Can You Microwave Cardboard?

It’s very convenient to just simply microwave leftover takeouts in their cardboard boxes, whether it’s pizza, Chinese, fried chicken, your favorite burger, or even Indian curry from the restaurant down the street. 

The same applies to beverages, too. Think about it. Wouldn’t be easier to warm up your cold cup of coffee or tea in the microwave? 

And while you normally hear people telling you not to put aluminum foil, styrofoam boxes or cups, and plastic bags in the microwave, they seem to be divided when it comes to cardboard.

So, can you or can you not microwave cardboard?

Read on as I answer everything related to microwaving cardboard.

Can you microwave cardboard?

Yes, you can microwave cardboard that contains food or beverage, provided that you don’t leave it unattended. However, you shouldn’t microwave cardboard that contains glue, plastic, ink, metal, or wax. You should also not microwave a cardboard box that doesn’t contain any drink or food to avoid overheating.

I know what you’re thinking – cardboard and paper are used to easily and quickly start fires. So how can you microwave it?

Well, it is because of the way microwaves work. When any food or drink is heated in the microwave, the molecules of water inside them move at a high frequency and heat up. But since cardboard doesn’t contain a lot of water, it doesn’t heat up as quickly as the food and water do. 

Even if some water molecules from the food or drink stick to the cardboard while it is in the microwave, the temperature won’t be so high as to cause a fire.  

Of course, this doesn’t mean that you can microwave just about any kind of cardboard. Some can catch fire, especially those not labeled microwave-safe, so you need to be careful.

Is it safe to microwave cardboard?  

It is both safe and unsafe to microwave cardboard, depending on the kind of cardboard in question. You can nuke pure cardboard without damaging the microwave or the container, or affecting the food. But it is not safe to microwave cardboard that contains metal, ink, plastic, wax, or glue.

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Some cardboard containers can contain these things as a coating either between the different layers or outside the box. This is why you should always read the label to know if it’s safe to microwave. 

It’s also important here to remember not to overheat cardboard. It usually contains chemicals that can be very harmful when subjected to the high heat of the microwave. These chemicals can even leach into your food and drink, creating a health hazard.

If your cardboard container is safe to microwave, there will be a tag or label somewhere. This means that the manufacturer has tested it. Similarly, if it is not safe, then there might be some kind of disclaimer on the box. Just make sure you read the box thoroughly to be absolutely sure of microwaving it.   

When should you not microwave cardboard?

There are some instances where you shouldn’t microwave cardboard, such as when it doesn’t have a microwave-safe label. Similarly, you shouldn’t microwave cardboard with metal (think Chinese takeout containers) or ink that is not safe to heat.

As mentioned earlier, you should always check if the container has a label certifying that it’s safe to microwave. If you don’t see any label, it’s probably best not to microwave it.

Also, make sure that you only microwave cardboard for a short amount of time – and never microwave an empty container. There should be at least some food in the container to absorb the microwave rays. 

And while there’s a very less chance of a microwave-safe container catching fire, you should still microwave it for short durations. 

In addition to all that, cardboard is usually made using recycled materials. This means there’s a chance that your cardboard contains some amount of materials that are not safe to microwave, like metal. And if the box itself doesn’t contain any metal, then it might have handles made of metal so you need to be careful.   

But metal isn’t the only thing you need to be wary of; there are some other materials to look out for, too. For instance, some cardboard boxes have a wax coating and that might leach harmful chemicals when heating. Similarly, some might contain glue or ink that could leach into your food. 

How long should you microwave cardboard?

There is no particular timeframe for which you should microwave cardboard. But for your safety, you shouldn’t microwave it for too long. You should only microwave it for 60 or, at most, 120 seconds. 

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In case you want to heat your food for longer, it’s better to do so in short 30 or 60-second intervals with a 5 or 10-second break in between. Make sure you never microwave it for more than 2 minutes in one go. 

In the case of food, you most likely won’t need more than 2 minutes. But that might not be the case when it comes to beverages. Usually, beverages in a cardboard container take a little longer to heat up. 

Tips to microwave container

As you’ve seen by now, you can’t microwave all cardboard boxes. So to make sure you don’t put your food – and yourself – at risk, here are some tips to remember when microwaving cardboard:

  • The most common cardboard box you’ll find yourself putting in a microwave is a pizza box. In most cases, there will be a label that mentions if it’s risky to microwave it. You can know if the box is safe to microwave with the microwave-safe symbol. This is a small icon with three waves. If you don’t see a symbol anywhere, look for texts or directions about how to heat the food.
  • As mentioned earlier, some cardboard includes traces of metal, making it unsafe for microwaving. Make sure you check that there are no traces of harmful materials like aluminum foil in the packaging.   
  • Never microwave cardboard bowls, soup containers, or cups that contain a waxy coating or plastic layers. Similarly, you shouldn’t nuke cardboard with any kind of styrofoam insulation or printed ink. Things like these can either melt or release potentially dangerous fumes into the food, which can be toxic for you. 
  • Before microwaving cardboard, check your microwave settings. Set it at the lowest possible power setting to ensure your safety. 
  • Avoid using cardboard for heating meals that contain a high water content. This is because heating such food generates heat that dissipates quite slowly, especially if the box is closed. This creates pressure and as that pressure tries to escape from the closed box, it can result in an explosion.
  • Be careful when heating liquids. This is because heated liquids can ruin the cardboard’s structural integrity and make it soggy. As a result, it can break down. 
  • Be careful when handling the box you just microwaved. Apart from the food or beverage, the cardboard container itself can get super-hot sometimes, so be cautious. 
  • Never nuke food in cardboard for more than 90 seconds. For drinks, you can stretch that limit to up to 120 seconds, but not more than that. 
  • Make sure you remove any metal, paper, or plastic pieces from the cardboard before microwaving it. For instance, make sure you get rid of the plastic lid that’s on top of disposable tea and coffee cups and remove the plastic pizza box holder (the table-like thing put in the center of pizza boxes). Similarly, make sure you take out any metal clips used to fasten the container’s lid.

The Bottom Line

You now know whether you can microwave cardboard or not. The key is to always look at the label to know what you’re dealing with, and never microwave for more than 2 minutes. Remember, cardboard is only safe to microwave as long as it says it is, so make sure you’re vigilant. Happy microwaving!

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