RE: [Vo]:Question about hot glass
Oh it's glowing all right at those temperatures. It's just clear. Now if you get a layer on the surface, even a very thin one, of something that reacts with the atmosphere, and it will go opaque because of the reacting compounds at the surface. Tin bloom on float glass, applied color or additives, nasty polluted kiln atmosphere from previous pottery glaze use, etc. will cause the surface to become rather opaque to some degree. Good clean clear glass though is always clear at process and furnace temperatures. I suppose if the background behind the clean glass is dark, then the glow might overwhelm what you actually see and it could look opaque, but it is still clear. Get some light behind it and it becomes readily apparent. R. -Original Message- From: Stephen A. Lawrence [mailto:sa...@pobox.com] Sent: Monday, July 27, 2009 6:57 AM To: vortex-l@eskimo.com Subject: Re: [Vo]:Question about hot glass Rick Monteverde wrote: The hot (1800+ degF) and warm (1450+ degF) glass I've worked with always stays clear. Glass from a furnace is extremely clear, you can look at the bottom of the pot and it looks like there's nothing in there. In this case it's presumably also not glowing, or at least not much, and that would seem to fit with the claim that it absorbs just as it radiates. The really weird thing is when gold metal gets translucent. Noticed it for years but never believed my eyes were telling me the truth. Say what?? Could you please provide more info on this? This teaser is a killer! R. -Original Message- From: Stephen A. Lawrence [mailto:sa...@pobox.com] Sent: Wednesday, July 22, 2009 3:38 AM To: vortex-l@eskimo.com Subject: [Vo]:Question about hot glass I ran across an explanation of a blackbody which I actually understood a week or so back (totally unexpected, it was in the introductory chapter to a QM book), and since then I've been fiddling around with gedanken experiments involving black boxes with little holes in them and the second law of thermodynamics. And it appears to me that, according to the second law of thermodynamics, if glass is heated red-hot or orange-hot, and it's actually seen to be glowing orange, it should also turn *opaque* to visible light while it's at that temperature.
Re: [Vo]:Question about hot glass
Horace Heffner wrote: On Jul 27, 2009, at 10:07 AM, Stephen A. Lawrence wrote: Suppose you took a lump of glass and placed it in an (evacuated) oven. Suppose further that the walls of the oven are dead black, absorbing (nearly) all radiation which falls on them, and assume that they radiate about as you'd expect a blackbody to radiate. Suppose further that the oven and the lump of glass are at the same orange-hot temperature (and let's ignore the fact that the glass has melted all over the bottom of the oven because that adds unnecessary complexity to the experiment -- maybe we put the whole thing in free-fall, or whatever). Now the walls of the oven are giving off a cheery orange glow. Assume the glass is glowing orange, too, and assume further that it's glowing just as brightly as the walls of the oven. (This is an assumption; we know glass glows *some* but we haven't confirmed that glass glows as brightly as something which starts out black.) I think there might be a misconception here about the difference in behavior between heated surfaces and heated black body cavities. Cavities, at least peep holes into cavities, act as almost perfect black bodies. See: http://en.wikipedia.org/wiki/Black_body In the laboratory, black-body radiation is approximated by the radiation from a small hole entrance to a large cavity, a hohlraum. (This technique leads to the alternative term cavity radiation.) Any light entering the hole would have to reflect off the walls of the cavity multiple times before it escaped, in which process it is nearly certain to be absorbed. This occurs regardless of the wavelength of the radiation entering (as long as it is small compared to the hole). The hole, then, is a close approximation of a theoretical black body and, if the cavity is heated, the spectrum of the hole's radiation (i.e., the amount of light emitted from the hole at each wavelength) will be continuous, and will not depend on the material in the cavity (compare with emission spectrum). Yes, I understand this. I know from personal observation that it goes beyond this. As a cavity and its contents heat up, everything in the cavity eventually disappears from view through the peep-hole. I