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Sylvester Graham
21-Mar-2007, 18:49
I live in New England. I've had the privilage of living in two 300+ year old homes. In my opinion one of the greatest charms of an old home, one that cannot be replicated, is its windows. They're always warped in the most interesting ways.

Assuming the disfigurment is indeed a product of time, and not of the poor production standards in the 18th century (I could be wrong), I'm fairly sure the reason windows warp is because glass is in fact a supercooled liquid, or some sort of amorphous solid. I'm a little patchy on the chemistry but you get the idea. Under the pull of gravity and over a long period of time, the molecules in a pane of glass move perceptibly.

If glass changes shape in a window, wouldn't it also change shape in a lens?

I ask this question because I wonder if the old lenses so desired by modern
wet-plate and alternative process photographers have changed their properties over time. Obviously many photographers enjoy vintage lenses, and use them often. But Eventually won't all lenses become merely nostalgic, and have no real value to a working photographer? Won't the characteristics of a certain lens from a certain period change over time?

-Alex

Ed Richards
21-Mar-2007, 18:54
While I had not thought about it before, of course you are right. You can measure a thickness change from top to bottom in old windows. Since lenses depend on close tolerances, they should also be affected through time. Maybe they should be rotated in storage.:-)

Sylvester Graham
21-Mar-2007, 19:01
On second thought this would probably only be problem if a lens were in the same position for a long, long time, since through active use it's orientation would be constantly changed. And even if it were in storage, like you said, 100 years of melt could be corrected by turning a lens upside down for another 100.

Dave Parker
21-Mar-2007, 19:03
Alex,

It takes an extreme amount of heat to reshape glass, the production standards in the 18th century, although were good for their time, we still very poor in comparison to what we have now a days, baring some massive fire over the last 300 years, the windows would not have changed structure over that amount of time, glass is pretty well inert once it cools back into a solid, one of the most devastating things that happen to glass over the years, it the drying out of the oils and moisture that is inherently in the glass when it is formed, hence it becomes brittle and subject to breakage. Another thing, is glass is actually quite a hard substance..on the Mohs scale it is a little over 5 on the hardness scale, Diamond being the hardest, but it is in line with many minerals that are used for gems in jewelry..the melting point of common glass is approximately 1723C or over 3000 degrees F depending on the chemical make up of the particular type of glass heated...this can be varied by mixing different types of minerals to create the particular type of composition that you want.

I would suspect your waves and bubbles in your windows is just due to the wide variances in the productions methods of that particular time period.

Dave Parker
Satin Snow Ground Glass

Dave_B
21-Mar-2007, 19:31
There is an urban legend that windows that have been around for hundreds of years are thicker at the bottom than at the top because of the flow of the glass. It turns out that this is not true. The glass will move very little because of gravity. The explanation is more mundane. The old techniques of making glass were not very precise and the plates they made were not uniform in thickness. It turns out that as they assembled the windows that the craftsmen put the thick side down for extra strength. The heaviest part of the glass would be supported by the frame and not the thin part of the glass itself. I think our lenses are safe in any orientation.
Cheers,
Dave B.

Greg Lockrey
21-Mar-2007, 20:24
At any rate, you aren't going to live long enough to care.:eek:

Mark Stahlke
21-Mar-2007, 20:27
What you were seeing in the old houses was drawn glass. The variations in thickness were a result of the manufacturing process. Here is what wikipedia (http://en.wikipedia.org/wiki/Architectural_glass) has to say about it.

Sheet glass (sometimes called window glass or drawn glass) was made by dipping a leader into a vat of molten glass then pulling that leader straight up while a film of glass hardened just out of the vat. This film or ribbon was pulled up continuously held by tractors on both edges while it cooled. After 12 meters or so it was cut off the vertical ribbon and tipped down to be further cut. This glass is clear but has thickness variations due to small temperature changes just out of the vat as it was hardening. These variations cause lines of slight distortions. You may still see this glass in older houses. Float glass replaced this process.
I think our lenses are safe.

Cheers,
Mark

Ole Tjugen
21-Mar-2007, 20:34
Drawn glass was a transition between the current float glass and the older blown glass.

The old way of making window panes was to blow a big "bottle" which was then cut open while the glass was still soft. This gave at best reasonably flat glass, and a bottle end which was called "crown". That's the origin of the "crown glass" designation, too.

Glass, once it's cooled below red hot, doesn't "flow". Nor does it "outgas" or "dry". But it does have a tendency to devitrify (crystallise) with age. That makes it liable to splinter into thousands of tiny sharp needles when it breaks, instead of just breaking up into shards.

The "flowing windows" is an urban myth.

