Old Wavy Glass Panes- Reproducible?

[todcaesar]

Are the old glass panes wavy because of flow downwards over time or were they made that way originally?

Could this be replicated by heating a new pane of glass?

Thanks,

Tod

 

[patdaly]

Doubtfully. New glass is produced by floating the glass over a molten Tin bath, and allowing gravity to do much of the work. In addition, they have come a long ways in Blister reduction ( tiny air pockets ) I am not sure exactly how they produced old window glass, but I would bet it involved some sort of a horizontal pulling process while molten, and the waves would be a result of slight temperature inconsistancies.

Flow downwards over time results in the pane being thicker at the bottom than at the top.

 

[ewh]

While this may not be a duplicate question, it has been answered in thread367-83108.

As far as glass “creeping” from top to bottom, here is EnglishMuffins response and links to back it up from that thread:

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

 

[patdaly]

ewh:

I have more or less satisfied myself, on 2 old farmhouses, one with windows pretty well documented by the family back to 1868. Using a Micrometer, I was able to document a relatively constant 6 to 7 thousands variation from top to bottom on the older one, and about 5 on the House I just tore down......dont ask, I just like wierd stuff like that.

So, knowing the Amorphous structure, knowing that Glaziers in the 1800's didnt run around with Micrometers, and the consistent top to bottom variations, I will stand that the cause is indeed super viscous flow.

 

[ewh]

patdaly,

I cannot argue with you. All I have to go by is what I have read in various articles claiming that any variation in thickness is due to manufacturing methods, not creep.

Here is a quote taken from the article in the link that follows:

“It is worth noting, too, that at room temperature the viscosity of metallic lead has been estimated to be about 10 to the11th power, poises, that is, perhaps a billion times less viscous-or a billion times more fluid, if you prefer than glass. Presumably, then, the lead caming that holds stained glass pieces in place should have flowed a billion times more readily than the glass. While lead caming often bends and buckles under the enormous architectural stresses imposed on it, one never hears that the lead has flowed like a liquid.”

http://www.ask.co.uk/ix.asp?q=old+g...fn=t&b=0&fo=2&r=10&io=1&fp=4&fr=1&url=http://

http://www.glassnotes.com/WindowPanes.html&adurl=

 

[patdaly]

ewh:

LOL, I cannot argue with the authors either! That is one thing I do love about Glass, it continues to amaze me after all these years.

Good links, I had not read much of that material.

 

[unclesyd]

Here is website that might help

http://www.oldhouseweb.com/

 

[dj1]

Before glass was floated on molten tin, it was forced upward through a channel buy molten glass under it and pulled by gears to get the right thickness. This process produced sheet glass and it had the wavy look. The process was also inconsistant on producing thickness, thus variations in measured thickness.

A few glass companies produce a pattern glass that closely imitates the old look just for the renovation market.

The issue with glass running over time is a theory based on the chemical properties of glass. Given the estimated rate of flow, there has not been any glass around long enough to measure it.