check out the pyroceram material that mcmaster-carr sells.
its under glass-high temperature.  i know it has decent ir transmission because it transmits a lot of heat.  and it has additives for the high temperature properties.  hope this helps.

I actually found this out myself-
There are over a dozen materials that are good for the 2-15 micron band.
They are all crystal-grown.
Si,Ge,BrF, sapphire, just to name a few, each good for diff. applications
Si is the cheapest, but none are cheap.
I went to www.almazoptics.com and they have a lot of info there on the different materials- or do a search on altavista
with different mixtures of the following words:
Si, Silicon, Ge, optic/s, window, lens, IR, crystal, transmis*

The best material to use is fused silica - it's good from the far UV 180 nm  upto 2500 nm. From 2600 to 2900 nm the transmission drops to about 10%. Then goes back to 90% +.
Quartz is OK but silica is better.