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Tierney Lab: Glass Does Not Flow. Except in Space?
http://tierneylab.blogs.nytimes.com/2008/07/28/glass-does-not-flow-except-in-space/

Glass Does Not Flow. Except in Space?

By Kenneth Chang

[This is a guest posting by Kenneth Chang, a Times science
reporter.]

In this week's Science Times section, I write that glass is not just
a slow-moving liquid and that the belief that old windows have
sagged over time is bunk, even though scientists still struggle to
explain why glass is so solid.

Then there's Buzz Aldrin's flashlight.

In 1999, Christie's East in Manhattan auctioned off an assortment of
space memorabilia, including a flashlight that Buzz Aldrin used
during a Gemini 12 spacewalk in 1966. The auction catalog mentions:

   The flashlight lens became deformed while in the vacuum of space.

I saw the flashlight in person. The lens is definitely deformed,
just as if the glass had flowed. It's not cracked. It's deformed.
Thus deceived, I wrote in an article for ABCNEWS.com, "Offering
persuasive testimony that glass really is a very slow-flowing
liquid, not a solid, the flashlight's thick lens was blistered and
warped by the powerful sucking force of the vacuum of space."

Phil Plait, at the blog Bad Astronomy, took me to task for calling
glass a liquid. (He also didn't like that I had described the glass
as being sucked out by the vacuum, arguing that it would have been
more correct to say that air in the lens had blown the glass
outward.)

But Phil admitted he didn't have an idea of what caused the glass to
deform. He invited readers to provide more information. As far as I
know, no one did.

I showed a picture of the flashlight to a couple of the researchers
while reporting my glass article. They too did not have any good
explanation to offer.

If nothing else, that shows that the nature of glass is still
puzzling to a lot of people, even the really smart ones.

As I write in the article, there's no magical dividing line between
liquid and glass. But any flowing movement in everyday glass , if it
did flow, would not be noticeable for geological time scales of
millions of years. (Ask a glass theorist, "If you wait a few million
years, would the glass flow?" and you often get a reply so
philosophical that you wonder if you had actually asked about the
nature of God.)

The flashlight sold for $9,000.

30 comments so far...

1. July 29th, 2008 1:05 am
I'm assuming that a great enough pressure differential acting on
a piece of glass on Earth would simply cause the glass to
shatter. I would also suspect that the cold temperature of space
would cause the glass to be more rigid, not more pliable, if it
had any noticable effect at all. The big question seems to be
what other factor(s) (assuming my assumptions are correct)
differs between the atmosphere of Earth and that of space that
could possibly explain the flashlight buldge. I doubt that such
an answer will appear here, but now my curiosity really wants an
answer.
-- Posted by Aaron

2. July 29th, 2008 1:28 am
Forgive my skepticism, but ... are you sure the lens is glass,
and that the deformation formed while the flashlight was exposed
to a vacuum?
If both of these things are true, than I suspect it is a heat
effect.
This is an incandescent light bulb, and the temperature of the
filament would be quite high. Heat from the filament flows
through the air inside the bulb to the glass, through the glass,
and then away from the glass to the surrounding environment.
When the flashlight is in an atmosphere, heat transfer away from
the outer surface of the glass
would be dominated by convection and diffusion. In a vacuum,
neither of these heat transfer mechanisms exist, and the only
heat transfer off the surface of the glass would be from
radiative heat transfer, and the total heat flow out of the
flashlight would be significantly lower.
As a result, the glass of the bulb and the air inside the bulb
would heat up. Heating the glass would soften it. Heating the
air inside the bulb would raise its pressure. The combination of
those two effects led to the deformation.
-- Posted by Amy

3. July 29th, 2008 6:33 am
Ack - just saw a typo in my post. In the atmosphere, heat
transfer away from the outer surface of the glass would be
dominated by convection and CONDUCTION.
That's what I get for posting at 1:30 am.
-- Posted by Amy

4. July 29th, 2008 7:41 am
An experiment on earth ie placing a NASA designed space
flashlight with the same glass lens in a temperature controlled
vacuum chamber, exposing it to the same temperature, pressure
and time used conditions as the Aldrin flashlight, might provide
insight into its condition but may still not answer the
questions, How and Why.
-- Posted by E.Patrick Mosman