have personally sat and watched through a gas forge observation port, which I kept open, the cover lifted, as that gas forge, which was about 1' by 2' by 2', heated up. Initially, I could clearly see the far walls of the forge and things in it through the port. When the temperature rose to an orange glow, suddenly nothing was visible inside the forge. There was a pure orange glow coming from the observation port that had nothing to do with the contents of the forge. One moment I could see the other side of the forge, which had some hot spots and dark spots on it, and the next it was replaced by flat orange glow. I could see nothing at all inside the port. It was as if the hole surface itself (which is not a physical thing) was radiating. Cool! I like your description -- I hadn't realized it would be so dramatic, but I suppose it must, when everything inside is glowing equally. Actually I do understand the distinction between heated ordinary bodies and an ideal blackbody or the peephole in an oven. Furthermore, the reason the peephole in an oven works that way, regardless of the material in the oven, is exactly the fact that a hot real-world object radiates exactly to the extent that it also absorbs. As a result, the sum of radiated, reflected, and transmitted radiation from the object (when it's in an oven of a fixed temperature) is identical to the radiation from an ideal blackbody of the same temperature, and since everything in the oven behaves that way, everything in the oven looks identical (when it's all glowing at a uniform temperature). And that's why I was wondering about the opacity of *glowing* glass. Best regards, Horace Heffner http://www.mtaonline.net/~hheffner/
RE: [Vo]:Question about hot glass
The hot (1800+ degF) and warm (1450+ degF) glass I've worked with always stays clear. Glass from a furnace is extremely clear, you can look at the bottom of the pot and it looks like there's nothing in there. The really weird thing is when gold metal gets translucent. Noticed it for years but never believed my eyes were telling me the truth. R. -Original Message- From: Stephen A. Lawrence [mailto:sa...@pobox.com] Sent: Wednesday, July 22, 2009 3:38 AM To: vortex-l@eskimo.com Subject: [Vo]:Question about hot glass I ran across an explanation of a blackbody which I actually understood a week or so back (totally unexpected, it was in the introductory chapter to a QM book), and since then I've been fiddling around with gedanken experiments involving black boxes with little holes in them and the second law of thermodynamics. And it appears to me that, according to the second law of thermodynamics, if glass is heated red-hot or orange-hot, and it's actually seen to be glowing orange, it should also turn *opaque* to visible light while it's at that temperature.
Re: [Vo]:Question about hot glass
Molten glass at red stage is generally crystal clear. you can find videos of glass blowing demenstrations on youtube and see for yourself. On Wed, Jul 22, 2009 at 6:38 AM, Stephen A. Lawrencesa...@pobox.com wrote: I ran across an explanation of a blackbody which I actually understood a week or so back (totally unexpected, it was in the introductory chapter to a QM book), and since then I've been fiddling around with gedanken experiments involving black boxes with little holes in them and the second law of thermodynamics. And it appears to me that, according to the second law of thermodynamics, if glass is heated red-hot or orange-hot, and it's actually seen to be glowing orange, it should also turn *opaque* to visible light while it's at that temperature. (If its glow is weaker than, say, steel at the same temp then it should be semitransparent rather than totally opaque but none the less it shouldn't be water-clear, as it is at room temperature.) I've seen lead-crystal (very clear) glass being worked at high temperatures, at Corning many years ago, and as far as I can recall it did indeed glow bright orange. Does anyone here happen to know if glass also turns opaque (or semi-opaque) when it's heated to high temperature? (If it is I'll be amused; if it's not I'll have to go figure out where my reasoning went off the tracks.) I know for a fact candle flames are transparent, but I don't have the facilities to heat a pane of glass until it produces a cheery glow while shining a bright beam of light through it (don't even own a propane torch at this point, and in any case hitting a windowpane with a propane torch would probably shatter it).
