For another example, consider a glass coffee mug: The bowl will be hot because heated by contact with the coffee - but the handle stays at a comfortable temperature.
I used to bend glass without gloves. True, it was little tubes and such for my Gilbert Chemistry set, but the bending part got good and orange. Never dawned on me that gloves would be needed. (I think the instruction book may have said it was okay if you held the ends.)
A different era.
Glass stretches too. I made glass pipettes in chemistry by heating and stretching the glass. That created the tiny tip at the end.
I regularly heat glass rods in a torch making glass beads. You can be melting a 1/4" diameter glass rod in a torch and holding the rod bare handed around 4" from the melty part.
Gloves are mostly because if you are working around hot glass and torches alot its very easy to brush something very very hot.
My Dad used to do that. Pipettes started as short, about half-centimetre wide glass cylinders. He’d heat the middle part up over a bunsen burner, rotating it to get it evenly hot, and then suddenly just pull the ends apart to make a foot-long pipette in the middle (then cut into normal-sized pipettes). I guess that the air in the cylinder keeps the centre of the pipette open during the process.
I do that all the time. You only need an inch or so between the spot where it melts and your fingers.
Glass is a terrible conductor of heat.
From Incropera and DeWitt’s “Fundamentals of Heat and Mass Transfer,” third edition:
Glass, K = 1.4 W/(m*K)
For comparison:
Iron, K = 80.2
Aluminum, K = 237
Where insulating a house is concerned, a glass window is terrible compared to a well-insulated wall; for fiberglass insulation, K = ~0.040, and exterior walls are usually several inches thick. Even a double- or triple-glazed window is going to be terrible compared to that.
But for bending glass tubes over a burner? glass is a crummy conductor. Once you take the tube out of the flame, most of the heat will get pissed away to atmosphere before it manages to get conducted down the tube to where your fingers are.
It’s enough to make a difference.
A mirrored-glass-lined vaccuum bottle is considerably more effective than a metal-lined vaccuum bottle. The difference would come down to thermal conduction losses of the metal liner.
Of course, a metal bottle won’t shatter if you look at it wrong. :eek:
I suspect the difference is in the quality of the mirror surface. That makes a massive difference to the emissivity. A stainless steel vacuum flask is very unlikely to get a mirror surface finish on the inside, not to the quality of aluminium deposited on glass.
That can be the difference between emissivity values of about 0.05 versus 0.5. That will totally dominate the heat transfer, whilst the differences in conductivity of the material in the neck of the double bottle is going to be very minor.
My freshman Chemistry professor, just after someone shrieked and dropped a beaker on the floor: “The most important lesson you will learn in Chem lab is that you cannot tell the difference between hot glass and cold glass by looking.”
Conduction of heat is physics.
Bending glass.
It is also why hot food isn’t served on glass plates!
Given that glass is a poor thermal conductor, I would think that makes it an ideal material for dinner plates. The opposite extreme, a plate made out of copper or aluminum, would be like attaching a giant cooling fin to your steak: your dinner would go cold far more quickly.
The latter phenomenon (rapid heat transfer facilitated by a high-conductivity, large-surface-area plate) is in fact the basis for thawing/defrosting trays. They’re just big aluminum plates, intended to get atmospheric warmth into your frozen food ASAP.
Glass can bend in a different, well, glassy state (“hardened”–there must be a word for this), over centuries–you can see it in old stained glass windows. So theoretically steady hands and inter-generational hand-offs might do it.
ETA: Wait, don’t tell me: vitreous?
Which is why dull (matte) surfaces in the bottom of a pan makes for easier clean-up (and food motion in cooking, to a tiny amount) than for the equivalent residue in a nice “spotless gleaming” one.
Since we mentioned food apps.
ETA: Certainly in tin.
Hot food isn’t served on copper or aluminum either, for the reasons you list. It is served on ceramic (stoneware, pottery, etc) which will keep it warm.
ETA: Salad otoh is perfectly suited for glass plates.
Thank you!! That’s where my confusion was coming from. Compared to insulated walls (or even the paneling on a car) glass is a great heat conductor. If a 1/8 - 1/4" (2 panes x 1/8") sheet of glass can’t keep out a ~30 temp differential, why would a few inches stop several hundred degrees of difference? It sounds like there’s difference between radiating/absorbing heat and conducting it to adjacent areas.
Also, your heat transfer numbers there what do the constants/variable represent? I’m guessing W=watts (cause I’m an electronics geek and can see energy transfer being expressed in watts), m meters from the heat source, and K is temperature difference in degrees Kelvin?
This is an urban legend, debunked right here on the SD!
What do you mean by “keep out?”
The fact that there is a 30° differential means that the glass is doing a pretty good job. The issue is one of degrees (pun intended), Glass is a poor insulator compared with dedicated insulators (like foam insulation), and it’s used in very thin sections, over large areas, which makes everything worse. Compared to metals, it’s a great insulator, though (but most metals aren’t transparent (at least not until ST IV)).