Jim Jones
21-Mar-2007, 20:40
The distortion of large glass lenses due to gravity can indeed be a problem. Traditionally it limited the diameter of telescope lenses to about 40 inches diameter, as in the Yerkes telescope, finished by Alvin Clark and son in 1895. While mirror lenses for telescopes made of more stable glass or other material are now much bigger, the Yerkes telescope is still the largest refractor.

Ole Tjugen
21-Mar-2007, 20:43
Yes, but that's just because glass is flexible: It sags under gravitu, but "bounces back" when the orientation is changed. It doesn't flow, which is a form of inelastic deformation.

Sylvester Graham
21-Mar-2007, 21:22
Ok, just wondering. Perhaps I shouldn't have been so artsy-fartsy about it. Everyone's shown off their intelligence nicely.

Although... my chemisty teacher enjoyed telling that urban myth quite often, accompanying the example of glass with wax.

Dave_B
22-Mar-2007, 03:15
It was not his fault. The Society Committee on Education of the American Chemical Society included it in a sourcebook for High School Chemistry teachers. They were wrong. He had been actively mislead by a group of experts he had every reason to trust. He should be forgiven for his error.
Cheers,
Dave B.

Steven Barall
22-Mar-2007, 08:16
Finally, a technical topic I can finally claim expertise on. I am a part time glass blower and I have talked about this exact issue with the now retired chief glass making engineer for Corning Incorporated. He was responsible for all the glass that Corning melted all over the world.

I asked him specifically about whether or not glass sags over time due to gravity. He told me that at Corning they did an accelerated age test for the affects of gravity. The test approximated many hundreds of years of gravity on a technically perfect rod of glass. The test itself took, I believe, something like 12 or 15 years. The thing just basically sat there in a corner of the office in a sealed chamber for all that time. The results are that gravity does not affect glass.

There are in fact still many factories that still make glass panes in the way that Ole described although with some variation to how he described the process. Most of the glass used in the stained glass industry is made this way.

There is another and much older process for making glass panes and its probably how the glass in question was made. Because of the way the glass blowing works, it's sort of easy to make round flat disks called rondels. An inherent result of the process is that the glass is much thicker in the center of the disk than the edges which is where the term "Bullseye" glass comes from. So you make one of these rondels that is perhaps four or five feet across and then simply cut rectangles out of it. The edges of the individual panes that were closer to the center of the original disk are thicker than it's opposite edges.

So that's it for my technical contribution to the site and I suspect that I won't be having another for some time to come.

GPS
22-Mar-2007, 08:58
...
The test approximated many hundreds of years of gravity on a technically perfect rod of glass. The test itself took, I believe, something like 12 or 15 years. The thing just basically sat there in a corner of the office in a sealed chamber for all that time. The results are that gravity does not affect glass.
...
So that's it for my technical contribution to the site and I suspect that I won't be having another for some time to come.

Don't be so sure, Steven -here is one for you... While I agree with your conclusion I doubt the technical details of the test. In what way the 12-15 years of just sitting there approximated many hundreds of years of gravity?? That doesn't make sense. Usually if you have this kind of tests, for ex. corrosion tests, fatigue tests etc. you have to make the leading cause of the tested characteristic exaggerated in some way or the other. Either you expose the part to stronger corrosive ambiance (in the above test) or the part gets more strain in a given time (the famous IKEA chairs tested in front of customers with constant weight up and down on them) etc. That would not be the case of this perfect rod at all. The way it could be tested for gravity would be if you put the rod in a centrifuge and rotated it for higher gravity under a shorter time (rather than 100 years). As you described it, it wouldn't say anything about sagging in 100 years.

GPS
22-Mar-2007, 09:21
Unless... they had some super duper method how to measure a submicroscopic change of the rod shape - even smaller than in the nanotechnology realm... hm...

Dave_B
22-Mar-2007, 09:28
The way you do such a test is that you keep gravity the same and measure very small changes in dimension and then do an extrapolation. If things are going to move a cm over centuries they must move ~.1mm over a period of years. The folks at Corning labs are quite clever and can easily measure ten to the minus twelve (a trillionith) of a meter. In a pinch they can probably measure ten to the minus fifteenth of a meter. You can also pick a geometry for the test that magnifes the effect of any flow that might occur. The failure to see any changes with such precise measurements over a period of 15 years can then be used to put an upper bound on the magnitude of any possible flow effect. Such tests are routinely done to measure changes in size with temperature, magnetic field, pressure, gravity, electric field, etc. One can measure both reversible and irreversible effects.
Cheers,
Dave B.

GPS
22-Mar-2007, 09:33
Sure, even if here we have changes that don't make even a mm in 100 years. Still, some kind of greater gravity imitation would be easier to measure. Hm. Would be interesting to know their measuring method.

