jgourlay
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Need some schooling on heating in glass
Gents,
I'm working the young 'uns through a series on historically important reactions. Sort'va "Cave Man" chemistry with other things thrown in.
We're up to 'cooking' or 'decomposing' pyrite. The thought being to 'cook' the pyrite in a clear vessel so they can *SEE* what's happening, piping
the gaseous product out through a tube and into water that has been adulterated with purple cabbage juice.
(if you didn't know, purple cabbage water is a great qualitative PH indicator that is non-toxic, comes out in the wash, and kid friendly)
The setup I have now is a boiling flask with the pyrite powder, tubed to an empty flask, which is tubed in series to a water column. I do this so
that if the system 'sucks back' it will suck into a room temperature vessel, not a hot one.
Here is my problem: I've broken a boiling flask. I seem to be faced with two situations. Either I use a butane burner which doesn't generate
sufficient heat to drive the decomposition of pyrite. Or, I use a more aggressive heat source and the vessel shatters. I broke one and decided to
stop and ask advice here: what is the right way to do this, remembering the key element that the kids need/want to *SEE* what is happening to the
pyrite?
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bahamuth
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A proper clamp with asbestos/glass wool in the claws and slow even heating is the key.
Perhaps a Kjeldahl tube (they use tubes and not flasks on the automated systems I've come across) would withstand the high temperatures, we heated
them to around 220-300 Celsius for prolonged periodes of time for hundresds of times until we saw crack formation in the bottom hemispheres without
ever one breaking. Plus in my experiance tubes withstands heating better through better stress resistance, flasks are complex shapes and must be
heated even equally.
Any sufficiently advanced technology is indistinguishable from magic.
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Mr. Wizard
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Why not switch to pyrex test tubes? The test tube will most likely be destroyed and melted by the temperature needed to break down pyrites, but the
cost is minimal. Another downside is the tube will be be approaching red heat and the glow from the glass in the flame will obscure viewing.
A cheap option would be a small stainless steel cooking pot with a lid. A small hole in the lid would allow gasses to escape whee they could be tested
with some filter paper soaked in your cabbage juice. If you must duct the gasses, use a steel tube (brake line) or 1/8" plumbing pipe fitted to a
hole in the pan lid.
Your glass is most likely breaking because of the difference in temperature between where you are heating it and the cooler parts induce a stress due
to expansion. I saw an example of this last night when my wife warmed up some cold liquid in a cold glass measuring cup with a microwave oven. The
cup snapped in two right at the liquid line. It really did a clean cut. Heating your material more slowly, and having a less steep heat gradient might
help.
Using a fused quartz test tube would solve your problem, but they are rather expensive.
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jgourlay
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Thanks gents!
I'll try the clamp/glass wool route first, because I have that. And I'll figure a way to make the heating from the propane torch more gradual. Maybe
I'll interpose an absbestos/steel wire 'sheeting' between fire and vessel until the vessel is pretty hot, then remove it.
If that fails, I'll go to Mr. Wizards option or the Kjeldahl tube (purchased items).
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peach
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Fond as I am of photographs, I'm not setting this up right now, it'll have to be the swiggly line interpretation from GIMP and my decade old roller
ball mouse.
I have used this method before to get test tubes extremely hot for an entire gas cartridges worth of time, when I was spending 24h odd trying to
diffuse manganese into zinc sulphide near the melting point of the glass.
Clamp the top (holding it in the clamp by hand isn't a great idea as this needs careful tweaking and patience). Build yourself a skirt of aluminium
foil around the top, starting from the clamped section and forming out into a bell. It must not touch the touch the tube on the way down. If you can
get two or three loose skirts on there, it works well.
Place your butane burner on the desk, not holding it. Light it and set the flame right down low, just barely on will do fine - so it's a soft flame
rising like a lighter, rather a blowtorch.
Gradually lower the tube down the stand towards the flame. If you go too quick, you may decompose the tube it's self. It is very easy to hit the
aluminium with the flame, which causes it to fall apart.
Hot air guns are useful, as those will reach 500C. The others are also right that you are up against the limits of the glass temperature wise. If it
doesn't crack, it is quite likely it'll come out warped or with bits of the sample stuck in the surface.
Not sure if this is going to be a particularly exciting demonstration for them. Depending on their age, and whether or not they're doing it
themselves, it may also be too fiddly. The chances of fingers getting burnt are high, but it has to happen sometime. It's amazing I have any fingers,
never mind all of them. My first self inflicted burns were in primary school, when I began messing around with soldering irons. There was also a close
call in primary school when the end of one was cut off, then surgically reattached. 
[Edited on 3-1-2012 by peach]
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zoombafu
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You could also try a sand bath, however im not sure if you can get a sand bath that hot, it would probably take hours to heat up.
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Sedit
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For fragile glasses I have found that sometimes placing them in water and allowing the water to boil away tends to ease the thermal shock on glass
that is prone to breaking. Once the water is gone the temperature can then rise and more often then not prevent the glass from shattering. I have been
able to do this with wine and beer bottles directly on an electric oven element and gain rather high temperatures without the majority of them
cracking. I don't know if this will be high enough for your experiment however.
Knowledge is useless to useless people...
"I see a lot of patterns in our behavior as a nation that parallel a lot of other historical processes. The fall of Rome, the fall of Germany — the
fall of the ruling country, the people who think they can do whatever they want without anybody else's consent. I've seen this story
before."~Maynard James Keenan
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zoombafu
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Quote: Originally posted by Sedit  | | For fragile glasses I have found that sometimes placing them in water and allowing the water to boil away tends to ease the thermal shock on glass
that is prone to breaking. Once the water is gone the temperature can then rise and more often then not prevent the glass from shattering. I have been
able to do this with wine and beer bottles directly on an electric oven element and gain rather high temperatures without the majority of them
cracking. I don't know if this will be high enough for your experiment however. |
What also works is using oil, such as peanut oil, mineral oil, etc.
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Hexavalent
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Speaking of sand baths, I just pop them in the oven for a while before they go onto the hotplate . . .they heat up much faster because the heat is
even all over the material, not just concentrated on the bottom. Works great!
"Success is going from failure to failure without loss of enthusiasm." Winston Churchill
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zoombafu
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| Quote: Originally posted by Hexavalent | | Speaking of sand baths, I just pop them in the oven for a while before they go onto the hotplate . . .they heat up much faster because the heat is
even all over the material, not just concentrated on the bottom. Works great! |
Never thought of that, Ill have to remember that in the future
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Arthur Dent
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Cool idea! It's the sum of all these neat little tricks that make home chemistry fun!
Also, it's always a good idea if the sand you use comes from the garden/hardware store to thoroughly wash it several times (until the water runs
clear) and then to cook the whole batch in a shallow pan in the oven for a few hours at high temp in order to "smoke it out".
That way, you won't stink up the place every time you use a sand bath. The sand (after cooling down) can be then stored in a large plastic container
(like a bleach bottle or a windshield washer bottle).
Robert
--- Art is making something out of nothing and selling it. - Frank Zappa ---
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jsc
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Doh, you don't heat glass over 600C unless it is pure silica or you want to do something horrible to the glass. The strain point of borosilicate glass
is 515C.
If you want to roast pyrites, do it in a crucible or refractory dish and interpose a plexigas viewing shield in case something explodes.
If you are teaching chemistry you need to research this stuff more carefully. Don't screw around with high temperatures, corrosives, poisons or vacuum
unless you know the material characteristics thoroughly.
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