Glass, Ceramics, Fibers & Fabrics

According to a Scientific American article some 45 years ago, it's liquid. And dissolves in water! Some old window panes look runny, and you can't get any good results from a glass cutter on them. I haven't tried this, but some guy said you could cut glass under water with metal shears.

Hello Tippycanoe,

Old glass was made in a different way to modern 'float' glass. It was either very small panes made by letting the glass flow from the rod it was collected with from the kiln and, these often had the part which had been attached to the glass blowers rod still in the centre where it was broken off. This I think is boss glass, and now it is made to order as it is costly. The old way of making flat glass panes was to grind it but this was mostly used for mansions and large expensive buildings, and was just ordinary glass from a kiln flattened out roughly on a sheet of steel or similar, or shallow tray.

http://en.wikipedia.org/wiki/Polished_plate

Modern 'float glass is molten then poured onto hot liquid mercury and other low melting point metals, which allow the glass while still liquid to flow to fill the size of the tank at one end, while the cooler end now solid, is continuously removed and stacked by automatic machines.

I read that glass was a liquid scientifically speaking, because it is not crystalline. Though no human would see it or perhaps notice it, the glass would eventually 'run' and 'drip' as natural glass from volcanoes does. The volcanic glass is black and is molten with the other constituents of the volcano, and it is thrown out or it pours out in lumps. This was often used as knifes thousands of years ago and even now is still used to make knives.

Check out the site above.

bb

Thank you babybear! That volcanic glass is Obsidion, yes? Sharpens like flint, with a deer antler. That same article I mentioned related how Sumerian decorative glass was made up of multipule strands of different colors stacked together. Water could enter one end and dissolve the glass as it flowed between the strands.

Are you sure about using mercury as the "flat" liquid metal for making float glass? Typically, glass melts at 1,425 to 1,600 C and mercury boils at 356 C. In addition to extreme toxicity, one would think that bubbles of mercury vapor under the molten glass would cause severe defects. Most glass mfg'ers use molten tin as the flat liquid.

Hi Casrdio07,

Thanks for the post. And no I am not sure but I seem to recall it from a Documentary several years ago, just after the invention of 'float' glass.

I may well have made a 'boo-boo' of a mistake for which I am sorry. It was years ago and I never knew the boiling point of mercury then! I do now but never allowed for it. Still a clever way of making glass though?

Take care and once again sorry to all for the mistake.

bb

They use mecury as it is heavier then glass so the glass technically floats as for boiling point that only going to happen if the pressure allows for it (ie different pressures different boiling points)

I have had the pleasure of working on 150 + year old houses, and I have replaced windows over the years that all had larger bottoms of their window panes. My father explained that glass is a liquid, and gave me one of my first lessons about gravity and showed me that evidence even early on in my youth when we were just replacing a few broken ones.

I have never researched that once in my life, for two reasons really, one that my father never made a damn thing up, and it was extremely obvious that every single original glass pane had a significantly larger bottom. I have been believing in that since the late seventies.

I will buck the difference here, I say solid, Reason?

Yes I have heard the stories of the windows that are thicker at the bottom in 100 year old houses and all that.

But I am also an optician and telescope maker. And there are telescopes that are well over 100 years old, and their optics are still perfect.

And these have to be exacting. the max error on the thickness and or curve of the glass is 1/4 wave of light! and that's the MAX.. More are often closer to 1/16th wave.

That is only 0.00031496062992 of an inch ! If glass is thicker at the bottom of a window, it is because it was made that way. I did not drool down.

Hello NSS,

how are you?

I agree and disagree.

I agree that the window glass which is apparently thicker at the bottom was made that way. It would make it stronger and less liable to brake as the old windows flexed when the wooden frames moved. Of course with the real old houses the wooden frames, made from whole trees, supported the whole house, bricks, and contents! And all relied on the accuracy of the joints.

I sometimes go to a coffee shop which has the 'trees' it is made from standing out from the walls about 4" (10 cm). This place was built in the 16 century. Some walls are plastered with laths and horse hair and you can see that in places as well. The wall follow the shape of the trees which make the frame, so the 'head room' may be a foot 12" (30 cm) or more less at one end of the room. Real character!

bb

I agree that the glass is still a liquid, though it would probably 'wear out' before anyone noticed any droop in the glass. It will happen over several thousand years.

Interesting though!

bb

Does optical glass not have a higher percentage of quartz than other glass?

Not really. Glass composition is depended end need and in this case what we want to keep and what we want to filter out will change the glass composition.

Chris, are you old enough to remember "cheap Japanese junk"? Zeiss once bought sand from Japan for use in lens making. Our friends from Nippon stopped selling it to them and now make the worlds finest optics. About the same time German optics became full of bubbles, and they were hard pressed to make a decent lens.

"Clever, these Orientals"

I'm 48. being somewhat trained in quality, I'm basically familiar with the story. (Deming, Toyota, Honda, etc.)

