Sciencemadness Discussion Board

Heating glassware, building heat gun powered source

thewanderer - 27-3-2013 at 07:30

I've been reading a lot about not directly heating glassware with open flames, etc. but I have a few questions...

Is it a problem to sit an Erlenmeyer flask directly upon a hotplate/stirrer? I've heated a 1L Erlenmeyer used for creating a yeast starter for homebrewing beer, directly on my glass top stove (as many other homebrewers do, some even directly upon the metal electric elements or in direct heat from a gas range burner) in the past and I've never had issues, but I don't know if this is good practice.

As far as heating round bottom flasks... I've read some threads mentioning use of heat guns as a heat source, but from what I've seen this is more of a manual process. I can get a cheap heat gun locally that has low (572°F) and high (1112°F) settings for under $10. I was thinking I could build a "heat gun powered mantle" of sorts by using a mesh strainer or metal colander and heating it from below with the heat gun on low (and/or cycling it on and off) I suppose I could even line the inside of the strainer/colander with fiberglass. Does this sound like a feasible idea or should I just stick with oil baths, etc. until I get a lab hotplate?

At the moment, I only have an inexpensive "kitchen" hot plate, that I've used with an oil bath to heat a RBF. I'm not sure an air bath would be a great idea with this hot plate as I doubt it has much in the way of temperature regulation and I could see it overheating and burning out.

Hockeydemon - 27-3-2013 at 07:40

What the glassware can take is much more dependent on the type & quality of the glass, not really the name given to the shape of it. What brand glassware do you have? I place my Kimax, & Pyrex stuff on heat sources all the time, but I would not heat my Chinese Erlenmeyer flask much above room temperature..

What heat sources you can get away with is also dependent on what you are doing. You can heat up a glass of water with a blowtorch if you want :)

hissingnoise - 27-3-2013 at 08:14

Quote:
. . . some even directly upon the metal electric elements . . .

Not a good idea!
Those older heating rings can get to red heat fairly quickly.
RBFs really shouldn't directly contact hot flat surfaces . . .

Dr.Bob - 27-3-2013 at 09:16

There are a few factors in heating glassware. First, if it is real borosilicate, if will do better then crap glass. But the quality of the fabrication (lack of internal stress in the glass from properly making and annealing the glass is key to that) including the variation in thickness and bubbles, etc in the glass. But the overall thickness is also important. Most flasks come in thin, medium and thick walled varieties. Some Chinese and US glassware is quite thin, which will hold up much BETTER to direct heating than thick glassware, which is great for surviving rough handling like being washed and put loose in a drawer full of other glassware, BUT undergoes more stress when heating, as glass is a great insulator, so heat does not move fast through glass.

Thus if you heat it quickly and unevenly, some spots will heat faster and slowed, which will cause uneven expansion and stress on the glass. The thicker the glass, the slower the heat moves from the outside to the inside and the bigger the difference in temperature from one place to another within the glass. Oil and water baths are good, both because they are not as hot as a flame or red hot element, plus, they have complete contact with the glass, stay more even in temperature, and transfer heat better than a metal to glass interface. So if you have a heavy walled flask or beaker, you should NOT heat it with a direct flame or on a cooktop coil burner.

Most hotplates are not so bad, as the porcelain helps to spread the heat evenly and does not get as hot as the metal heating element. This does not mean it will break, only that it might break, as there are a number of variables involved. But the last thing I would want is a flask full of ether or some other solvent cracking on a hotplate. I've seen it happen and it makes a really big fire quickly. (PS, those ABC fire extinguishers make a real mess of yellow powder everywhere when used. And no, I was not the person who started it, but I did put it out.)

Endimion17 - 27-3-2013 at 09:31

Quote: Originally posted by thewanderer  
Is it a problem to sit an Erlenmeyer flask directly upon a hotplate/stirrer?

Depends on the temperature difference and the size of the flask. Large flasks are more prone to breaking from thermal induced stress.
If a metallic surface is red hot, it should not touch the glass. I'd put some asbestos or artificial mineral wool below, just make sure it won't melt (glass wool would melt if pressed against red hot steel).
If it's mildly hot, you're ok, but always remember what can happen if there's something viciously flammable inside, like gasoline or ether.

Heat gun is a nice option, but make sure the hot air flows around the bottom part, only. If distilling something, heated air rushing by the thermometer will cause large errors.

When heating round bottom flasks on a hotplate, it's always better to cut a small cushion of earlier mentioned materials on which you can place the bottom. Glass stretches when heated, and it's good to ease those movements in order to reduce the stress. After all, when all is clamped, there aren't a lot of places where glassware can stretch, and the apparatus often partially rests on the first flask.
Don't forget to make an aluminium foil tent over the flask's bulbous part. It will trap heated air and IR rays, reducing the need for higher temperature settings and errors in temperature reading.

If you're distilling a nonflammable liquid, use a tripod or a ring stand with asbestos or ceramic (again, cushion the flask) plate and heat the plate with flame.
Don't be afraid of burners. I see that today everyone's shy of using them like they're some devilish contraption. Hotplates and mantles are for flammable stuff.
Quote: Originally posted by Hockeydemon  
What heat sources you can get away with is also dependent on what you are doing. You can heat up a glass of water with a blowtorch if you want :)


Yeah, he can (I can jump into a lava lake, too), but what would happen if he did?

Hockeydemon - 27-3-2013 at 10:22

Quote: Originally posted by Endimion17  


If you're distilling a nonflammable liquid, use a tripod or a ring stand with asbestos or ceramic (again, cushion the flask) plate and heat the plate with flame.
Don't be afraid of burners. I see that today everyone's shy of using them like they're some devilish contraption. Hotplates and mantles are for flammable stuff.



This was more my point rather than what he was physically capable of doing.. albeit a poor attempt on my part.

[Edited on 27-3-2013 by Hockeydemon]

thewanderer - 27-3-2013 at 10:22

I've never heated any glass directly on a metal element, although I've seen others do this. A lot of brewers will even take the flask straight from boiling into an ice bath, which to me seems like you are just asking for trouble. I have heated my 1L Erlenmeyer on my glass top stove, which obviously has a glass and air layer above the heating element. The only way I've ever heated RBFs is in an oil bath. The reason I was asking about heating an Erlenmeyer directly (on a hot plate, not a coil) is because it has a larger surface area that's flat.

My 1L erlenmeyer is from "United", made in India. It's rather thin glass. My other smaller erlenmeyers are German Pyrex and thicker (I've not tried heating these on my glass top)

I am planning on picking up a lab hot plate w/ stirrer in the near future, so I can heat RBFs in a foil-covered air bath by clamping it suspended just over the plate with a foil skirt. I was hoping I could also use it to heat erlenmeyers for yeast "starters" as they benefit from being continuously stirred, even after heating. Simply being able to place an erlenmeyer on top of the hot plate and turn it on would be easier than clamping it suspended above, making a foil skirt, etc.

Dr.Bob - 27-3-2013 at 12:53

I heat erlenmeyers and beakers on a hotplate often enough. As long as the heat is on 1-5 range, it should be fine. With water, I am not nearly as worried, both as it is not flammable, and it conducts heat well, so the flask heats more evenly. I would really like some fused quartz beakers or Vycor, with those you CAN take a flask from ice to red hot with little worry. I remember seeing demos of that years back. I do still have a few small quartz beakers, but don't want to risk them.