Nickbb
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Cracks disappearing
I recently posted a topic on how to fix a retort I had and before I could fix it, I had gotten it out and lo and behold the crack wasn't there! Is it
possible for glass to just fuse back together by itself? I've never seen that before.
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Oscilllator
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I have done it, mind you it wasn't at room temp though. My one and only test tube had a crack in the bottom so I whipped out my propane lamp and
heated it to softening point. The crack literally disappeared before my eyes, although once it had cooled down there was still a faint trace of were
it had been before.
I dont think it can fuse together by itself, though.
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zed
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No longer visible, is not the same as non-existent. You can't see the crack right now.
Unless it has been hundreds of years since you last used the glasswear, it is unlikely that it has healed itself.
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hissingnoise
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I think glass flows slower even than that . . .
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Siggebo
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Glass does not flow! This myth needs to be beaten to beyond death...
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hissingnoise
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Glass flows at a slow rate that makes proof of flow impossible to demonstrate . . .
But this doesn't mean glass is not a super-cooled liquid!
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jock88
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Is it not true that old stain-glass windows sag due to flow of the glass.
Ice flows in icebergs. Of course the pressure is large.
Retorts etc, when cracked, do not fix themselves.
I wonder would the stuff that is used to fix cracks in car window screens be used to 'fix' the crack. The stuff is sucked through the crack from one
side of the window. I would be very very wary of a retort or any glass ware 'fixed' that way.
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Endimion17
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Glass is not a supercooled liquid. It's not that simple. It has crystalized domains. It is an atypical type of matter which we call - glass.
Metals can be turn into metallic glasses by rapid cooling, but that doesn't mean they're liquids.
Liquids aren't just characterized by their long range disorder. It's the ability to flow, however small that rate is. Liquids are fluids, and fluids
flow. If it doesn't flow, it isn't a fluid, therefore it isn't a liquid.
Glass was never ever proven to flow (unlike extremely viscous organic liquids). Those old chubby church windows - it's a myth. They're chubby upwards
or downwards, depending on how they were installed. It's connected to the lousy flattening process which was used long time ago.
@jock88: The liquid used for repairing car windshields just has the same/similar refraction index. They don't repair anything.
Cracks on flasks can be repaired by heating it slightly above its softening range. The pieces join because they're polar and they just exclude air.
Sometimes minute air bubbles might remain, but that's a minor problem. What is important is to anneal the glass properly immediately after. Of course,
such flask will never have the same strenght, but it can be used for less demanding things.
[Edited on 4-1-2013 by Endimion17]
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jock88
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http://www.glassnotes.com/WindowPanes.html
No flow!
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sargent1015
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Hmmm, my distinguished Inorganic Chemistry professor told us that glass flows. I'm going to trust her over a website...
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Vargouille
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Would an ACS article stating that glass doesn't flow be better?
http://pubs.acs.org/doi/abs/10.1021/ed077p846
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Siggebo
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Then she needs to be undistinguished at once.
My analytical chemistry professor repeatedly wrote Fl- for fluoride. We winced every single time.
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kavu
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Professors are humans too, they do not posses the ultimate knowledge. Many proffs have said that rather than having exact answers they can only
provide rather good estimations based on their academic experience. Moral of the story, don't believe everything they say!
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jock88
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Ask the distinguished professor where she found that out.
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turd
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Quote: Originally posted by Endimion17  | | Glass is not a supercooled liquid. It's not that simple. It has crystalized domains. It is an atypical type of matter which we call - glass.
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And even if it hadn't any crystalline
domains it'd still be an amorphous solid and not a slowly flowing fluid.
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12AX7
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In principle, a crack could reseat itself -- the molecules are still there, grasping for electrons, happy to get what they can. I think the actual
physics of a glass fracture are more complicated than that, though (of course!).
Besides the obvious nanometer scale realignment required for a large contact area, air is exposed to the fresh surfaces, oxidizing some hanging Si
bonds; if there's any humidity, water reacts, forming Si-H and Si-OH ends (which won't stick back together), and any adsorbed water will essentially
form a lubricating monolayer (or more).
The classical solution to adsorbed substances, hydroxyls and bonding things in general, of course, is fire. And lots of it.
I wonder if any studies have been done on glass fracture in a vacuum. It might be a practicable experiment to take a glass fiber, break it, and mash
the ends back together precisely enough to restore it.
But in general, yeah, cracks are always there. By geometry or by lighting condition, you might not be able to see them, but a polarimeter will see
the gap and the strain around it.
Tim
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Fantasma4500
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about this.... liquid glass.. im still reading, but im just saying.. i was explained the reason that it flows is because it in between the sheets of
apparently SiO2 its still a tiny tiny bit liquid, also as i read abit down (going to read further, still) i think surface tension would play a role in
this, the thinner the glass, the less it sinks together, also if you have a window thats really thick, then theres more pressure per cm3 in the bottom
than in the top, because of gravity..
if you take 100 metre tall tube of water, the pressure in the bottom will be higher than just 10 cm down from the top..
im definately not denying facts, but by this it could sound reasonable, and also making the exambles used with the very fine glass pieces unusable
just once again so you dont forget, no im not denying what it says.
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12AX7
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As "liquid between sheets" goes, there's a certain grain of truth to that, just not at this temperature. At surprisingly low temperatures, sodium
ions do become mobile -- in fact, if you have sensitive enough measurements, I guess they are mobile at room temperature. (That doesn't mean the ions
will "drip out" over time, or that the silicon lattice is necessarily weakened -- it can still be rigid, say, like water percolating through
concrete.)
I say this because some early semiconductors had problems with sodium ion impurities: indeed, the MOSFET (which today dominates use in almost every
chip produced today) was discovered theoretically very early (before 1920 I think), but practical devices wasn't created until the 60s because the
process had to be refined, purified and perfected so thoroughly until they were finally marketable.
The biggest problem they had was sodium ions (and something called surface states, also related to impurities), some of which would get trapped in a
vital layer of (amorphous -- glass!) silicon dioxide. When a voltage was applied to this layer, turning on the transistor, the electric field would
pull sodium ions through it, displacing them and changing the static electric field in the layer. Next time you went to use the transistor, it would
turn on at a different voltage! This is like turning on your kitchen faucet and not knowing whether the knob has to be turned 30 degrees or 120
degrees to get a gentle flow rate, and worse, the flow of water causes the threshold to change over time.
(Just to close the story -- modern devices, of course, are made at such extraordinary purity standards that, not only are those threshold voltages
highly repeatable within a given device, but a metal terminal, isolated by glass, can be charged at the factory to a particular voltage, and just left
there, insulated, a capacitor with no resistance to discharge it, for *decades*, even at elevated temperatures. These have been used as *voltage
references* before. On a more crude, digital level, this is what Flash memory does, except with billions of ones and zeros per chip!)
Tim
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