Maui3
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DIY Heating Blocks For Flasks?
By heating blocks, I mean something more or less like this:
I was wondering if anyone have made heating blocks for round bottom flasks
themselves?
I know absolutely nothing about casting metal, so I was hoping someone could give me tips on that.. or at least a few reasons why I am going to get
hurt trying to, lol 
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jackchem2001
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In my opinion, oil, water, molten metal, or sand baths (or other solutions like metal heating blocks) are not needed. A simple air bath with aluminium
foil works very well for most applications. Advantages of this approach:
Very low thermal capacitance means a fast heating response. If something is running too hot then you can rip off the foil to
rapidly cool the flask. With something like a sand bath, you would need to physically raise the flask to have a rapid cooling response which might not
be feasible with large scale reactions.
Moreover, the low thermal capacitance means you can fine tune the amount of heat supplied. If you are running a fractional distillation and
notice the column temperature rising (use an IR thermometer for this), you can cut the heat and see a rapid response before the component reaches the
distillation head and becomes a problem.
Non-toxic, less flammability hazard than with oil
No mess or odour as with oil
Easy to setup the apparatus because it is resting on the ground and not suspended in the air
Not a very flattering picture but you get the idea 
There is not very much thermal loss provided you put some foil over the top of the flask (the outer walls of the aluminium become only slightly warm).
The heating is very even too - I have not had problems with bumping. Note the small foil ring in the picture - the RBF rests on this and avoids direct
contact with the hotplate.
There are some disadvantages though:
Thermal shock - if the flask is too big to fit in the air bath and is pressed up against the foil, then the flask is likely to
crack once cooled because this area will cool much faster. Avoid direct foil on glass contact.
Aluminium foil is reactive with halogens, strong acids/bases
Obviously there are times where a foil air bath is not such a good idea. If you need to maintain a precise temperature then the additional thermal
capacitance from an oil/water bath can be helpful. Also, if the flask runs dry with an air bath then it will rapidly heat. This can cause cracking if
say, a drop of liquid falls down from a distillation head. This is much less likely to happen when using a water bath.
[Edited on 4-5-2025 by jackchem2001]
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Sulaiman
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I've used an aluminium foil hot air bath a few times,
most useful when I distilled sulphuric acid at atmospheric pressure using gas flames,
generally useful as a wind shield to reduce heat loss/fluctuations due to wind/breeze etc. - whatever heat source used.
I would be careful using an electric hotplate as a heat source for higher temperatures (>100oC) because
the hotplate will need to run very hot to transfer heat at a reasonable rate.
I burned a hole in a table like this 
_____________________________________
If making a metal heating block, be aware that RBFs of similar nominal capacity may be of significantly different diameters.
______________________________________
My own preferences are
electric heating mantle
foil air bath / windshield
oil bath
hotplate
spirit lamp
candle flame
CAUTION : Hobby Chemist, not Professional or even Amateur
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Maui3
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Thank you both!
I will take this into consideration
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chempyre235
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If you have a 3D printer, you could print a full-scale model out of PLA, then make a plaster cast with the plastic model. Once the plaster is set,
then you can get rid of the plastic by heating upside-down in an oven. This will leave only the plaster cast. You can then use the plaster cast as a
mold to cast metal.
I've been looking at this method to build a durable heating mantle, using inductive heating as the heating method of choice.
[Edited on 5/5/2025 by chempyre235]
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davidfetter
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DIY heating bath
I'm given to understand that one can make a pretty good heating bath with PEG 3350, which is the main ingredient in "osmotic laxatives," as the label
indicates. m.p. is 64-66, b.p. is over 250. If you need higher temps than that, you might be using flame directly.
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R-industrialist
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Quote: Originally posted by chempyre235  | If you have a 3D printer, you could print a full-scale model out of PLA, then make a plaster cast with the plastic model. Once the plaster is set,
then you can get rid of the plastic by heating upside-down in an oven. This will leave only the plaster cast. You can then use the plaster cast as a
mold to cast metal.
[Edited on 5/5/2025 by chempyre235] |
- Make sure you account for shrinkage, It will be dependent on the alloy used, and you'll need to scale the model accordingly.
- You should also burn out the plastic/wax as their residues can leave dissolved hydrogen bubbles in aluminum and copper alloys
[My solution]
Although a bit odd, I personally use a dry soapstone (~talcum) powder to heat my vessels in a large metal pot. It won't scratch your glassware until
you've heated it above 600-700C but I'd think the glass would break before that.
[Edited on 7-5-2025 by R-industrialist]
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MrDoctor
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if you add common hardware store high temp refractory mortar to some rather coarse sand, youll get something that binds the sand rather well and more
or less behaves the same as an uncovered sand bath, minus especially good contact since youll need some clearance to account for the discrepancies
between flasks. some chicken wire and a sheet of either alu foil or fiberglass would seal the deal to ensure it doesnt crumble, it will be very
crumbly.
Im told its possible to acquire large aluminum beads with which one can produce something similar to a sand bath with that has better distributed
heating despite.
overall id recommend going to a machining workshop and asking them to make one for you using their cheapest aluminum block stock.
If you have the means to cast metal. casting a bowl to put inside a large mantle sleeve oversized for your flask may be desirable as an option as
well. mantle sleeves (the element) are available on ali for $15-35, and they themselves can just be put in a teracotta pot and call it a day, mine
have lasted years like that, i use the 2L mantle sleeve like its an oven sometimes.
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Radiums Lab
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Guys I am plannig to make Bromine but Al block is a big no. Should I really use a block for 1l RB?
[Edited on 9-5-2025 by Radiums Lab]
Water is dangerous if you don't know how to handle it, elemental fluorine (F₂) on the other hand is pretty tame if you know what you are doing.
