Hot Dishes, Cold Food, Microwave Perplexity

Dear Home Ec 101,

Why is it sometimes when I heat foods in the microwave the bowl is screaming hot and the food is still a block of ice?


Scalded in Scranton*

Heather says:

Here on Home Ec 101 I’ve written previously about how microwaves work, if you missed it the first time, here’s the short version.

Think of your microwave as a box built to hold in waves of energy. These microwaves bounce all over the inside of this box when the appliance is running. When items are placed in the microwave three things can happen: the waves can either pass through object, bounce off the object, or be absorbed by the object. When conditions in the microwave oven are perfect the dish is pretty much ignored by the microwaves which  reach food and excite the water molecules which heats the food.

What do we know about real life and optimal conditions? I know, I know! The two rarely occur at the same time outside of a laboratory. You don’t live in a lab, do you?

Of course you don’t, your bowl is hot and food is cold. Why?

In your case the microwaves are being absorbed by the dish which is pretty much shielding your food from the microwaves. If you used a plate, the shielding effect would be less noticeable, as less of the surface area of the food would be blocked by microwave absorbing material.

So, what kind of dishware can be used in the microwave?

Skip metal which may cause arcing or the buildup of charge that jumps from one point to another. It’s like lightning on a domestic scale.

Glassware should be safe, but it isn’t always. The best test is to check the manufacturer’s suggested use.

Engineers and other sticklers, I know I’m using some terms a little loosely, we’re just working on basic concepts today.

It’s important to remember that not all ceramic is created equally. Dry, unglazed ceramic is usually microwave safe, but there is a caveat. The material is porous and can absorb water. When this happens the microwaves heat the water in the ceramic dish. If the water gets hot enough it can cause uneven thermal expansion (some things get bigger when they are heated) which can break your dish.

When it comes to ceramic glazes a few other factors come into play. Some ceramic glazes may may contain dangerous chemicals like lead that can leach into food. These containers should not be labelled food safe. Caveat emptor is the rule of the day. Most retail establishments would not knowingly sell consumers items not intended for food use without a clear label. However it has happened in the past. Use extra caution with decorative bowls, that label not for food use is VERY important.

With food safe glazes, no dangerous chemicals should leach into your food. That doesn’t mean your dish should be used in the microwave. If the bowl gets hot, before the food, the microwaves are exciting molecules in the glaze. While this may not be dangerous from a chemical standpoint, burns still hurt. Throw in the the fact that you’re not getting the most efficient use of  your microwave and I’d quit using that container to reheat food.

Plastics are a whole ‘nother ball of wax. Personally, I sidestep the issue and stick to microwave safe glass and ceramic. Your mileage may vary, but please ensure the plastic used is rated microwave safe. Once the plastic-ware shows signs of wear, it should be recycled.

Got it?

*Today’s question actually came from a tweet by @TJeffrey: Don’t you hate it when you open the microwave and pull out a bowl that’s 1,000 degrees, but the food inside is ice cold? I just couldn’t think of a good synonym for hot or burnt that went with a local town. It happens.

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  1. Joan Carrel on June 4, 2016 at 7:15 am

    Recently, my microwave is heating the dishes before heating the contents. I have used these same dishes before and the contents were heated like they were supposed to. What is happening to my microwave? An example is my coffee cup with cold coffee is heated as usual for 115 seconds. Before that is all I needed to do. Now the cup burns my hand. I have to remove it with a pot holder. Please help me with this issue.
    Thank you,

    • Sallie Campbell on January 22, 2018 at 9:43 am

      I’m having the same issue with dishes that I have using in the microwave for years. Can constant use be affecting the dishes? I can see no visual change in the dishes, glaze, etc. to account for this.

  2. aho on December 17, 2013 at 12:20 pm

    I would like to say that the problem in most cases is not with a safe glazem!
    I had a feeling and would like some real pro will look into it- we know that some ceramic dishes that imported are made from ceramic containing lead to lower its curing temperature. – with dishes that heats up fast before the food, is it a possible sign that the dish is also containes heavy toxic metals??

  3. sharon meek on April 9, 2013 at 11:13 am

    I have two homes with two different microwaves. In one microwave my ceramic dishes dont’ get hot. In the other microwave the same dishes get burning hot. What is the difference in the microwaves?

  4. Laura on March 3, 2011 at 12:52 pm

    At work there are some communal dishes that are glazed ceramic and NOT microwave safe, which we all learned the hard way. Who knew that the microwave was reacting to the molecules in the glaze? Thanks for the explanation & the tips!
    My recent post Psssst! Nameisms looks a little different

  5. Jackie on March 3, 2011 at 12:47 am

    I have two very pretty LARGE Frankoma mugs. I can't heat much of anything in them. The cups get so hot they will fry your fingers, but the contents are barely warm. BEWARE!

    • HeatherSolos on March 3, 2011 at 8:05 am

      We have a pair of mugs like that, too. They are big and perfect for a big cup of coffee, but it's pointless to put them in the microwave.
      Mugs with certain glazes are especially difficult because the microwaves only have that opening at the top as their access point to what their trying to heat.

    • Bobcat on May 4, 2011 at 3:19 pm

      No more s***. All posts of this qulitay from now on

  6. Daisy on March 2, 2011 at 7:23 pm

    Even when plastic is supposed to be microwave safe and looks pretty sturdy, I hate microwaving it. Sooner or later it always gets this sort of microwave disease ring of roughed-up plastic on the inside. I've been mostly using pyrex bowls in the microwave for the past year or so since I got tired of replacing plastic and being vaguely skeeved out by the thought of what might be leaching into my food once the plastic starts to get damaged.
    My recent post Yay patchwork!

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