microwaveable glass - bubbles?

Richard Dubois · Post by **Richard Dubois** » Mon Mar 29, 2004 12:56 am

I've made a cereal bowl and was wondering if there is an issue with putting the glass into the microwave (for oatmeal, if you must know

).

Might there be any problem with air trapped in bubbles? I can't imagine there's any mosture left in them.

Richard

Tim Swann · Post by **Tim Swann** » Mon Mar 29, 2004 2:01 am

If you like the bowl a lot, I would not recommend putting it in the microwave for anything.

Tim

Carla Fox · Post by **Carla Fox** » Mon Mar 29, 2004 2:08 am

Hummm, I must be living on the edge. I have plates with bubbles I microwave them, dishwasher them, bang em around. They are fine. The only problem I had was after running one under the broiler for the 10th or so time, it broke. But the microwave has never affected them.

Carla

Phil Hoppes · Post by **Phil Hoppes** » Mon Mar 29, 2004 9:50 am

Microwave ovens work simply because the frequency they operate is at a high absorption band for water. Since most, if not all of our food contains water, it will heat it up. That is not to say that if you put in a dry glass container it will not heat up, it will, it just won't heat up as efficintly as something containing water. The tricky part is when you fuse glass or you have a ceramic cup lets say and you put it into a microwave. Unless you can guarantee that the material either of these items is exactly 100% uniform throughout there will be uneven heating in spots that may contain moisture or the material in one spot (a bubble) is not the same as another spot (no bubble). I can't tell you what that difference is, one would either have to have an analysis or experiment to determine if these difference were significant enough to cause a problem or not. My guess is probably not but as Tim mentioned, if you really like what you made, I would not suggest putting it in a microwave. (Especially if you used an irid or fused a metallic coating on to the glass!!).

Remember too that even if your glass has been properly annealed when you made it, it is still glass. You can thermal shock it and break it if you are not careful.

Phil

Carla Fox · Post by **Carla Fox** » Tue Mar 30, 2004 3:00 pm

I'm still puzzled by this, since I microwave on my plates all the time and never have had problems.

Is there really any water left in the bubbles after all they have gone thru in the kiln. I would think any water is long gone before the glass ever fuses.

So is the air in the glass bubbles expanding?

Has anyone out there in Warm Glass Land ever had any troubles microwaving on their glass?

Carla

Richard Dubois · Post by **Richard Dubois** » Tue Mar 30, 2004 3:06 pm

A bit of closure from me. As suggested, I did a test on a piece I didn't care about and it survived the microwave just fine. As did the cereal bowl this morning.

My take on what's in the bubble is less-than-atmospheric-pressure dry air. The bubble expands due to air pressure at heat, which must be reduced as the glass is cooled. So even if the air was to be heated up by the microwave, there should be "room" for it in the bubble. But I also see no reason why the air should heat up, since the waves in microwaves are tuned to resonate water molecules.

I just hope the bowl continues to survive in the face of this logic.

Richard

Tim Swann · Post by **Tim Swann** » Wed Mar 31, 2004 12:48 am

I would be more concerned about thermal shock.

Tim

Carla Fox · Post by **Carla Fox** » Wed Mar 31, 2004 1:10 am

I filled my bubbly pasta bowl with tap water and heated it to boiling in the mw. It didn't break. I was afraid of thermo shock too. But it lived to be used another day.

Carla

Brock · Post by **Brock** » Wed Mar 31, 2004 1:13 am

CM Fox wrote:I filled my bubbly pasta bowl with tap water and heated it to boiling in the mw. It didn't break. I was afraid of thermo shock too. But it lived to be used another day.

Carla

I guess, theoretically, because thermal shock needs a hot/not hot interface to occur, and microwaves heat uniformly and evenly, thermal shock may be no consideration at all. Or not . . . Bill Nye

Tim Swann · Post by **Tim Swann** » Wed Mar 31, 2004 1:27 am

When I was younger we use to do a neat trick of boiling water in a paper cup over an open camp fire. I believe the same principle is at play when you only boil water. The glass will not heat up significantly as long as there is water in contact with the glass surface. If you place food into the bowl and the food does not have enough liquid you will get localized conduction heating of the glass surface. The heat will be highest were solid food touches the glass surface and lowest were no food is in contact. I have taken many microwaveable bowls out of the microwave that have one side so hot you can not pick it up. Oatmeal may work just fine, but carrots may not.

Tim

Steve Immerman · Post by **Steve Immerman** » Wed Mar 31, 2004 9:32 am

I gave my twin nieces small plates with their names on them 2 or 3 years ago. Last year, one twin put the other twin's plate in the microwave with some pancakes on it - and it exploded. I assumed it was thermal shock from uneven heating.

Needless to say there was much finger pointing and recriminations between the nieces (they were seven) so Uncle Steve had to make Brie a new one.

So, I think this is more or less a random thing, and if it is a piece you really care about, microwaving probably isn't safe.

Steve

Phil Hoppes · Post by **Phil Hoppes** » Wed Mar 31, 2004 11:35 am

Most true microwaveable glass is made from quartz not glass. (Pyrex) This stuff is almost unshockable. You can take it out hot and put it in cold water and it will not crack. As mentioned above, I believe is a root cause was searched for it would be found to be uneven localized heating which is causing an extreme temperature differential across a very small area. Do this and "BOING" normal glass is going to break.

Phil

watershed · Post by **watershed** » Thu Apr 01, 2004 6:25 pm

I think that I would not Sell glass as microwaveable, but I Would use it in my own kitchen. Big difference.

I think the carrots vs oatmeal scenario is the best I've heard. I have old (vintage 1975) plates, that really don't like the microwave. Basically they develop cracks due to intense heating under the food (usually brats).

Greg