According to PhysOrg.com news:
A team of engineers and artists working at the University of Washington's Solheim Rapid Manufacturing Laboratory has developed a way to create glass objects using a conventional 3-D printer. The technique allows a new material to be used in such devices.
The key to the new Vitraglyphic process was the realization that if a material could be powdered to less than 20 microns in diameter, the material could likely be successfully fed through 3D printers. The glass powder is mixed with a binder and then objects are printed. After printing completes, the object is then fired to fuse the glass bits together.