Re: [Vo]:Question about hot glass
Rick Monteverde wrote: The hot (1800+ degF) and warm (1450+ degF) glass I've worked with always stays clear. Glass from a furnace is extremely clear, you can look at the bottom of the pot and it looks like there's nothing in there. In this case it's presumably also not glowing, or at least not much, and that would seem to fit with the claim that it absorbs just as it radiates. The really weird thing is when gold metal gets translucent. Noticed it for years but never believed my eyes were telling me the truth. Say what?? Could you please provide more info on this? This teaser is a killer! R. -Original Message- From: Stephen A. Lawrence [mailto:sa...@pobox.com] Sent: Wednesday, July 22, 2009 3:38 AM To: vortex-l@eskimo.com Subject: [Vo]:Question about hot glass I ran across an explanation of a blackbody which I actually understood a week or so back (totally unexpected, it was in the introductory chapter to a QM book), and since then I've been fiddling around with gedanken experiments involving black boxes with little holes in them and the second law of thermodynamics. And it appears to me that, according to the second law of thermodynamics, if glass is heated red-hot or orange-hot, and it's actually seen to be glowing orange, it should also turn *opaque* to visible light while it's at that temperature.
Re: [Vo]:Question about hot glass
Alexander Hollins wrote: Molten glass at red stage is generally crystal clear. you can find videos of glass blowing demenstrations on youtube and see for yourself. Thanks -- I'll have to look them up. Issue is that if it's radiating, say, 10% as much as molten platinum would (with roughly the same melting point), then it would produce a visible glow *and* appear transparent, particularly in thin section. This is squishier than I expected, so to speak (and candle flames turn out not to be *fully* transparent BTW but that's another story). Sigh I may have to get hold of a torch and do my own experiments here -- or find a good thermo textbook (ugh, I hated thermo in college, which is one reason I still don't understand it)... One interesting sidelight: Bill Beatty has posted at least one video in the past showing that hot glass turns opaque to microwaves. That was glass heated well below the glow point (until the microwaves hit it, of course) but that would make sense as microwaves are much lower frequency than light (obviously). On Wed, Jul 22, 2009 at 6:38 AM, Stephen A. Lawrencesa...@pobox.com wrote: I ran across an explanation of a blackbody which I actually understood a week or so back (totally unexpected, it was in the introductory chapter to a QM book), and since then I've been fiddling around with gedanken experiments involving black boxes with little holes in them and the second law of thermodynamics. And it appears to me that, according to the second law of thermodynamics, if glass is heated red-hot or orange-hot, and it's actually seen to be glowing orange, it should also turn *opaque* to visible light while it's at that temperature. (If its glow is weaker than, say, steel at the same temp then it should be semitransparent rather than totally opaque but none the less it shouldn't be water-clear, as it is at room temperature.) I've seen lead-crystal (very clear) glass being worked at high temperatures, at Corning many years ago, and as far as I can recall it did indeed glow bright orange. Does anyone here happen to know if glass also turns opaque (or semi-opaque) when it's heated to high temperature? (If it is I'll be amused; if it's not I'll have to go figure out where my reasoning went off the tracks.) I know for a fact candle flames are transparent, but I don't have the facilities to heat a pane of glass until it produces a cheery glow while shining a bright beam of light through it (don't even own a propane torch at this point, and in any case hitting a windowpane with a propane torch would probably shatter it).
Re: [Vo]:Question about hot glass
Stephen A. Lawrence wrote: Rick Monteverde wrote: The hot (1800+ degF) and warm (1450+ degF) glass I've worked with always stays clear. Glass from a furnace is extremely clear, you can look at the bottom of the pot and it looks like there's nothing in there. In this case it's presumably also not glowing, or at least not much, and that would seem to fit with the claim that it absorbs just as it radiates. I don't think I explained the reasoning here, and perhaps I should. Suppose you took a lump of glass and placed it in an (evacuated) oven. Suppose further that the walls of the oven are dead black, absorbing (nearly) all radiation which falls on them, and assume that they radiate about as you'd expect a blackbody to radiate. Suppose further that the oven and the lump of glass are at the same orange-hot temperature (and let's ignore the fact that the glass has melted all over the bottom of the oven because that adds unnecessary complexity to the experiment -- maybe we put the whole thing in free-fall, or whatever). Now the walls of the oven are giving off a cheery orange glow. Assume the glass is glowing orange, too, and assume further that it's glowing just as brightly as the walls of the oven. (This is an assumption; we know glass glows *some* but we haven't confirmed that glass glows as brightly as something which starts out black.) If the glass is still FULLY TRANSPARENT, so that the radiation from the walls is passing through the glass without being absorbed, then the glass must also be COOLING OFF, because it's radiating more than it's absorbing, and the oven walls must be WARMING UP, because they're receiving (and absorbing) all their own radiation *plus* the glow from the glass. Now, let's run heat pipes to the walls of the oven and to the lump of hot glass, and lead the other ends of the pipes to a Stirling motor. As the glass cools, the Stirling motor finds itself with a warm source and a cool source and so it runs, and heat flows from the oven walls through the Stirling motor and on to the lump of glass through the heat pipes, thus keeping the glass hot enough to continue to radiate. Now we enclose the whole rig in a perfectly mirrored box so no radiation gets in or out, and we run the shaft of the Stirling motor out through a hole in the box (with careful friction-free seals around the shaft). Voila, we have a permanent energy source, which consumes nothing and produces mechanical energy until the motor wears out. That's what the second law of thermodynamics says you can't do. What's worse, if the motor's bearings aren't perfect, the inside of the box will actually get warmer due to friction in the bearings, and that violates the first law as well as the second law (and there isn't any ZPE running around in this scenario to let us explain that violation away). This is clearly a very evil box.
