Pressure weld the two very clean, smooth, optically-wavelength-matched surfaces in a vacuum. No binder, low temperature (depends on how long you want to wait) makes an optically invisible joint. Cons: difficult to keep surfaces clean and precisely fitting. May need RF sputtering. Also, experimentation needed to get it right. Other people might call it cold welding.
It is impossible to bond the quartz glass by melting method with the 850 C environment condtion.
Becacuse generally the quartz glass temperature should be reached to the softening point, 1530 C to be bonded each other.
So, try to chage the materail from quartz glass to Vyco, the hard glass.
Even if its mechanical specification is slightly lower than the one of quartz glass, with 850 C it may be softened, and became workable condition.