Bruce Watson
22-Mar-2007, 09:39
Assuming the disfigurment is indeed a product of time, and not of the poor production standards in the 18th century (I could be wrong), I'm fairly sure the reason windows warp is because glass is in fact a supercooled liquid, or some sort of amorphous solid. I'm a little patchy on the chemistry but you get the idea. Under the pull of gravity and over a long period of time, the molecules in a pane of glass move perceptibly.


My training in engineering, in particular strength of materials, indicates that your assumption is wrong. Typical window glass at normal environmental temperatures does not change shape over time due to gravity. Perceptibly or imperceptibly.

What you are seeing is old glass plate production techniques. They didn't have float glass manufacturing 300 years ago.

The structure of glass is amorphous. In this case that means that it does not form a crystal structure like most solids do. Yet, it has sufficient cohesive attraction to reach a rigid state. Put another way, its viscosity is sufficiently low at room temperature that it's considered a solid.

So don't worry about your camera lenses. OTOH, it's better to use them for their intended purpose than to leave them laying around for hundreds of years! ;)

Richard Årlin
22-Mar-2007, 10:00
I did not bother to read the whole string even. If your house had stood since the last glaciation maybe and if your measuring devices were good enough (very good) you'd perhaps be able to measure the infiniesmally slight thickening in the bottom. It's a myth quite simply. Windowglass was mouthblown at the time and cut open to cool as sheets. Naturally such glass has not an absolute equal thickness and would obviously be put in the framework with the thickest and thus heaviest edge downwards... gee

Richard

Ed Richards
22-Mar-2007, 10:07
> The way you do such a test is that you keep gravity the same and measure very small changes in dimension and then do an extrapolation.

Or you put a weight on the rod to simulate a higher force of gravity.

I am certainly willing to listen to better scientific evidence on this, like a Corning scientist, but Wikapedia does not qualify.:-)

jnantz
22-Mar-2007, 10:08
a lot of older glass was made by blowing a big glass ball and spinning it to make it compress and flatten. the window glass was cut from the large "plate" and the place where the tube was became the lantern-lites that one sees over the doors. ...

GPS
22-Mar-2007, 10:39
> The way you do such a test is that you keep gravity the same and measure very small changes in dimension and then do an extrapolation.

Or you put a weight on the rod to simulate a higher force of gravity.
...


Oh Ed, you took it from my mouth before the hand could write it,:) Why to do things simply when we can do them in the complicated way?
Speaking glass - I was absolutely fascinated when polishing my first grounded lens I heard our teacher saying - nobody knows for sure, how the polishing in fact works. There are some theories, the leading one says that the upper glass layer (the microscopic summits) are melting away by the heat generated in the polishing process. But nobody knows for sure...
In fact, the mystery holds still today (and the leading theory too). But how come the melted glass doesn't cover the polishing medium? Who knows?

Jim Ewins
22-Mar-2007, 10:44
Did anyone take physics?

Ed Richards
22-Mar-2007, 11:28
Checked with a fabricator of really high precision big mirrors - glass does not flow, at least at macroscopic levels. Does flex, and the big telescopes use real time actuators to counteract the flex.

Dave_B
22-Mar-2007, 11:36
Did anyone take physics?

I have a Ph.D. in Physics from Cornell and have worked at Bell Labs for 28 years. Putting a weight on some part of a piece of glass does not well-simulate the effects of gravity. A weight can put different forces on different parts of the piece of glass. It can create stresses and strains. Gravity acts uniformly on all the atoms in the piece of glass. For example, stress gradients can make materials act differently than they would under a uniform force. One needs to do the experiments carefully to actually measure what you wish to. Elastic properties and creep are different phenomena and get measured differently.
Cheers,
Dave B.

GPS
22-Mar-2007, 11:43
Ok, back to the centrifuge idea then??

Struan Gray
22-Mar-2007, 12:35
http://math.ucr.edu/home/baez/physics/General/Glass/glass.html

Executive summary: no.

Greg Lockrey
22-Mar-2007, 12:42
I have a Ph.D. in Physics from Cornell and have worked at Bell Labs for 28 years.

A little OT: Dave, I have a friend whose first job out of college was working for the Bell Labs. This would be in the early 70's. He said that his "purpose in life" at Bell was to find a way to send a body through the phone lines. He stated that the real purpose was the discoveries that they would make along the way. Are they still trying to do that? He is now at Johns Hopkins in the Physics department and does calculations for projects like NEAR.