Re: [Vo]:Question about hot glass
Stephen A. Lawrence wrote: Stephen A. Lawrence wrote: Rick Monteverde wrote: The hot (1800+ degF) and warm (1450+ degF) glass I've worked with always stays clear. Glass from a furnace is extremely clear, you can look at the bottom of the pot and it looks like there's nothing in there. In this case it's presumably also not glowing, or at least not much, and that would seem to fit with the claim that it absorbs just as it radiates. I don't think I explained the reasoning here, and perhaps I should. Suppose you took a lump of glass and placed it in an (evacuated) oven. Suppose further that the walls of the oven are dead black, absorbing (nearly) all radiation which falls on them, and assume that they radiate about as you'd expect a blackbody to radiate. Suppose further that the oven and the lump of glass are at the same orange-hot temperature (and let's ignore the fact that the glass has melted all over the bottom of the oven because that adds unnecessary complexity to the experiment -- maybe we put the whole thing in free-fall, or whatever). Now the walls of the oven are giving off a cheery orange glow. Assume the glass is glowing orange, too, and assume further that it's glowing just as brightly as the walls of the oven. (This is an assumption; we know glass glows *some* but we haven't confirmed that glass glows as brightly as something which starts out black.) If the glass is still FULLY TRANSPARENT, so that the radiation from the walls is passing through the glass without being absorbed, then the glass must also be COOLING OFF, because it's radiating more than it's absorbing, and the oven walls must be WARMING UP, because they're receiving (and absorbing) all their own radiation *plus* the glow from the glass. Now, let's run heat pipes to the walls of the oven and to the lump of hot glass, and lead the other ends of the pipes to a Stirling motor. As the glass cools, the Stirling motor finds itself with a warm source and a cool source and so it runs, and heat flows from the oven walls through the Stirling motor and on to the lump of glass through the heat pipes, thus keeping the glass hot enough to continue to radiate. Now we enclose the whole rig in a perfectly mirrored box so no radiation gets in or out, and we run the shaft of the Stirling motor out through a hole in the box (with careful friction-free seals around the shaft). Voila, we have a permanent energy source, which consumes nothing and produces mechanical energy until the motor wears out. Or ... err ... until the whole thing cools off so much it stops running... You can't use this arrangement to violate conservation of energy; I'm clearly wrong about that. Mechanical energy extracted from the system will show up as a loss of total heat inside the box. Anyhow the second law says you can't do that, either. That's what the second law of thermodynamics says you can't do. Indeed.