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Mateo_swe
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Or you can use a DIY heating mantle that you make from replacement heating mantle sleeves found online.
You need to power theese with a variac or some other way, there are power modules you can buy.
Be careful with outlet power.
The sleeves look like this and are basicly the insulated heating wire.
Then put the sleeve in a flowerpot with vermiculite or other heat safe material and you have a heating mantle.
You can build it as good looking as a real heating mantle if you are skilled.
If your skills permit you can build temp control and maybe even stirring.
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Belowzero
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| Quote: Originally posted by jackchem2001 | In my opinion, oil, water, molten metal, or sand baths (or other solutions like metal heating blocks) are not needed. A simple air bath with aluminium
foil works very well for most applications. Advantages of this approach:
Very low thermal capacitance means a fast heating response. If something is running too hot then you can rip off the foil to
rapidly cool the flask. With something like a sand bath, you would need to physically raise the flask to have a rapid cooling response which might not
be feasible with large scale reactions.
Moreover, the low thermal capacitance means you can fine tune the amount of heat supplied. If you are running a fractional distillation and
notice the column temperature rising (use an IR thermometer for this), you can cut the heat and see a rapid response before the component reaches the
distillation head and becomes a problem.
Non-toxic, less flammability hazard than with oil
No mess or odour as with oil
Easy to setup the apparatus because it is resting on the ground and not suspended in the air
Not a very flattering picture but you get the idea
There is not very much thermal loss provided you put some foil over the top of the flask (the outer walls of the aluminium become only slightly warm).
The heating is very even too - I have not had problems with bumping. Note the small foil ring in the picture - the RBF rests on this and avoids direct
contact with the hotplate.
There are some disadvantages though:
Thermal shock - if the flask is too big to fit in the air bath and is pressed up against the foil, then the flask is likely to
crack once cooled because this area will cool much faster. Avoid direct foil on glass contact.
Aluminium foil is reactive with halogens, strong acids/bases
Obviously there are times where a foil air bath is not such a good idea. If you need to maintain a precise temperature then the additional thermal
capacitance from an oil/water bath can be helpful. Also, if the flask runs dry with an air bath then it will rapidly heat. This can cause cracking if
say, a drop of liquid falls down from a distillation head. This is much less likely to happen when using a water bath.
[Edited on 4-5-2025 by jackchem2001] |
I've done mostly the same thing and for relatively small batches and low temperature this works fine.
Most of my hotplates are at some point not able to maintain 250+ degrees C with such a setup.
The boiling takes away so much heat that things can seemingly take forever and the stability of the distillation is quite poor.
Once over a certain threshold the temp can rapidly increase and cause problems.
I think the primary role of a block, sand bath or alike is to buffer the temperature, this is a very useful feat. imo.
Maybe I am overstating the obvious.
Perhaps as Mateo_swe points out it is quite possible to use use special furnace cement and cast(casting is probably the wrong word here) a spare
heating coil into it.
Depending on where you are located, I think chemland has relatively cheap spare heating coils and I don't really see why this can't be done.
One would have a solid mass with a high degree of temperature control all on a tight budget.
The chemical resistance would be pretty good too.
Presumably this would not conduct electricty.
Thermo shocking could be an issue, I have no idea how well such a material would be able to handle this.
Perhaps such a thing can be reinforced with high temp resistant fiber or even rockwool.
I am just throwing some thoughts out here but this might well be worth a shot.
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MrDoctor
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| Quote: Originally posted by Belowzero |
Depending on where you are located, I think chemland has relatively cheap spare heating coils and I don't really see why this can't be done.
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i made a narrow vertical retort/quartz test tube heater, just using old discarded space heater nichrome coils. you know the kind, with the foggy
quartz/ceramic tubes? its just a regular nichrome coil in that, and conveniently, all are 500W, the power rating you want for your heating element,
people toss them out though at the drop of a hat when one cracks or blows.
i cased a coil in a wax spiral, cast that, then melted the was out, it failed terribly because i forgot how gravity works and the tension of the coil
changes with heat cycling, but it was just a goof, cast into a paint tin. In hindsight, what was always an option was just breaking the quartz tube
into segments and casting that. doesnt even matter if it crumbles apart as long as it happens after the refractory sets. the quartz can be a resistive
heating cylinder, or it can radiate IR, its fine either way, if it cracks or air gets in, it wont really burn out unless you are running it red hot or
more.
[Edited on 27-5-2025 by MrDoctor]
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carhole chemistry
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I've always found flat bottom flasks a lot easier to use. but if the heating needs to be more even then use a foil air bath like everyone else
suggested
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jackchem2001
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| Quote: Originally posted by Belowzero |
I've done mostly the same thing and for relatively small batches and low temperature this works fine.
Most of my hotplates are at some point not able to maintain 250+ degrees C with such a setup.
The boiling takes away so much heat that things can seemingly take forever and the stability of the distillation is quite poor.
Once over a certain threshold the temp can rapidly increase and cause problems.
I think the primary role of a block, sand bath or alike is to buffer the temperature, this is a very useful feat. imo. |
Yes, the lack of thermal capacitance is definitely a two way street and can be disadvantageous. I should have mentioned in my post that the highest
temperature chemistry I have done with that setup is ~200 degrees C (phenol prep from salicylic acid and distillation), which went very smoothly but I
have not tested higher temperature chemistry.
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MrDoctor
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i just had an idea, instead of taxing your hot plate, why not air-bath using a standalone stovetop coil? you could even sit it on your hotplate if you
need stirring.
it would be simple to just buy a replacement coil but i know ive seen them come as portable units with just a basic little stainless frame to keep it
off the ground and a little thermostat box, otherwise its pretty much just a coil on the end of a cable.
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