Can you microwave cardboard?

Cardboard itself is mostly cellulose and heats slowly, so a short run is not the problem. The problem is what is attached to it. Metal staples arc, foil-lined susceptor patches in popcorn and pizza packaging are designed to get very hot and can scorch, and glued seams and wax coatings can melt or smoke. Dry cardboard left running for many minutes will eventually reach its ignition point around 220-250 °C at a hot spot, and there is no water in it to hold the temperature down.

How to do it properly

1. Remove staples, tape and any plastic window first
2. Keep runs under about three minutes and stay in the room
3. Do not microwave cardboard on its own with no food in it
4. Use only plain, dry, uncoated board

What goes wrong

• Staples and metallic printing arc
• Dry board with no food to absorb energy can smoulder and catch fire
• Glue and wax coatings melt into the food

Other options

• Move the food onto a plate — it takes ten seconds
• Use parchment paper if you need something disposable underneath

Other "can you microwave" questions

• Can you microwave styrofoam? It depends — Only if the container is labeled microwave-safe, and most takeout foam is not.
• Can you microwave aluminum foil? No — No — foil reflects microwaves and the edges arc, which can start a fire.
• Can you microwave plastic containers? It depends — Only ones labeled microwave-safe — cold-storage tubs are not among them.
• Can you microwave eggs? It depends — Beaten or pricked eggs, yes. A whole egg in its shell will explode.
• Can you microwave Tupperware? It depends — Yes, if that specific piece carries the microwave-safe symbol on its base.
• Can you microwave paper plates? It depends — Plain white paper plates are fine; anything with metallic trim or a foam layer is not.
• Can you microwave metal? No — No — metal reflects microwaves, arcs at edges, and can damage the oven or start a fire.
• Can you microwave breast milk? No — No — microwaves create hot spots that scald a baby even when the bottle feels cool.
• Can you microwave takeout containers? It depends — Only if the container is marked microwave-safe — most are not, and metal-handled ones never are.
• Can you microwave glass? It depends — Yes for ordinary heatproof glass; no for leaded crystal, and never straight from the freezer.
• Can you microwave water? It depends — Yes, but very clean water can superheat and erupt when you move the cup.
• Can you microwave ceramic? It depends — Yes for plain glazed ceramic; no if it has metallic trim or is unglazed and porous.