Re: [Vo]:Question about hot glass
Stephen A. Lawrence wrote: I've seen lead-crystal (very clear) glass being worked at high temperatures, at Corning many years ago, and as far as I can recall it did indeed glow bright orange. After the conversation here I searched through my old slides and found a photo of what I remembered. Turns out my memory was wrong -- the end of the metal rod, clearly visible through the (transparent!) hot glass, was glowing *bright* red. The glass itself was glowing *dim* orange (dim by comparison with the rod's glow). The rod and the glass must have been at about the same temp, or more likely, the glass was hotter than the rod, as the glass had been in direct contact with the environment of the furnace while the rod was protected from it by the glass. Guessing the rod is steel but I don't know for sure. Anyhow a blowup of a small piece of the original picture is attached. It's a scan from a slide, which was shot in dim light and wasn't super sharp, unfortunately; it's been despeckled and unsharp-masked heavily but still looks a bit fuzzy. So it looks like hot glass doesn't go particularly opaque, but also doesn't glow very brightly at all in comparison with metal. The double image is due to a reflection: the glass knob, on the end of the rod, is being rolled along a very smooth, shiny surface to give the shape the glass worker wants. This was the Steuben workshop, where they mostly made blobby animals and things like that, so rounded knobby shapes were kind of the order of the day. inline: 1983-may-roll-7-33-b.knob-annotated.lowres.jpg
Re: [Vo]:Question about hot glass
On Jul 27, 2009, at 10:07 AM, Stephen A. Lawrence wrote: Suppose you took a lump of glass and placed it in an (evacuated) oven. Suppose further that the walls of the oven are dead black, absorbing (nearly) all radiation which falls on them, and assume that they radiate about as you'd expect a blackbody to radiate. Suppose further that the oven and the lump of glass are at the same orange-hot temperature (and let's ignore the fact that the glass has melted all over the bottom of the oven because that adds unnecessary complexity to the experiment -- maybe we put the whole thing in free-fall, or whatever). Now the walls of the oven are giving off a cheery orange glow. Assume the glass is glowing orange, too, and assume further that it's glowing just as brightly as the walls of the oven. (This is an assumption; we know glass glows *some* but we haven't confirmed that glass glows as brightly as something which starts out black.) I think there might be a misconception here about the difference in behavior between heated surfaces and heated black body cavities. Cavities, at least peep holes into cavities, act as almost perfect black bodies. See: http://en.wikipedia.org/wiki/Black_body In the laboratory, black-body radiation is approximated by the radiation from a small hole entrance to a large cavity, a hohlraum. (This technique leads to the alternative term cavity radiation.) Any light entering the hole would have to reflect off the walls of the cavity multiple times before it escaped, in which process it is nearly certain to be absorbed. This occurs regardless of the wavelength of the radiation entering (as long as it is small compared to the hole). The hole, then, is a close approximation of a theoretical black body and, if the cavity is heated, the spectrum of the hole's radiation (i.e., the amount of light emitted from the hole at each wavelength) will be continuous, and will not depend on the material in the cavity (compare with emission spectrum). I know from personal observation that it goes beyond this. As a cavity and its contents heat up, everything in the cavity eventually disappears from view through the peep-hole. I have personally sat and watched through a gas forge observation port, which I kept open, the cover lifted, as that gas forge, which was about 1' by 2' by 2', heated up. Initially, I could clearly see the far walls of the forge and things in it through the port. When the temperature rose to an orange glow, suddenly nothing was visible inside the forge. There was a pure orange glow coming from the observation port that had nothing to do with the contents of the forge. One moment I could see the other side of the forge, which had some hot spots and dark spots on it, and the next it was replaced by flat orange glow. I could see nothing at all inside the port. It was as if the hole surface itself (which is not a physical thing) was radiating. Best regards, Horace Heffner http://www.mtaonline.net/~hheffner/
Re: [Vo]:Question about hot glass
Some great history of black body radiating cavity hole physics: http://Galileo.phys.Virginia.EDU/classes/252/black_body_radiation.html http://tinyurl.com/mbra5q Best regards, Horace Heffner http://www.mtaonline.net/~hheffner/
Re: [Vo]:Question about hot glass
In reply to Stephen A. Lawrence's message of Wed, 22 Jul 2009 09:38:15 -0400: Hi, [snip] Does anyone here happen to know if glass also turns opaque (or semi-opaque) when it's heated to high temperature? (If it is I'll be amused; if it's not I'll have to go figure out where my reasoning went off the tracks.) ...going on vague recollections, I think it is at least translucent, and possibly opaque. Regards, Robin van Spaandonk http://rvanspaa.freehostia.com/Project.html
