Hi Ludwik,
Couldn’t find VETROPYREX in searching the Internet (even tried Vitropyrex, etc.).  However, judging from the name, it might be similar to Corning’s Pyrex® for which there is lots of information.  Pyrex is a borosilicate glass much used in chemical laboratories and cookware, although it is inferior to pure silica glass in terms of temperature resistance.  Information on the glass chemistry, softening temperature and operating temperature is given in the following references.  Although silica glass has a higher operating temperature and a lower coefficient of thermal expansion than Pyrex®, this is perhaps less important than the method of mounting and sealing the sightglass.  It is also important for safety to temper or chemically treat the glass in order to induce compressive stress within the glass surface.

Who is your vendor for this Vitropyrex?  I would expect them to furnish information to you.

Some references on glass:

‘NOTES ON GLASS’ (John M. Rusin, Edmonds Community College, Lynnwood, WA 98036)
 http://depts.washington.edu/mti/1999/labs/glass_ceramics/glass_notes.html
A primer on glasses and glassmaking, compositions, thermal data tables.

Valley Design http://www.valleydesign.com/
Properties of various glasses.  Slicing, lapping and polishing for optical and semiconductors, etc.

Technical Glass catalog http://www.technicalglass.com/tgphiqu.pdf
Technical data for FUSED QUARTZ (silica), commercial forms available.

Hope this helps,
Ken

NEVER HEAR OF VECTROPYREX. PYREX IS A BOROSILICATE GLASS. IT HAS VERY LOW THERMAL EXAPANSION (3.2 PPM/C). I WOULD NOT USE IT OVER 500C.

If you need a glass for higher temperarures than borosilicate, fused quartz or fused silica is the solution.

Fused quartz/silica will withstand temperatures of over 1000 degrees C and have virtually zero thermal expansion (0.54 ppm) so no problems with thermal shock.

George