This is one of interesting topic came after longtime. Definition of glass I had fun discussion and still not clear how to define. Let me give some basic definition from few discussion I was involved on this topic:

Definition # 1:

Glass is supper cooled liquid. This definition is first one and historically this is after observation of couple year old Churches glasses which increased thickness at bottom and got thinner on top. This is also called now sagging. This happened since the window glass was high viscosity liquid and took few hundred years to observe measurable change and definition of the glass.

Definition # 2

Glass is solid materials with short range order. This definition is even used to-day to calculate macromolecular size.

Definition # 3

This is the current and still evolving. Glass is Nano disordered composite.

Masyood Ph.D.

Hello Massyood,

Right, so in other words it is an amorphous substance with no granular order.

The thickening of the bottom of windows is not where the glass has 'run' or melted, it was designed like that to make it stronger.

When glass was first used for windows, and it is not that long ago, it was made from all kinds of sand etc. So it had very little intrinsic strength, which is why it was made thicker at one edge..............to allow it to be picked up and transported without it breaking.

I am going to try to find some info to prove this, I mean where the glass is made thicker at one edge.

bb

Glass flow is know to scientific community from last more than 50 years. The flow is at room temperature also only thing is it is exponentially lower than high temperature flow.

Low temperature of glass flow is commonly measured at room temperature. If you have not experimented with this then you can draw fiber or you can buy glass fiber

Add weight and then after few days measure it length change and this common way to measure low temperature viscosity of glass.

When I was saying thickening it took about two hundred years to see the measurable change

Masyood Akhtar Ph.D. (Glass Engineering, MS Ceramic engineering, BS Metallurgical Engineering)

I still will say this is all impossible. For the simple reason that every telescope ever made would become useless in a very short time if glass flowed at all.

Especially if it was soo much that a person could see it with their eyes after only a couple of hundred years of time. If glass flowed at that rate. Then any telescope would be more or less useless in a week or so after it was finished. And i know of many that are over a hundred years old that are just as perfect as the day they were made.


Joe

Glass flow can be decreased by known technologies. since this is good topic and good discussion I just want to add few thing how things can stay good for few hundred years.

If you take soda-lime silicate glass it will flow close to 50 to 100 microns every 100 years. This can be decreased to less than 1 micron by putting surface coating. This process is call ion exchange of surface sodium by potassium ions. This does the surface strengthen of the window glass.

Telescope glass is hard glass and then to protect it from flow and distortion we do the coating and surface treatment.

Engineering allows to do the same thing as in steel business we did when steel was corroding we end up developing dopant like chromium and increased the resistance to corrosion.

This true with every materials we can make ceramic as flexible as metal by proper formulation.

Same way we can have the glass which will flow less. We developed glass which will hold highly radio active waste for 200 years before fixing it again and for sure by that time Technology will improve and we will be able to store for 400 years that is the reason we say US is young country since we work to improve our to-morrow better than to-day.

Masyood

Masyood

'Telescope glass is hard glass and then to protect it from flow and distortion we do the coating and surface treatment."

Not all the time. The 100" mirror was made from busted wine bottles.

The 200" mirror is made of Pyrex.

Do a Google for John Dobson, and look what he litterally revolutionized telescope making in the 70's His scopes were made from the bottoms of like culligan man water jugs.

With the HIGH cost of Pyrex now, most amateur telescope makers have gone to simple plate glass the exact same stuff you get in your windows.

And

NO in a telescope, the glass has to be uniform throughought, it can NOT have any treatment on it's surface in any way. doing so it will not give good images.

The glass in a telescope to show you how perfect in it's curve it has to be, even a change in local temperature of 5 degrees, will ruin the figure of the telescope, until the telescope has become thermal equivalent to it's surroundings simply to it's expansion coefficient. and thats tiny..

You can take scrap window glass and remelt with additive to make it hard glass which will decrease it room temperature viscosity and flow to less than micron a year and will give one more than few hundred years of usable life.

This is glass property changing to make it usable under different needs and atmospheric conditions

Very short time will be few hundred years for optical lens before it gets bad

Hi to all,

I just want to make my position clear with regard to this Blogs entry on glass.

1) I do not think glass 'flows' at all.

2) It is a fact that glass is not 'crystalline', so (in theory) may not be a solid. But, by all ways of measurement over the short time since window panes were invented, there has not been, to my knowledge, any panes which have flowed in any way at all!

3) I have not noticed glass which was thicker at the bottom of the pane than at the top. I was trying to convince myself they made it this way but I have not had time to confirm it yet.

4) As a poster has said, any glass lens which flowed would be useless in no time at all!

5) Any 'wear and tear' on window panes may happen from the top down by the various 'rays' and 'particles' which rain down on us on Earth, and, to some extent, it may be possible that some glass, where it is open to the 'elements', could be worn then way. I know that some if not all old glass is somewhat 'softer' than modern glass, and solar radiation breaks it down as it removes colours from fabric etc. The glass becomes like powder to touch......................BUT, this does not mean it 'flows' in any way.

This is not meant as an insult to anyone who posts to this blog, OK.

bb

In technology conversation we do not insult any one but we get opions and facts. Try to explain and grow in knowledge.

