I came across this earlier:
www.doobybrain.com/2012/11/08/how-heat-resistant-borosilicate-glassware-is-manufactured (http://www.doobybrain.com/2012/11/08/how-heat-resistant-borosilicate-glassware-is-manufactured)
I'm not sure that Pyrex is still generally made out of borosilicate glass, but it was fun to watch anyway - but then, I'm always a fan of watching melting glass ;D ;D
Cool video, I quite fancy the outfit they use when cleaning the hot moulds! :D
Fascinating ;D