Hey, OK! Cite me!
Or tell me me look it up myself!
I will!
Part of it is time. If you left part of a glass tube in the flame for a long time, eventually you’d feel some heat where you are gripping the glass. The window in your house sits there for hours and hours, equilibrating to whatever temperature it’s exposed to.
And a double-glazed window does a decent job of retaining heat. On the coldest winter days, you might see some condensation on the inside of a double-glazed window if your indoor humidity is too high, but you aren’t likely to see frost.
The equation for conductive heat transfer from point 1 to point 2 is:
q = K*A/X(T[sub]1[/sub] - T[sub]2[/sub])
Where:
q = heat transfer rate, watts
K = conduction coefficient of the material through which heat is being conducted, w/(m*K)
A = cross sectional area of the material, m[sup]2[/sup]
X = thickness of material (distance between points 1 and 2)
T[sub]1[/sub] = temperature at the surface heat is being conducted from
T[sub]2[/sub] = temperature at the surface heat is being conducted to
Since you’re into electronics, you can view X/(K*A) as a resistance, and the temperature difference as the driving potential; note that building insulation is often described by its R-value, with a higher number denoting better resistance to heat transfer. Heat transfer scenarios are sometimes modeled as resistance networks: heat may get conducted through sequential layers of materials (resistors in series), and then carried away to the environment through a combination of convection and radiation (resistors in parallel). Chunks of material can even be modeled as capacitors, since their temperature increases as larger amounts of thermal energy get stuffed into them.
While it’s a lot better than metal, it’s not nearly as good as ceramics, so it’s not ideal. You can sometimes find glass dinner plates, however.
Very good discussion, so far. There are some points I’d like to add from the chemical engineering perspective :
1> The “bendiness” of glass depends on the chemical composition of glass. Look at page 3 of this presentation. Soda glass, the common glass is good for doing what the OP calls for - the glass utensils used for baking are not.
2> Unlike metals, the thermal conductivity of glasses generally increases with increasing temperature. It increases even more rapidly when radiation is applied. See page 6 of the same presentation above.
If fact glass is such a poor conducter that you have to spread out the heat over a larger area than you’re working on. Or it will have too sharp a tempreture gradient and break.
Thats the combined effect of poor conductance and good expansion.
Consider the Following :
Thermal conductivity of Carbon Steel (at 25 deg C) = 54 W/m-K
Thermal conductivity of Glass (at 25 deg C) = 1 W/m-K
Volumetric thermal exp coefficient of Carbon Steel (at 20 deg C) = 35.5 …10^-6/K
Volumetric thermal exp coefficient of Glass (at 20 deg C) = 25.5 …10^-6/K
So as you can see while the thermal conductivity is poor compared with steel - the volumetric expansion is comparable!!! giving rise to high stresses
Now consider Borosilicate Glass (the glass bakeware and lab equipment is made of) - it has a Volumetric thermal exp coefficient of 10 (10^-6/K) - 2.5 times less than regular glass and thus produces less thermal stress.
And that clearly explains the difference between a glass tube (low A, high X,) to a window pane (High A, low x)
I can personally verify this.
In undergrad chemistry, we used to “blow” small vials from largish glass tubes. After heating one end, we would directly blow on the other end. I was once distracted after heating, but before blowing. I guess I reheated the other end and went to blow on the normal-looking cool end. :smack:
I don’t specifically remember if my lips even touched the glass; I suppose not. I actually don’t remember even how much it hurt, except that it was a LOT (my lips screamed at me)! (So much so, that many of the details of this otherwise memorable experience are merifully lost to me.) Luckily, I did not experience more than a minor burn. Also fortunately, noone noticed, and I was able to continue the class.
the back of your hand or fingers placed slowly progressively closer can be a useful temperature sensing safety method.
It is what it appears to be…
You see a guy working glass with his bare hands and you see the size of the tube and you see how far his hands are away from the molten part. The only answer for how this is possible is that it’s due to the heat conductive properties of glass.
Another factor at work is that glass torches burn much hotter than say a typical propane torch you might be familiar with and so they’re able the heat the glass to a molten state quickly, before the heat has time to radiate very far along the rod. Obviously, it will eventually migrate along the rod, but you’re usually done with what you’re doing before that’s an issue. Glass workers typically work quickly because glass doesn’t stay molten for long once it’s out of the torch and that quick pace also limits the amount of time the heat has to move down the rod. I can go from a room temperature rod to molten, to blown and finish shaped in just a few minutes. It all simply happens too fast for the heat to have much time to travel.
I used to work as a graphic designer at a mom & pop neon sign shop, so I drew up all the patterns for hand-bent neon tube. They had a guy from Colombia that did the glass bending etc. – he was really good, and it’s a bit of a dying art. I tried it a few times, and it’s WAY harder than it looks…but I didn’t wear gloves.
You have a rubber hose in your mouth that attaches to an open end of the tube so you can blow air in to keep the hose from pinching when you bend it – you have to blow in just the right amount of pressure, while trying to heat the glass just the right amount (don’t want it too soft, but pliable), and trying to make a complicated bend and match your pattern. Those guys are true artists. The glass-bending guy now owns his own neon company.
Excellent username/post combination.
Windows are lousy as keeping heat out on a sunny day because the energy passes through the window as visible light. Glass is a very good conductor of visible light, what with being transparent and all. The light then absorbed by anything inside the room/car that isn’t a mirror, and converted to heat. The glass then prevents that heat from escaping, because it’s not a very good conductor of heat. This is known as the greenhouse effect.