There is book Introduction to ceramics by Kingry and introduction to glass by William LaCourse.

I have also enclosed link you can read and get few new leads also

http://dearplanetaryastronomermike.blogspot.com/2009/02/another-followup-does-glass-flow-as.html

I have done this during my Ph.D. thesis work and will try to explain more

Here are some good points:

http://www.glassnotes.com/WindowPanes.html

This paper is same as Dalton Theory which we all know was not the true statement of building block of material. He made atom as smallest piece of material which we all now know is not correct. Albert Einstein gave us theory of relativity and now we all know till to-day it is correct.

As science grows so does our understanding and refinement of experimental practice. I will strongly suggest you if you happen to be in up state New York in corning area visit School of Ceramics at Alfred University and they have experimental set up to measure elongation of glass (window glass) at room temperature and for cast the sag in 200 years. This is well know experimental practice.

When we design the product we take into consideration to minimize this sag and predict the usable life of the glass.

You can make the elongation go to zero by converting glass to glass ceramics. You can minimize the sag by ion exchange strengthening. You can decrease the sag also by creating medium range order in the glass and more covalent bonds in the glass structure.

Some technology we know know and understand to the extend we can control the bad effect and sag is one for the glass by measuring its room temperature elongation rate

MAsyood

Hello Guest,

I fully agree with what your referenced site has to say!

Well pointed out!

bb

technically speaking. all things in existance start as gas and then are "cooled or frozen" into various other states (liquid, solid). we work within those other states. the form of glass we use is the frozen form of a liquid. The same way mercury is liquid at room temperature, glass is frozen at room temperature. Is it solid? it is as solid as frozen water. I guess the real question is what does solid/liquid actually mean?

Artisteroi has made by far the most sensible post of all, askin the question what is a solid?

OK first up I'm a materials scientist so I know a small amount about this. Amourphous, crystalline, its irrelavent, is glass solid? Yes. But all solids flow over time, its called creep, if you raise the temperature then the rate of creep increases, if you reduce the temp then the rate of creep reduces. Different materials creep due to different mechanisms, and will creep by different amounts.

A solid has a stable physical state, has higher density than a liquid or a gas, and is the most energetically favourable state for those particles to be in. It is really characterised by a phase change from liquid, where as it cools from its liquid state it takes in energy and starts to solidfy. Again different materials solidify in differnet ways, some gradually thicken, others make a more sudden step.

Many solids exhibit solid - solid phase changes, like the glas transition in polymers, or the ductile to brittle transition in metals, or rearrangement of crystal structure like BCC to FCC in iron.

So in reality as Artisteroi intimates in his question, its harder to define what a solid is, and its just a convenient abritary descriptor which fits our general world view.

Discussions like these are common amongst people without any science training, a background in science gives you the grounding to know that these thinsg are often abritary. It really doesnt matter what a solid is or isnt, glass will creep, but there are mechanisms to reduce this, just as there are in metals and polymers.....

Here in the UK we have examples of glass which was made 500-100 years ago, some of these exhibit creep but you are not going to see it in 100 year old glass.

Glass defination is changing and that change is still under discussion. The historical facts are

1. First glass defination was solidified liquid --> This defination came under believe that liquid does not have any structure.

2. We invented X-ray and used to experiment with glass powder and then came up with new defination of glass and that is "glass is macromolecules and half the area under hump is the size of marcomolecules"

3. Size and peak angle of the hump differed from composition to compostion and this was good fit

4. When angle at the peak is less than 10o we end up saying it is metal or ceramic or glass ceramics

5. Both glass as well as metal, ceramic and glass ceramic posess creep behavior the difference is in mechanism

6. In metal it is movement of dislocation. When thes dislocation impinged we say work hardning

7. In glass it is elongation of micromolecules under its own weight

8. Under both case creep increased with temperature.

9. In metal high temperature unpinged dislocation and increased movement

10. In glass decreased high temperature viscosity and hence sag increased

New defination which is evovlving is based on Nano and TEM ( transmission electron microscopy and other tools based information and it provides the micro molecules size and shape.

Glass is an amorphous solid. In cases of old homes where the glass is thicker at the bottom. That is do to how the glass was made. If blown from a pipe the glass will be thicker the closer you get to the pipe. If the glass bubble is cut and laid flat the outer edges will be thicker. Same as if the glass was rolled with a pin like dough. Both are methods old flat glass was made. When the glass was glazed into the windows the worker found it set better with thicker edge down. Ripples in the glass are the imprint of the surface that it was flatten on.

Today flat glass is poured on molten tin.

For you telescope and optics types, can you comment on this idea please? ( I didn't know where else to post the idea on CR4)

Chris

At first it seems a novel concept, But I'm thinking of two possible problems,

Vibration, which is a major problem with scopes at all. even with something as small as a miffin fan. never mind spinning the main objective.

Secondly to achive the full aperture 30 rpm isn't gonna cut it. i'm thinking the slowest part of the "Slice" (the center) will need to spin at the speed of light to get full sweep. but then of course that's not possible, for then the outer edge would be going faster than he speed of light