Edgar Dutra Zanotto of the Federal University of Sao Carlos in Brazil calculated the time needed for viscous flow to change the thickness of different types of glass by a noticeable amount. Suffice it to say that the glass could not have thickened since the 12th century. Zanotto reports his finding in the May American Journal of Physics. The study demonstrates dramatically what many scientists had reasoned earlier. You would have to bring normal glass to 350 deg.
Viscosity depends on the chemical composition of the glass.
Medieval stained glass contains impurities that could lower the viscosity and speed the flow, but even a significant reduction wouldn't alter the conclusion, he remarks, since the age of the universe is only 10 to the 10th power. That's 100,000,000,000,000,000,000,000,000,000,000 years folks. Try saying that number with a mouthful of marbles. The difference in thickness sometimes observed in antique windows probably results from glass manufacturing methods, says LaCourse. Until the 19th century, the only way to make window glass was to blow molten glass into a large globe then flatten it into a disk.