Dan:
These are machine times. I used one of the old one-armed bandits we have at the shop, a nice name for basicly an ac driven spindle , with a manual eccentric. Using a 1/3 stroke, with a 12 lb wieght on top, flipping mirror and tool half of every grit run time. As for silicon vrs pyrex, silicon is much harder (I believe 7-8 on mohr scale), pyrex would take even less time.
Bill Marriott Forest Knolls, Ca. btk@ix.netcom.com