Since Shapeways introduced glass to the range of materials available the team doing the process have come across the barriers of possibility, seen some amazing results like the Vase below which was hand painted with leaded glass enamels of the sort that traditional stained glass artist use. This is a secondary process, sort of like glazing pottery.
GlenG is part of the team responsible and is generous enough to post really helpful information in the forums on a regular basis about the glass process.
We have had very few problems with small solid parts . Failures are mostly due to bad design practices like overly thin sections, radical sectional changes, and lack of fillets. These problems tend to become magnified as model size increases. When designing hollow parts accommodations must be made to allow the support powder to completely fill all empty spaces. The support powder acts as a mold during firing. The glass becomes like soft toffee during firing and without support it will move in unexpected (usually disastrous) ways.
These are things we know for fact.
A big thanks to Glen for his time and effort in sharing his knowledge and experience from the coalface.