Can You Microwave Cardboard?

Cardboard is a popular choice for many restaurants because it reduces plastic usage and is a practical, sturdy, and durable way to transfer foods. Heating food in the microwave is a quick and easy way to reheat a meal, and it’s even easier if you don’t have to take the food out of its original container. But before you put your leftovers in the microwave, check to make sure it’s safe to do so if they’re in a cardboard container. 

Cardboard is flammable; however, it’s generally microwaveable. You should only microwave cardboard with food or liquid in it so the cardboard absorbs the microwave’s energy, or else it could catch fire. Avoid microwaving cardboard coated in plastic or wax and any cardboard that contains metal.

Throwing your cardboard container in the microwave is an easy solution for reheating your food, reducing the number of dishes you have to wash, and wasting paper plates. Most cardboard is safe to heat in the microwave; however, be sure to inspect your cardboard and check the label on the container to ensure it’s safe to heat in the microwave. Avoid heating cardboard that contains glue or other hazardous materials that can leak into your food.

Can You Microwave Pure Cardboard?

Cardboard is safe to microwave when made from uncontaminated fiber or cellulose material, meaning when the cardboard isn’t mixed with any other materials. 

Although cardboard is flammable and a heat conductor, it’s unlikely that pure cardboard will catch fire in a microwave when it has food in it. 

How Pure Cardboard Reacts in the Microwave

Be careful not to overheat cardboard in the microwave. Cardboard is made from wood fibers similar to paper, and it has low moisture levels that range from 9-12%. Those moisture levels inside the cardboard quickly drop in a microwave because it dries out the contents. 

If the cardboard’s moisture levels dip below 1%, it can suddenly catch fire because it is so dry. If you need to dry out cardboard, it’s best to let it air dry or put it under a fan. Never microwave cardboard to dry it out.

The longer you microwave cardboard, the more you decrease its moisture. Overheating your cardboard container could result in a fire if the cardboard dries out. You shouldn’t have to worry about your cardboard catching fire if you’re heating it for less than two minutes. Additionally, the moisture from the foods in the cardboard will reduce the risk of the cardboard drying out.

Important Safety Tips for Heating Pure Cardboard in the Microwave

However, keep an eye on your cardboard if you’re heating it for several minutes in the microwave. Additionally, if you microwave cardboard for too long, you could alter its structural integrity. Don’t keep it in the microwave for too long, or else the box can collapse or fall apart and create a mess in your microwave.

If you need to microwave your cardboard container for a long time, do it in spurts of several seconds or one minute at a time. Make sure your cardboard is flat or in its original shape. Don’t bunch up your cardboard because it increases the risk of catching fire. Be sure not to microwave an empty cardboard container without any food in it.

Always check the label on your cardboard container to see if the manufacturer has microwave-tested it. If it’s not safe to heat in the microwave, it should have a label or disclaimer saying so. If you’re unsure if your cardboard container is pure cardboard, then err on the safe side and transfer your food into a microwave-safe container to heat it.

Can You Microwave Cardboard That Contains a Wax or Plastic Coating?

Many restaurants distribute cardboard containers coated with wax or plastic to make it waterproof and prevent the food from leaking and staining your clothes or car seats on the way home. Grease tends to soak through traditional uncoated cardboard containers, and it can be messy if it gets on your hands or other belongings. 

You shouldn’t microwave cardboard with a plastic or wax coating. Check the cardboard to see if it’s shiny or feels slick. If the cardboard is polished or glossy, it most likely has a wax or plastic coating. 

When you heat plastic and wax in the microwave, it can release toxins into your food that are harmful if ingested. Some plastic isn’t microwave-safe and will melt or contaminate your food if you heat it in the microwave. Check the cardboard label to see if it’s a microwave-safe container or not before heating it in the microwave.

Can You Microwave Cardboard That Contains Metal?

Metal and the microwave don’t mix well, so you should never microwave any cardboard that has metal in it. Microwaves work by electromagnetic radiation, and some metal reflects these radiation waves, which can be dangerous. 

A small transmitter in the microwave sends out tiny waves of radiation. When your food absorbs these radiation waves, it cooks in the microwave. 

When metal reflects those waves instead of absorbing them, it can create sparks in your microwave. These sparks can ruin your microwave, and if it happens enough times, you’ll eventually have to purchase a new one.

Metal also heats exceptionally quickly, and it can catch fire in the microwave. Some cardboard containers have metal handles or screws that hold the container together. Check the cardboard before you microwave it to ensure that it doesn’t have any metal. 

Recycled cardboard likely contains tiny bits of metal. It’s difficult to see this metal with the naked eye, so be sure to check the label before putting it in the microwave to ensure that it’s microwave-safe. 

Other Materials in Cardboard You Should Avoid When Microwaving

In addition to wax, plastic, or metal, some cardboard containers contain glue that holds them together. Much glue is flammable and can release toxins and other chemicals when heated in the microwave. These toxins can easily seep into your food and contaminate it, and harm you if you ingest them.

Be sure to check for ink or writing on your cardboard container before microwaving it. The ink can bleed or leak into your food and contaminate it as well. Always check the label on your cardboard container before heating it in the microwave because it could contain materials and contaminants that you can’t see that are harmful to you.

Tips for Microwaving Cardboard

Follow these simple tips for safely microwaving cardboard: 

  • Use mitts when removing cardboard from the microwave. Cardboard can get extremely hot in the microwave. Be sure to take the proper precautions when removing the cardboard from the microwave by using oven mitts or potholders.
  • Maintain your cardboard’s structural integrity. Make sure that the cardboard is in its original shape. If you’re using cardboard torn from a pizza box or something similar, be sure to lay it flat. Don’t bunch up your cardboard because it increases the risk of catching on fire.
  • Check on your food often. Make sure your food isn’t burning or smoking in the microwave. Heat your food in short increments to ensure the best results.

Common Cardboard Containers You Shouldn’t Microwave

Take a look at this list of common cardboard food containers to make sure you don’t microwave something that could harm your food or your microwave.

  • Empty cardboard containers: They’re more likely to quickly catch fire.
  • Pizza boxes: They’re generally too big to fit in the microwave. Try reheating pizza in the oven instead.
  • Chinese takeout: They typically have metal rings, which can harm your microwave.
  • Boxes with writing, ink, or glue on them: The materials are most likely hazardous and can seep into your food.
  • Cardboard cookie containers: These often have plastic on the top of them so that you can see inside the box. Plastic can contaminate your food.
  • Cardboard sandwich containers: These often contain the same clear plastic as cookie boxes, and you don’t want that plastic to melt into your food.