5. July 29th, 2008 10:26 am
I agree with Phil Plait, that distortion of this lens is not
proof that glass well below the glass transition temperature is
still a liquid. It very well may be answered by considering what
is seen in the photo and considering alternatives. The photo
shows us that the glass is thin. Many solids and glass is no
exception can experience a plastic deformation process called
creep. Creep is found in many glass fiber failure experiments
where strength is measured before a suitable flaw is formed or
expanded to result in a failure. It is also found in thin films
and have also been shown in forming glass sheets for use in flat
panel displays. In fact most of us may not be aware of this but
you can find this property of glass in most plastic drink
bottles. Many of these bottles have a thin layer of glass on the
inside surface. The layer is considered a coating but they
deform during manufacturing and distort during filling and yet
do not fail because the film is able to creep.
-- Posted by Bob

6. July 29th, 2008 10:40 am
Why was Buzz Aldrin walking around in space with a flashlight?
So that he could avoid snakes?
-- Posted by Huh

7. July 29th, 2008 10:51 am
Thanks for dispelling the notion of the flowing medieval glass
window panes. You called it "bunk" and it is indeed erroneous,
but I don't know if it would qualify as genuine "bunk" as nobody
really promotes that old notion as an example of glasses
flowing...at least not any more, though it would be easy to see
that early researchers and artisans might take it for gospel
since they would actually know how sheet glass was made from
progressively more molten batches of increasingly hot mixtures
and what it looked like in its ultimate end use application, and
of course, you can actually see and feel the frozen flow lines
in the glass. It's a natural assumption only disproved when a
larger and more objective and scientific (in the modern sense)
examination of old windows reveals that there were lots of old
fragments of sheet glass in the design that were thicker at the
top than at the bottom, and that the likely reason most were
actually thicker at the bottom was because the process of making
a stained glass window involves an artisan making decisions by
holding the glass, comparing it to the design for suitability,
arranging it for both aesthetics and structural integrity, and
so the thicker part would naturally rest more stabily up against
the tilted easel on which the design was placed and used as a
guide in the fabrication of leaded glass windows. Thickness on
the bottom, unless there was a complelling reason to not place
it that way (color or useable shape), would be the natural
choice.
I remain fascinated by the astronaut's flashlight. I wonder if
the lense itself is in the family of boro-silicate glasses like
pyrex or some other low-expansion formula specifically designed
for its expected extreme range of conditions. I suspect it is
and that the lense softened due to heating perhaps while exposed
to the intense power of sunlight undiminished by atmoshpere and
perhaps even redoubled by the very geometry of the lense itself
in relationship to the sun's position for some time or maybe
some kind of impurity invisible and unexpedted to the naked eye
or some density arrangement acting as an internally refracting
region. Ordinarily, if there is anything ordinary about the
equipment and conditions of space, that would facture most
common glasses but if it were a low-thermal-expansion glass like
a boro-silicate pyrex kind of glass, it could endure both the
heating and more critically the cooling without fracturing.
-- Posted by doug l

8. July 29th, 2008 10:57 am
The bubbles in the lens may indicate a "sublimation" (transition
directly from solid to liquid) from glass to liquid in the
profound vacuum of space.
-- Posted by Gail

9. July 29th, 2008 11:04 am
I would like to know the properties of the glass lens before
answering the question. I think the solution lies therein. Of
course, one must use the laws of heat transfer in addition to
the material properties.
Regarding Amy's (#2 & #3) comments on the heat transfer
processes at work, I am agreement with the comment that
radiative heat transfer is dominant in outer space (when one
considers heat transfer from an object to the surrounding
vacuum.)
But one must also consider how the glass itself gets heated.
Presumably, it is heated more or less uniformly by the infrared
energy emitted by the lamp's hot filament. While this was going
on, conduction and radiative heat transfer processes would be
acting to cool the glass.
Also, I like to make a comment on the relative importance of
radiative cooling versus convective cooling of objects in the
normal environment. Because radiative cooling goes as
Temperature to the fourth power, it can soon dominate.
-- Posted by Joe (Burnaby)

10. July 29th, 2008 11:11 am
This one seems easy. If the glass "flowed" due to the pressure
differential, the process should have been reversible when he
returned the lamp to atmospheric pressure. Whats the explanation
of the deformation? Its simple. Aldrin used the lamp, the bulb
heated the glass which causes to "flow faster", i.e. become more
malleable and more susceptible to the pressure differential. It
does not return back to shape since he turned the light off
before bringing back in his ship. Its clear that the thing got
pretty hot by looking at the photo.
-- Posted by Enrique

