[time-nuts] Lifetime of glass containers

[Lux, James P]

> -----Original Message-----
> From: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] On Behalf Of Dave Carlson
> Sent: Monday, June 15, 2009 12:57 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Lifetime of glass containers
> 
> Not to charge in, but I've looked at ordinary window pane 
> glass in very old buildings and you can actually see the 
> rippling effect that occurred over time, showing the "flow" 
> of the glass toward the lower edge of the pane. One presumes 
> that the panes were relatively uniform when installed 120 
> years earlier. Sounds liquid to me.
>
Nope..   120 years ago, I don't think they had modern float glass or even continuous casting processes.

You blew a large cylinder, cut it open, and laid it flat in an oven, or took molten glass, poured it onto a flat surface, rolled it flat, then polished it (with a "plate" hence the name "plate glass")

Sometime early in the 1900's they started making glass in a sort of continuous casting process with slots or rollers or some such scheme to make sheets, but it wasn't very flat in an optical sense.

After WW II, they developed the float glass process, where the molten glass is floated across liquid metal, giving you continuous production AND flat surfaces.

So, what you're seeing in old buildings is the fact that flat glass was really hard to make and expensive.  You might use it in a mirror, for instance, if you didn't use polished metal instead.

I'm sure wikipedia has more than anyone would want to know about sheet glass manufacture..