques like jetting and sintering, but the products turn out opaque. Now researchers at MIT have demonstrated the first-ever machine that can print molten glass through a nozzle and make transparent glass objects layer by layer according to digital instructions.
The most challenging aspect of printing glass is that it must happen at extremely high temperatures. To flow well enough to be extruded through a nozzle, the material must be kept at a temperature greater than 1000 °C.
https://www.technologyreview.com/s/540926/3-d-printing-breaks-the-glass-barrier/