11. July 29th, 2008 11:45 am
Interesting piece. Considering Buzz aldrin is alive, he should
be able to comment on the light's usage history. While one may
be quick to say, well, that was 42 years ago, I would counter
that walking in space must not be easy to forget.
Items I would consider:
Composition of the lens glass. It would certainly contribute to
any practical explanation of so gross a deformation.
Exposure. Which would also depend on composition. If the glass
were `wet' or had some hydrated quality to it, and then exposed
to microwave radiation (transmitter to houston) the glass
`could' have absorbed enough heat to reach a softening point in
only several minutes. (duration of space walk?)
The bulb theory works, but you'd wonder about the bulb itself
not expanding (back to composition)
Creep certainly an observable phenomenon in glass, and not
surprisingly when creep occurs in other materials, such as
metals, and ceramics, when you start to look real close, voila,
glass. The region between grains of crystal growth (and in any
polycrystalline material you will have grains, which necessarily
have boundaries) tends toward an amorphous character, the
molecules are not wholly aligned to any of the neighboring
crystal grains. This is the home of creep.
One glass, which perhaps lends weight to Dr. Wolynes theory, but
sort of in the reverse, is the metallic family of glasses. To
form a metallic glass the melt must be quenched faster than the
molecules can crystallize. Thus, as you decrease the time period
to approach an infinitely short cool down, you would be
maximizing entropy into a material that really, really, Really
prefers to be ordered.
Metallic glasses are one of the most interesting materials out
there, but their bulk manufacture is still perhaps decades or
further away. They will do what the steel I-Beam did for the
modern world: change it.
Thanks for the flashlight story. In your primary Article dated
the 29th of July, I would have enjoyed hearing from actual Glass
Scientists, such as may be found at any of the following
institutions: Rutgers, Missouri-Rolla, Leeds University of
England, the university of St. Petersburg, Russia, and Alfred
University, (my alma mater; B.S. Glass Science, 2006).
My view, amorphous as glass itself, is that any substance that
exhibits bonds between the primary molecules composing said
substance, is a solid. Amorphous, but solid. The graphic in your
article illustrates precisely this. Most flow in solid glass
occurs across centuries, or millenia, and if it is strictly at
or near room temperature, will not be humanly discernible. The
Deborah number of a glass signifies its rate of flow, more or
less, over time. It is a biblical reference, something to the
extent of "And the mountains shall flow like glass before the
Lord". Though hot glass is the object of comparison here. As we
can see them, mountains do not flow.
Sorry, Way too long for a reply!!!
-- Posted by michael

12. July 29th, 2008 11:52 am
That the lens sagged or became dominated by its surface
tensionunder the influence of external forces once it became
heated is most probable, and that the distortion is not due
specifically to some pressure gradient alone or even in a very
significan way. Once the glass is softened enough to move,
gravity, pressure or inertial would suffice as the distorting
force. The likelyhood that the incandescent element in the
device was the cause of the heating doesn't seem that good to me
since if it could happen like that I'd expect that we'd see it
happen in other lighting devices in space and I can't recall
ever reading of this before. One thing that the space
environment does have in abundance is extremely powerfull
electrical equipment ostensibly shielded from casual human
contact presumably, but the work environment in space, like all
workspaces has a tendency to become complex,messy and has a
potential for a lot of energy to concentrate and I would include
the intense solar power, especially so since in space condtions,
90% of it has not been filtered out by the atmosphere as we
experience it here on the surface of the planet. I can easily
imagine that the flashlight could have been resting in a
position in the workspace, illuminated by the sun or in contact
with a current forming condition, for a period of time that
would allow for that amount of heating...keep in mind that the
lense is a relatively small amount of mass and that both vacuum
and the glass itself are insulators, but when glass reaches a
certain temperature it can become conductive and it would only
have to do this over a relatively short time and distance.
-- Posted by doug l

13. July 29th, 2008 1:29 pm
Explanations for the deformed flashlight lens while in the
vacuum of space assume that the flashlight lens is made of glass
and that there was air in the flashlight. (I don't know why
there would be air in the flashlight.) Pressure and temperature
differentials are also offered to help explain the deformed
flashlight lens. If this is the case, then I have three
questions: (1) Would not windows (if they are made of glass) on
space craft deform, bulge out or become concave? (2) Also, would
not windows (if they are made of glass) on deep diving
submersibles deform, bulge in or become concave? Maybe this
phenomena is avoided on deep diving submersibles by having extra
thick or tempered glass. But for space craft, as weight is a
problem, maybe some other stronger material is used for space
craft windows. (3) Have the glass lenses in cameras used in
space also deformed???
-- Posted by Gerry O'Brien, Ottawa, Canada

