I tried to explode a cup in the microwave

[davidh4976]

I tried to explode a cup in the microwave. The test was to see if a cup with water absorbed into the body would explode when microwaved. My cup got really hot, but did not explode.

I started with two nearly identical cups made from a white stoneware with an estimated absorption of 3.7% as previously measured with test bars. Each cup has a foot ring and was glazed inside and out with a clear glaze. The foot ring and the bottom area inside the foot ring was not glazed.

One cup was set aside and was not subjected to any water.  It weighed 433.9g.

For the other cup, I kept it upside down on my desk and kept filling the area inside of the foot ring with water for a few weeks. My thought is this would simulate water absorption from being subject to multiple rounds in a dishwasher. Periodically, I would check its weight. After a couple of weeks, the water absorption stabilized and the weight of the cup went from 426.0 grams to 441.8 grams, a gain of 15.8g or about 3.7% (which matches my test bar estimate). This means it absorbed about a tablespoon of water. It did not gain any additional weight/water with more time beyond a couple of weeks.

Then, I microwaved both cups (both empty) at the same time in a 1.4kw microwave with rotating platform for 15 minutes. I very quickly measured their temperature after each minute of microwaving. The cup with absorbed water reached a maximum temperature of 433F. The cup without absorbed water maxed at 250F. Neither exploded. The cup with absorbed water crazed, but no other damage.  The cup with no absorbed water did not craze and was not damaged. The graph shows the temperature in degrees Fahrenheit over time in minutes.

After microwaving letting the cups cool, I remeasured their weight. The cup with water applied went down to 425.4g indicating that all of the water was boiled out. The cup without applied water was 425.3. After microwaving, both cups weighed a less than their original weights.

So, I was not able to explode my cup. This does not mean that your cup won't explode. This was for one particular clay and may not be representative of other clay bodies.  If others have time, it would be interesting to repeat the test with other clay bodies.

Notes: Microwaving empty pieces is not recommended because it might damage the microwave oven.  I use an old microwave that I don’t care if it breaks. I used an infrared Infrared Thermometer Temperature Gun to be able to very quickly measure the temps. If you decide to repeat this experiment, note that I got 30F difference between the top and the bottom of the cup and averaged the reading.

Edited January 9 by davidh4976

[Kelly in AK]

This prompts me to share two microwave experiences in which pots cracked. Very different clays and experiences.

The first was an earthenware bean pot I made. It had regular use on the stovetop. Always soaked in water before use, heated slowly over gas, and made the best beans. I was fastidious about cleaning it. Occasionally it grew mold. I would scrub with baking soda, fill it with water and boil for a couple hours, then rinse and dry, which kept it sanitary for a while. Once after this process I had the bright idea to put it in the microwave to dry it better, mold needs humidity. After a couple or three minutes I heard a clearly audible crack and discontinued that experiment. The pot made three or four rounds of beans after that before I decided it was ready to fall apart and tossed it.

The next was trying to gauge how the microwave affected vitreous ware, mostly interested in iron content relative to heating. I tested a cup, iron rich clay, interior glazed only, for absorption. Heated in the (regular) oven at 185° to make sure the dry measurement was accurate. Two hour boil, then sat in the pot of water another four hours. Came out at less than 1%. Then I put it in the microwave (empty) to see how hot it got after two minutes. Before the time was up I heard an audible crack and stopped it. The cup was too hot to touch. It has a crack you can find if you really search for it in good light. I still use it.

These are the only two times I’ve had a microwave wreck a pot. Both were empty and dry on the exterior but were also saturated, though one had below 1% absorption. No explosions. 

[neilestrick]

On 1/9/2024 at 7:24 PM, Kelly in AK said:

Then I put it in the microwave (empty) to see how hot it got after two minutes. Before the time was up I heard an audible crack and stopped it.

I don't know the science here, but my gut tells me it cracked because it was empty. Could simply be that any liquid in it would slow down the heating and make it heat more evenly.