A while back we had a discussion debunking the idea that glass changes its shape slowly over the years under the influence of gravity.
That led me to question another thing I’ve been believing about old glass, that it gets more brittle and harder to cut without breaking. Does glass change with time in that way, or was glazing just a lot harder to do long ago?
— J.S.
Replies
What do you mean, debunking? Glass is influenced by gravity. In very old windows, the glass is thicker at the bottom of the pane than at the top. That's why it is wavy, too. I was at Paul Revere's house in Boston when I was a kid, and the guide talked about the glass being a super-cooled liquid, and not a true solid.
Bear
yeah, I can't say that I understand it, but glass is in a liquid state, & old glasswill be thicker at the bottom cuz of gravity!(I know thats what you just said, but it still blows me away, & I really get a kick out of saying things that I know are true, but are beyond my understanding!)
glass is an amorphous solid, meaning it does not have any ordered crystalline structure. I've heard it refered to as a super high viscosity liquid, and given the fact that it flows more easily (lower viscosity) with heating, that makes sense. Materials with an ordered crystalline structure like ice or iron, will not flow (under small forces) until they melt. As they melt, they continue to absorb heat without any increase in temperature until all the solid has melted. This heat is "shaking" the atoms loose from their happy home in the crystal lattice. I believe (haven't tried it myself :) that glasses do not show any such distinct heat of fusion. I would be very curious to know if anyone has actually mic'd an old piece of window glass to see if it was thicker at the bottom.
Scott
Yes a Brazilian professor did in 1998; he looked at 12th century glass from cathedrals; his findings--it would take longer than the age of the universe for the thickening to be caused by the "flowing" of the glass. He attributed the "thicker" at the bottom to be due both to manufacturing methods and installation. An MIT professor joked, "it's true, half are thicker at the bottom, and half at the top".
Regards,
Rework
I'm way too much of a rookie at cutting glass to answer the questions about brittlenes vs age. I did recut some salvage glass for replacement windows last week. I had taken big old plate glass thermopane out of a building last spring and split them and salvaged glass. The cuts went well in general but any sliver cuts were challenging. I'd love any glass cutting tips others would like to share.
joe d
PS I'm single handing a 3'X5' 1/4 plate glass window into its frame and the peanut gallery comes up right behind me talkng nonstop. When he says what'll you do if doesn't fit? I growl it'll fit.
joe d,
Buy a stained glass oil filled cutter with carbide wheel. One smooth stroke to cut. On old glass I sometimes half to flip and recut the other side. Make a pattern for curved cuts and slice off small pieces from original cut line. Duckbills help with sliver cuts.
KK
If the glass moves in our old windows, should we remove it and turn it over every 10 or 20 years?
Have a nice day..............elsewhere.
yeah, Boss, thats right up there with rotating the air in your tires...
If the glass moves in our old windows, should we remove it and turn it over every 10 or 20 years?
I'd guess once a century should do it. You did turn yours in 2,000, didn't you?
kk gave good advise. Also, clean well before you try to cut it and cut with a single solid stroke - retracing leads to bad cuts.
PS: the etched surface on the insides of old thermopanes makes for poor cutting, so scribe on what had been the outside.
PPS: refer to the Corning glass web site for a full de-bunk of glass flow "urban legend" or "high school BS" - it does, but over many millenia and not that you or I could measure, the Corning site has the viscosity numbers and calculations.
Nah. The urband legends sites love this one. A google search on "glass flow urban legend" turns up lots of articles, and I thought this was a pretty good one.
http://www.glassnotes.com/WindowPanes.html
Pete
Gosh darn it - (Sorry for the strong language)
We had a good story going, and now you have to try to confuse things with facts.
What is this website coming to ???
Now there's a man with an open mind - you can feel the breeze from here.
Whoa. You're right, I should be more careful. And so should you. What seems innocent could be downright dangerous. You meet some girl in a bar, have a few drinks, and you wake up in a hotel room in a bathtub full of ice with your kidneys missing. I know this is true 'cause a friend of mine heard it from a friend who's a nurse. Please send this warning in an email to everyone you know.
I think I'll head over to the tavern now...
Pete
Thanks,
That is what i was looking for. That is pretty much how i do it now but I'll have to add the cut on both sides of a sliver, and double check surface cleanliness. I did specificaly ask my client if they would like to save a buck on recycled glass before installing used glass.
joe d
Everything has a degree of fluidity. It's all moving, sometimes we have the instruments to measure the movement, for a lot of materials it's not worth the expense of trying to develop instruments to measure extremely minute movements. Nothing is absolute, everything is relative.