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.
Finding a nozzle suitable for printing glass was a complicated task, according to Peter Houk, head of MIT’s Glass Lab. To work it must be made of a material that can both handle high temperatures and resist the glass sticking to it. “At these elevated temperatures almost everything wants to stick to glass,” says Houk. Platinum nozzles are used in industry for some glass manufacturing processes, but are very expensive. Instead the group settled on a custom-made nozzle made of aluminum oxide.
The machine prints soda lime glass, a family of glasses used in everything from water glasses to windows. But glasses like Pyrex could in principle be printed this way too, albeit at much higher temperatures
SOURCES - MIT Technology Review