Chip bags in the microwave

Why do potato chip bags spark and shrink when you put them in a microwave?

If they’re sparking, you have chip bags that are lined with a thin sheet of metal, most likely aluminum (or lead, if the bags are made in China). The shrinking is caused by heat.

I had a similar reaction with Pepperidge Farm cookies. The snack packs with 2 cookies. The arcing scorched the cookies and I had to throw them out.

A lot of snacks have a metal or foil liner in the packaging. I’m not sure why.

Probably makes them air-tight.

As well as blocking light better than thin plastic.

Microwave ovens induce electrical currents in conductive objects. If you have thin conductive objects, they can get pretty warm pretty quick; if you have conductive objects separated by a small gap, you can get voltage differentials high enough to cause arcing.

Foil-lined paper or foil-lined plastic is good for this. The foil gets hot via electrical currents/arcing, and the plastic shrivels up because of the heat. Any grease on the bag remaining after the chips are gone will also absorb microwaves and help heat things up, further causing plastic shrinkage.

Note that “conductive” is not exclusive to metal. Want some excitement? Kids, try this at home without adult supervision:

  1. Slice a grape almost in half. Leave a tiny bridge of skin connecting the two halves.
  2. lay the grape on the floor of the microwave oven, with the two halves flat-side up, their tiny bridge of skin still connecting them.
  3. Repeat steps 1-2 half a dozen times.
  4. turn on the oven and observe.

The microwaves will produce electrical currents between the two halves of each grape, across that tiny bridge of grape-skin. The skin-bridge heats up, vaporizes, and the electrical current becomes an arc that follows the rising plasma that was the skin-bridge. You should enjoy a dramatic light show.

If you can’t find time when your parents aren’t home, browse YouTube for videos of this (and similar) experiments.

Ha, I have done that grape experiment before. It is worth trying, especially in a powerful microwave.


Biaxially oriented PET film can be metallized by vapor deposition of a thin film of evaporated aluminum, gold, or other metal onto it. The result is much less permeable to gases (important in food packaging) and reflects up to 99% of light, including much of the infrared spectrum. For some applications like food packaging, the aluminized boPET film can be laminated with a layer of polyethylene, which provides sealability and improves puncture resistance. The polyethylene side of such a laminate appears dull and the PET side shiny.