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Author: Subject: Thwarting uneven heating in glass?
deadrush
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[*] posted on 14-12-2024 at 18:05
Thwarting uneven heating in glass?


I have long wondered if covering a glass item in metallic foil would at least mitigate uneven thermal expansion. Basically the metal is against the hot plate/ whatever and so would disperse the heat more evenly. Thoughts?
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Chemetix
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[*] posted on 14-12-2024 at 21:51


It's a problem that has been around for a while and naturally there's been some developments along those lines.

You'll find closely fitting metal hemispheres as a hotplate surface. Alternatively, fine aluminium granules act as a surface conforming heat transfer medium and are found quite commonly in labs these days.

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Sulaiman
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[*] posted on 15-12-2024 at 03:29


For working temperatures below about 200C, an oil bath is useful.
https://en.m.wikipedia.org/wiki/Oil_bath

Above that a sand bath can be used,
but thermal conductivity is so low that to get high temperatures inside the glassware
the hotplate will be at or above it's maximum working temperature.

For higher temperatures I like a hot air bath
- a chimney surrounding an rbf that is above, but not in, a gas flame
This should work over a hotplate, but monitor your hotplate temperature.

My favourite for higher temperatures is a heating mantle.

......................
Due to differing thermal expansion coefficients,
it is impractical to have a metal 'skin' thick enough to distribute heat efficiently
attached to glass that is subject to large temperature changes.

Not all RBFs of the same capacity have the same diameter.


__________________________________
PS. SORRY ! .... I forgot about the fire hazard of an oil bath, as mentioned in the post below.
Personally : not high on my risk list - especially when compared to my typical RBF contents.
But IT IS AN EXTRA RISK
Plus
Cleaning up after using an oil bath is extra work.
And
'emergency cooling' is not easy, maybe a lab jack would work.
BUT
A big benefit of heating baths is that they can be designed/filled to have the capacity to catch anything that may drop from a broken RBF
Inconvenient, not pretty, but often safer.
BUT BUT BUT
It was mentioned by a member (sorry, forgot who or where) that the temperature and heat of the bath may
flash-boil liquid from a broken RBF - which is terrifying !

[Edited on 16-12-2024 by Sulaiman]




CAUTION : Hobby Chemist, not Professional or even Amateur
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jackchem2001
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[*] posted on 15-12-2024 at 03:30


My setup for heating round bottom flasks evenly on a hotplate uses an aluminium foil air bath around the flask which rests on a small, solid aluminium foil ring (just so the clamp doesn't have to bear the weight and so the flask doesn't directly touch the hot plate). I have never experienced bumping (outside of flasks with lots of solid material which is not fair). You can sort of see the setup here: https://www.sciencemadness.org/whisper/viewthread.php?tid=16...

Make sure the aluminium foil does not press directly on this glass as this will cause cracking due to thermal shock on cooling (since the foil will conduct heat away from the spot it is pressed against): https://www.sciencemadness.org/whisper/viewthread.php?tid=61...

Much better than an oil bath which presents a fire hazard.
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