14. July 29th, 2008 2:13 pm
Joe,
Typical flashlights are designed so that during normal,
terrestrial operation, the glass of a light bulb don't get that
hot. (Think product safety..)
It's well established that conduction and convection are the
dominant modes of heat transfer at low to moderate temperatures.
At moderate temperatures, convection dominates heat transfer
from a hot surface to the surrounding air. In a vacuum, there
would be no convective cooling of the outside of the glass, and
when the flashlight is switched on, the glass would become much
hotter than if it were being operated under "normal"
circumstances.
The heat transfer mechanism from the filament to the interior of
the glass is of secondary importance. Whether the dominant mode
is radiative or convective heat transfer doesn't change the
fundamental reason why the glass is heating up.
-- Posted by Amy

15. July 29th, 2008 2:33 pm
Gerry,
Incandescent bulbs contain an inert gas. It extends the life of
the bulb by suppressing sublimation of the tungsten filament. (I
misspoke when I called this air. I really shouldn't treat
insomnia by posting on blogs.)
If my explanation is correct, then a glass window on a manned
spacecraft would not deform the way that the flashlight did. The
interior of the spacecraft would be near room temperature, and
the window glass would not get hot enough to soften. Camera
lenses would not deform for the same reason.
Personally, I am skeptical that this deformed flashlight is
purely a pressure effect. The gas pressure inside a light bulb
is low -- I doubt it's more than an atmosphere -- and the
pressure difference across the glass would on the order of 1 atm
-- not be particularly large. As you pointed out, it is
certainly an issue in deep sea vessels, where the water pressure
is several hundred times the atmospheric pressure.
-- Posted by Amy

16. July 29th, 2008 2:38 pm
The lack of any discoloration at the forward edges of the
surrounding hi-tech fender material tell me that the
temperatures required to deform glass (of the purely pedestrian
definition) were not reached. The asymmetry of deformed lens
seems to indicate a material of high insulating power with a
much narrower and lower melting temperature threshold. If your
going to take the time to wrap your flashlight in carpet strips
to avoid bumps and nicks, you might also consider a shatterproof
polycarbonate lens. Do we know for sure what the lens is made
of?
[No, I don't know for sure what the lens is made of. But
polycarbonate is a glassy material, too, so there would still be
the question of how the lens deformed. -- Kenneth Chang]
-- Posted by Matt DuBeau

17. July 29th, 2008 3:23 pm
I think previous posts about the combination of heat build-up
and pressure differential are probably on the right track. I
just think it must have happened at a much lower temperature
than required to melt common glass.
My personal experience is in a glass blowing studio, where I've
seen hot glass char wood and set fabric aflame with minor
contact, well before it gets hot enough to deform.
I would guess that pressure alone would have shattered a glass
lens rather than deform it, and the heat required to allow glass
to deform to that degree would have had some visible effect on
the surrounding materials.
-- Posted by Matt DuBeau

18. July 29th, 2008 6:04 pm
A flashlight generally has a bulb (evacuated) and a lens. The
bulb is glass and the lens is plastic. (Why it is called a lens
is a mystery lost in the fog of time and a subject for another
thread). Bulbs do get hot, but generally survive in a vacuum as
the radiative cooling is sufficient. On the other hand the
plastic in a vacuum would not survive.
That said, there's not enough info here to be conclusive and I
doubt the problem escaped NASA's attention. They are still using
flashlights in space, no doubt of a more suitable design!
Bub
-- Posted by Peter Sieck

19. July 29th, 2008 6:04 pm
Amy,
Yes, I agree with what you have said. Radiative heat transfer
from the glass to outer space dominates the rate of overall heat
loss and will determine the amount of temperature rise.
I took the simple Stefan-Boltzmann Law for radiative heat
transfer and calculated that the temperature of the glass if it
acts as ideal blackbody medium (in the infrared) and if it
radiates at the rate of 1 Watt per square centimeter. I
determined the temperature to be 648 kelvin (or 375 C or 707 F).
This is fairly hot. And it depends upon my assumptions.
In trying to figure out whether my assumed emission rate of 1 W
/cm^2 is too high or too low, I would need to know the power
emitted by the light filament, the details of the parabolic
mirror, and most importantly, the properties of the glass lens
(absorption/emission of radiation as a function of wavelength.)
Joe
-- Posted by Joe (Burnaby)

20. July 29th, 2008 6:46 pm
Gail called the transition from glass to liquid "sublimation."
That is not sublimation but melting. Sublimation is a phase
change from solid to gas.
-- Posted by Morton