Dave_B
22-Mar-2007, 13:06
Yes, we are still working on teleportation. We are starting small-we know how to do it with a photon. It will be a while before we can do it with a starship. Your friend was correct, the process of figuring out how to do it teaches us a lot. For example, a lot of our work in nanotechnology has this as a long range goal. The breakthoughs along the way are useful in their own right.
Cheers,
Dave B.

Martin Courtenay-Blake
22-Mar-2007, 14:37
Most glass is in fact not a solid but a very very very viscous liquid. Under the right conditions and given sufficient time it will flow. This is the main reason that refracting telescopes are not made over about 40" diameter. Large lenses have been known to deform slightly over time. (It's also very difficult to cast blanks that size)

The reference to window glass is interesting as there are examples in Europe of medieval glass windows having bowed and definately thickened at the base because of this phenonema. In England in particular, when flat glass was produced by spinning, the very thick centres, called bulls eyes, were used for windows especially in inns. These can be commonly seen today and many have indeed deformed by sagging under gravity.

Better keep an eye on those new fast lenses.

Martin

GPS
22-Mar-2007, 14:43
A good joke, Martin. Now, next time you promise you read the thread from the beginning and find a new joke... ;-))

Martin Courtenay-Blake
22-Mar-2007, 14:58
You're probably right GPS. Coming to think of it I think it was an article about the old telescopes, including the Yerkes, that I read as well as Jim.

Must get out of this habit of reading the first two posts then jumping to the last page

Cheers
Martin

GPS
22-Mar-2007, 15:05
Never mind, me I read the article too - that's why I started to laugh in the first place...

Jim Jones
22-Mar-2007, 16:47
A real-life observation on flow [?] of glass: in northern Greenland barracks window panes seem to be slightly concave when viewed from the outside. There can be a considerable temperature gradation through one layer of glass, sometimes over 50 degrees C. I could understand a temporary deflection because of the temperature gradation, but didn't expect it to become permanent. However, unless I remember incorrectly, the panes remained slightly concave, even at a uniform temperature.

Martin Courtenay-Blake
23-Mar-2007, 02:07
As well as the Yerkes telescope article I recall a visit to the Lick Observatory above San Jose on one of my frequent trips accross the pond, They have a 36inch refractor and again the size was limited by potential glass deformation. After the usual tourist talk I managed to get hold of one of the astronomers based there to talk about the scope in some more detail and it seems that this approx. 40" diameter is an accepted limit in astronomical circles for refractors before things go pear shaped (in time of course)

Martin

GPS
23-Mar-2007, 02:12
Yes, but that's just because glass is flexible: It sags under gravitu, but "bounces back" when the orientation is changed. It doesn't flow, which is a form of inelastic deformation.

Kevin Crisp
23-Mar-2007, 05:52
Maybe all the new aspheric front element lenses from Schneider are just old ones they stored nose down for awhile?

GPS
23-Mar-2007, 05:58
Just wait when Goertz opens his old cave! All aberrations eliminated!

Brian C. Miller
23-Mar-2007, 06:46
Honestly, when I saw the subject to this thread I thought, "Oh, somebody is melting a Coke bottle to make a new kind of LF lens!" Sigh.

Beware the Wicked Witch of the West brand lenses: they melt when exposed to water.

Rakesh Malik
23-Mar-2007, 07:26
Checked with a fabricator of really high precision big mirrors - glass does not flow, at least at macroscopic levels. Does flex, and the big telescopes use real time actuators to counteract the flex.

Actually, the real-time actuators aren't there to counteract the flex, they're there to USE the flex. Their purpose is to deform the mirror to counteract the effects of atmospheric turbulence to enable a clearer image from terrestrial 'scopes. It requires a clear enough night to be able to obtain a sharp image of something bright enough to use as a reference.

An astronomer who had a chance to visit Mt. Palomar showed us how well it worked... it's quite amazing. There are times when they can't get a sharp enough image of anything for the adaptive optics to work; the controllers need SOMETHING sharp as a reference to determine what to alter in order to correct the image. :)

GPS
23-Mar-2007, 07:35
Panta reis! (Heraclitus)

James E Galvin
23-Mar-2007, 07:51
Telescope lenses are limited to 40 inches or less for several reasons: the difficulty of making a large piece of optical quality glass, the difficulty of strain free support for the weight, the loss of light from absorption through the considerable thickness, and secondary spectrum (the compensation for chromatic aberration is not perfect.) In my reading, secondary spectrum is the main problem. I have looked through the 36 inch Lick telescope, there is a considerable blue-violet glow around a star (secondary spectrum). Secondary spectrum gets worse as the lens size is increased. The only cure (that is before ED glass) is to increase the f#. But the telescope is already very long at f/19, 57 feet. For glass flow, do a Yahoo on