OT:Heating pyrex on electric elements

Hey all, ive got an off topic question for everyone. I currently heat a large pyrex carboy on my electric range. I have always been very careful about heating it very very slowly. So slow, it drives me nuts. Does anyone know of any tricks for heating pyrex this way? Ive heard that I could possibly use a copper plate as an insulator. Any advise? Thanks

matt

What temperature are you heating to and how slow do you want to heat it?

Is the temperature of the elements that's the problem causing the glass to crack?

How about a large saucepan filled with water and placing the carboy (demijohn I presume) inside the pan. The water temperature will never exceed 100 degrees C.

What about the metal to glass contact on the bottom of the carboy? Isnt this more or less the same as a copper plate with a water bath? Yes, what im worried about is the glass cracking. I know you can fire pyrex flasks on burners pretty hard, but have heard over and over that putting directly onto elements is a recipe for distaster. I havent thought about setting the carboy inside of a pan (which is copper plated on the bottom too btw). Am I just paranoid, or if I crank the heat, will the glass break?

I dont think that it would be a problem as the water would quickly dissipate the heat from the base of the pan but you could sit the carboy on a ceramic plate if you're worried.

You could then crank the heat high enough to keep the water at a boil (any higher and you're wasting energy), just remember not to let the water boil dry - keep topping up from a kettle.

im heating to less than boiling too btw. So, that sounds like a good idea. The copper plate on the pan and the water should pull away the heat and insulate the pyrex. Now I just need enough courage to try it on my oh so precious pyrex carboy.

When you first mentioned carboy I was thinking wine or similar, usually then you want the heat applied slowly.

As mentioned you could place it in water using a strainer or stand to keep it above the bottom. As mentioned using water you only get to boiling point.

If you know the brand you usually can get the property data for it, ex: http://www.corning.com/Lifesciences/technical_information/techDocs/thermalprop.asp

What is inside the Pyrex you are heating? What are you doing just heating distillation etc etc? What is the desired temp of stuff you are heating?

Pyrex is used in lab glassware often heated with bunsen burner or electric heating element. Since you have already heated and thus normalized it you can put very large amounts of heat into it with high temperature and have a high rate of temperature change of pyrex and contents.

DO NOT START EMPTY and add to hot container.
DO NOT LET BOIL DRY

You can add some cold to hot contents and be OK though.

I would start off at a low temp and work my way up to about a middle setting on range. Dont think I would run range heater red hot though.

Dan Bentler

I agree with Ron on this one. If you are really worried, stand-offs of some kind in water would be the way to go. They would be much more likely to dissipate heat in their centers than a solid sheet of anything. If your copper plate has a rim around the bottom that would do the trick too.

What you have for 'Pyrex' makes a big difference too. If this is the old-fashioned trade-name 'Pyrex' then it is borosilicate glass. That is very thermally tough. It's pretty hard to heat stress. If this is just general glass cookware or servingware you do need to be careful.

Keith

its the very slow heat up time (I take about 1 hour to raise the element to 50% output), and then it takes another hour or so to bring the liquid to boil (at about 168F or so). What I want to do is put the heating element to 100 fairly quickly so I can bring the contents to boil faster. But, im afraid to break the pyrex, which ive heard can break if placed directly on electric elements. Is this not true? Can I get more aggressive in brining the CV up?

It is borosilicate too btw. So am I being paranoid?

I'd hate to be the guy who breaks your vessel, but I've had real good luck with the real-life Pyrex. Both hot to cold and cold to hot.


I believe the warnings come from the newer Pyrex. Apparently the Pyrex you buy today is no longer borosilicate glass. So it tends to break with temperature change pretty easily. I haven't used anything made of Pyrex for quite some time so I don't know if this is true or not. But I do know the old stuff was pretty good.

Keith