21. July 29th, 2008 7:27 pm
I've worked with hot glass quite a bit and I just don't buy that
the lens is ordinary glass. I sincerely doubt it is glass at
all. There is a method of making sculptures with glass called
"slumping" which is pretty much what it sounds like: A pile of
glass is heated until it becomes soft and slumps into a form.
This is a fairly long process, even for small pieces, and it
requires a great deal of heat. 700 degrees F is around the
temperature that most glass becomes soft but when most types of
glass becomes hard again it needs to be cooled gradually to
allow the crystalline structure to become stable. Otherwise the
glass will shatter and crumble.
It also seems odd to me that the solution to this problem isn't
easy to find. There are exterior lights on the space shuttle and
space walking astronauts always have lights on their suits (the
way I understand it in space the areas in the light are very
bright and the areas in the shade are very dark). Obviously if
the heat of an ordinary 6volt flashlight bulb could melt glass,
that would be an enormous problem in space.
I know someone that works as an engineer in the aerospace
industry. I'll ask him what he thinks about this.
-- Posted by Alan Evil

22. July 29th, 2008 7:32 pm
No one has mentioned the reflector within the flashlight. If
pointed directly at the sun, it should focus the sun's light and
heat on the bulb, but at a slight angle, it might focus it on
the lens. If held there for a short period while pointing at the
dark side of the spacecraft, the lens might get hot enough to
soften the glass, at least in part. If hot enough, movement of
flashlight could cause that soften glass to deform simply from
inertial effects.
-- Posted by Dave

23. July 29th, 2008 8:16 pm
You blew it. Lets ask if Aldren pointed the Lens towards the
flying saucer he reported! Lets conspire and say he did! Wow,
Flashlight lens deformed by UFO! All other scientific
explanations are "Bunk", says respected primo NYT science
colomnist. Lets see how far up the "fool" chain this goes.
-- Posted by Jay Rosenberg

24. July 29th, 2008 11:12 pm
Posted by Alan Evil ...."I've worked with hot glass quite a bit"
..."700 degrees F is around the temperature that most glass
becomes soft"
Alan ..It's time to recalibrate your pyrometer. Even a soft
soda-lime glass is harder then most rocks at 987 F
-- Posted by kb richard

25. July 30th, 2008 9:46 am
Here's an extract from the Gemini 12 mission report (linked from
Wikipaedia entry)
One of the penlights stowed external to the tool pouch on the
adapter work station showed signs of overheating. The plexiglass
lens had bulged
outward, and the paint near the lens showed fingerprints which
were matched with paint marks on the EVA thermal gloves. This
penlight was
stowed with the lens end exposed, and it is probable that direct
solar heating caused the damage. The softening point of the
plastic lens
material is about 190 ° F.
Ron Palmer
[Thanks. I think that's a pretty definitive answer. Just to
note: Plexiglas is also a glass in the sense that I was writing
about (an amorphous solid). -- Kenneth Chang]
-- Posted by Ron Palmer

26. July 30th, 2008 10:43 am
I like the idea that the sun's rays would focus off the
reflector in the flashlight and cause the heat needed to do
this. From Dave at 7:32 pm.
Given that the flashlight is so carefully wrapped to minimize
damage from it, I assume, it seems unlikely that the lens would
be made of glass, as pieces of broken glass would be very
problematic in the shuttle environment.
It must be a plastic.
By the way I'm sure Mr. Evil meant Celsius.
-- Posted by Always take a flashlight on your space walks.

27. July 30th, 2008 1:26 pm
Thanks to Morton for pointing out that sublimation is the
transition from solid to vapor. The bubbles would be filled with
glass "vapor".
Gail
-- Posted by Gail

28. July 30th, 2008 1:28 pm
Maybe dark energy and glass have something in common and
interact in zero gravity.
-- Posted by Jane

29. July 30th, 2008 1:40 pm
Imagine the feeling of dread as you're walking through the void
of space, look down and see that your NASA-engineered FLASHLIGHT
is not up to the challenge. Think he had second thoughts
stepping back into the shuttle?
-- Posted by Serena

30. July 30th, 2008 2:11 pm
seriously, Aldrin was up there with a flashlight? Doesn't NASA
have something a little slicker he could use? something that
sounds a little fancier: spectrophotometer or defragulator or
something like that? did they pack their snackmaster sandwich
toaster, too? oh dear, NASA.
-- Posted by mercedes benj


About TierneyLab

John Tierney always wanted to be a scientist but went into
journalism because its peer-review process was a great deal
easier to sneak through. Now a columnist for the Science Times
section, Tierney previously wrote columns for the Op-Ed page,
the Metro section and the Times Magazine. Before that he covered
science for magazines like Discover, Hippocrates and Science 86.
With your help, he's using TierneyLab to check out new research
and rethink conventional wisdom about science and society. The
Lab's work is guided by two founding principles:
    + 1. Just because an idea appeals to a lot of people doesn't
      mean it's wrong.
    + 2. But that's a good working theory.
Comments and suggestions are welcome, particularly from
researchers with new findings. E-mail tierneylab at nytimes.com.