Is Window Pane Glass really Glass? 5

[todcaesar]

I wanted to reproduce the wavy glass in my parents old house with a similar pane(and yes it is because I broke it). Can this be done by heating the glass in an oven?

I have heard that newer glass panes are different. Is this true?

tod

 

[NickE]

The way the glass is manufactured is very different. Modern glass is made by floating molten glass on a liquid metal bath. This produces the very fine surfaces and consistant thickness of modern glass. Depending on the age of the glass there are likely a variety of methods used to produce it. I recall a method whereby the glassblower would take a boule (hot gob of molten glass) and spin it while pressing onto a flat plate. The place where the shaft attached to the boule created a "bullseye" thus bullseye glass.

To exactly replicate the wavyness of the original is likely going to be expensive and difficult. (I would go looking for abandoned structures of similar age, get a piece larger and have it cut to fit)

Im not sure whether or not the chemistry of modern window glass (the simple kind not the really ultra new super efficient etc. glasses) is that much different from old glass.

nick

 

[IRstuff]

Your other choice is to replace ALL the glass to match ;-)

TTFN

 

[edrush]

Cool! What metal do they use for the "liquid metal"? Surely not mercury!

 

[kenvlach]

todcaesar,
Old window glass appears wavy due to both the original method of manufacture, and because it slowly creeps under its own weight. Use a micrometer and compare the thickness top and bottom.
It won’t cost much to do some oven experiments. Heat with panes ~vertical, let cool (unless you have some very high temperature gloves), rotate 90^o and repeat. The older glass may also have a faint bluish color characteristic of less pure flint glass [more experimentation].

edrush,
"liquid metal" is liquid tin at about 1000^oF. Mercury is unsuitable due to its high vapor pressure/low boiling point, 356.7^oC. The properties which make tin suitable for the ‘float glass’ or ‘Pilkington process’ are a moderately low melting point, extremely low vapor pressure (doesn’t boil until 2602^oC), high density (otherwise, no float glass!), non-wettability and inertness to molten glass. The non-wettability/inertness is because tin is more noble than Si, Ca and Na. Window glass is composed of about 72% SiO₂, 14.3% Na₂O, 8.2% CaO, 3.5% MgO, 1.3% Al₂O₃ and traces of other oxides. Liquid Al would be unsuitable as a float base since it can reduce SiO₂ and Na₂O, plus it is only slightly more dense than molten glass, so more difficult to maintain a flat base.

The float glass process used for window glass manufacture is described and shown at

http://www.pilkington.com/corporate/english/education/float+process/step+by+step/default.htm

Hope this helps,
Ken

 

[drawoh]

todcaesar,

Check out

http://math.ucr.edu/home/baez/physics/General/Glass/glass.html

This is an interesting article on whether glass is a liquid or a solid. It answers part of your question.

JHG

 

[usjbh]

A few other sugestions are to look on e-Bay using a google search for antique wavy glass window panes. They are a hot commodity.
A more enjoyable and fun way to search for your glass panes is to go to the spring antiques shows held in May at Brimfield Mass.

http://www.brimfieldshow.com/

or look up some of the restoration salvage companies that stock pile old windows with wavy glass. There are some antique shops that also specialize in old replacement glass windows and trim. Happy hunting. I bet your parents were really pissed.

 

[EnglishMuffin]

kenvlach:

"Old window glass ........ slowly creeps under its own weight. Use a micrometer and compare the thickness top and bottom."

I have never believed this - the rate of creep, even if it were a fact, would be too small to be visually obvious over the relatively brief time span since the mediaeval period, and it was cursory observations of such crudely made glass that first gave rise to the idea. I am also skeptical that anyone has ever demonstrated the effect by using a micrometer on more modern glass. I think it's one of those things that gets repeated in textbooks over and over until someone actually investigates it carefully, and I think it has been somewhat debunked. Of course, I could be wrong, but see the following:

http://www.glasslinks.com/newsinfo/supercooled.htm

http://www.math.ucr.edu/home/baez/physics/General/Glass/glass.html

 

[usjbh]

You brought up a good point about the variation of the glass thickness when originally made. If I were putting glass like this into a new window I would put the thick end down as this would provide more strength when shutting the window and would also provide a better slope for shedding rain water.