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

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 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 (>100^oC) 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

Thank you both!
I will take this into consideration

DIY heating bath

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]

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.

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]

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.

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.