Kevin,
Glass often doesn't work the way you want it to. For common US boro, a 1200 Hammer is heating at about 664C, which is well above the thermal stress point (which "just" has to be figured in). G is correct that there are many reasons, but for a simple one, anything as thick as you ask for would probably fracture very quickly from thermal stress. When I used glass, it was common to switch to more expensive quartz if you were going above 400C (something to avoid for many other reasons too). There is a lot to go though and many sources for information, but a good start might be to note the first warning (as well as the rest) at:
http://www.udel.edu/chem/GlassShop/PhysicalProperties.htm
Where something becomes "thick" isn't said (there), but 5 or 9mm would count. Also while a torch flame is hotter than a 664C Hammer, the temperature is above the stress point and hopefully the exterior is cool enough to also be greater than the thermal shock differential - So thicker glass would be more likely to fracture when cooling.
It might be nice to protect it somehow, but if the Holy Grail is "all glass path", this looks like it delivers.
-NDA