Are Thermoses Microwave Safe?

There is some pervading controversy as to whether you can put a thermos safely in a microwave or not. Theoretically, because many thermos flasks are made from steel, you should not put them in a microwave, but that is not the end of the story.

Thermoses are microwave safe even if made from steel because the design of the thermos is smooth and round. This reduces and prevents the sparks and arcing effects that sharp-edged materials cause in a microwave and can lead to microwave damage or even fire or explosion.

Let’s examine the debate in more detail so you can understand why thermoses are microwave safe, how to use them safely, how some thermos materials are safer than others, and which is better to use in a microwave.

Why Are Thermoses Microwave Safe

The first thing about these appliances you learned when you got your very first microwave whether by panic attack when you saw sparks or by reading the instructions, was that you DON’T PUT STEEL OR METAL OBJECTS IN A MICROWAVE!

So, if that’s the case, why can you put a steel Thermos in a microwave and not have it explode or spark? The reason you can do this lies in the design and shape of the thermos. For example, if you consider a crumpled piece of aluminum or a fork, each of those has sharp edges, and this is where the difference lies.

Sharp-edged steel instruments cause the flow of charged particles in the microwave will flow and congregate in the corners and on the edges, and this will cause the sparking and arcing that you often see when these items are microwaved.

The design and shape of the thermos are round and flowing with no corners or edges, preventing the abovementioned charged particles from gathering and causing sparks and arcs.

Thermoses Are Microwave Safe Because Of Their Thickness

Another reason is the thickness of the steel used in a thermos. Other steel dishes or implements are thinner, which means less charged particle absorption and more reflection. This will cause the sparks to fly!

But the thermos construction is different due to the insulation requirements and the vacuum seal that prevents air from moving through the layers to keep the temperature of the contents constant.

The thicker steel of the thermos means more microwave radiation will be absorbed rather than reflected.

This is why you can see videos of thermoses in microwaves without sparking or arcing because the combination of rounded, smooth design and thickness of the steel all reduce the risk and incidence of radiation reflecting and causing damage to the magnetron or the thermos.

Do You Need To Put A Thermos In A Microwave

The real question here is not whether Thermoses are microwave safe, but rather why would you want to put a thermos in a microwave first? The fact that you can put a thermos in a microwave with little risk doesn’t mean you should.

There are instances when this is needed; for example, you have made a soup or stew to take with you on your camping trip, and you have put some in a thermos, but you need it hotter-so you want to pop it in the microwave to heat it.

It would be a better option, where possible, to heat your thermos content first to the required temperature and then add it to the thermos for transport and for keeping warm, rather than put the thermos in the microwave to heat the contents, but if you need to do it, here is how to do it safely.

How To Use A Thermos In A Microwave Safely

It would help if you consider that a thermos is a very well-insulated container and, as such, is designed not to allow external temperature through the vacuum seal. This is an air-free space between the contents and the outer layer.

This is the key to keeping the contents warm or cold, as this vacuum does not allow natural thermodynamics to occur. The contents lose their heat or cold as the temperature equalizes with the surrounding ambient temperature.

When putting the thermos in the microwave, always leave the lid off, as this will allow the microwave radiation to penetrate the contents and warm it up. You should know that it will take longer to warm up a substance in a thermos than if it was in a regular glass or plastic dish or cup due to the thermos’ structure.

Another consideration is to check if there are any damaged or sharp edges on the thermos before putting it in a microwave, as this could result in sparks that could harm both the microwave and the thermos.

Where the outer layer of the thermos is ceramic, the risk of damage in a microwave is even less, so if you need a thermos that is microwave safe, then opt for a ceramic or other non-metallic material.

Another tip is not to have the thermos full when you microwave it and to leave some space as anything that is heated will expand, and if the thermos is full and you heat it, the contents could spill out, which would be a mess to clean up!

Perhaps leave an inch or two from the top of the thermos to accommodate the expanding contents, but because the contents will heat up slower, you can also keep an eye on it while heating and stop the microwave if the contents start to spill over.

If you see sparks or arcing off the thermos, stop the microwave immediately to prevent damage, and also make sure you use a cloth or oven gloves to take the thermos out, as the steel cover could be hot enough to burn you.


Thermoses are microwave safe but should be used with caution and care nevertheless, as even though they are less likely to create sparking, it’s better to keep a close eye during the heating process.

Overall, it would be better to heat your content before putting it in the thermos, but if you absolutely and unavoidably have to, you know how to do it safely and what danger signs to look out for.


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