# Microwave Metal (DD-589) > A microwave oven's magnetron fills the metal cooking cavity with microwaves at about 2,450 MHz, with a wavelength of about 12 centimeters. Food absorbs them, mostly through its water, fats and sugars. Metal reflects them instead, and a conductor in the cavity acts like an antenna: the oscillating field induces currents and voltages in it. The field is strongest at sharp points, edges, corners and narrow gaps. There it can become strong enough to ionize the surrounding air, which then conducts and glows as a spark or arc, jumping to the oven wall or to another piece of metal. MIT materials scientist Caroline Ross compares it to lightning. That's why a pile of nails or screws, crumpled foil, metal twist ties or gold-trimmed china spark so readily. Smooth, rounded metal is less prone to arcing but not risk-free. The USDA says small amounts of new, smooth foil can be used to shield food, if the manufacturer's manual allows it, the foil is shaped smoothly to the food, and it stays at least an inch from the oven walls. It also says to remove it at once if arcing starts. Arcing can damage the oven and start a fire. The footage shows a heap of nails inside a microwave, sparking and glowing as the plate turns. - Department: Science - First published: 2025-06-05 - Last updated: 2025-06-05 - Permalink: https://detailenjoyer.com/case/microwave-metal/ - Author: Marcus Easterling (DetailEnjoyer), The Detail Department ## Quick facts - **DD-589-F01**: In a microwave, electric fields concentrate at sharp points and edges of metal. _(Write-up · Exhibit B (Points and edges))_ - **DD-589-F02**: The USDA says to keep shielding foil at least an inch from the oven walls. _(Write-up · Exhibit E (Smooth foil, carefully))_ ## Write-up Lightning in a microwave. Wrinkled foil can spark in a microwave. Smooth foil, used carefully, usually doesn't. 1. **Metal reflects**: Food absorbs microwaves, but metal reflects them and acts like an antenna. 2. **Points and edges**: The induced electric field is strongest at sharp points, edges and narrow gaps. 3. **Ionized air**: A strong enough field ionizes the air, which conducts and glows as an arc. 4. **Wrinkles spark**: The USDA warns that wrinkled foil can cause arcing. 5. **Smooth foil, carefully**: The USDA says small amounts of smooth foil can shield food, kept an inch from the walls. ## Related case files - [(Z) Pinch Me (DD-741)](https://detailenjoyer.com/case/z-pinch-me/): I paired it with plasma at a much bigger scale - [Point Curie (DD-669)](https://detailenjoyer.com/case/point-curie/): I linked another case about metal and fields - [Feeling Salty (DD-285)](https://detailenjoyer.com/case/feeling-salty/): I paired it with another dramatic reaction people try at home - [Making Plasma in the Microwave (DD-287)](https://detailenjoyer.com/case/making-plasma-in-the-microwave/): I paired it with the case on why metal sparks in a microwave ## Reel vs. record - **Note**: 'Smooth metal surfaces like a spoon might reflect microwaves harmlessly': often, but I wouldn't count on it. Smooth metal can still arc near the oven walls or another piece of metal, so the USDA says to follow the oven's manual and stop at once if arcing starts. - **Note**: Otherwise the Reel's physics is right: the field concentrates at points and edges, ionizes the air and makes an arc. - **Note**: My Reel vs. Record note: the footage is a heap of nails, which is an extreme exemplar of sharp points. I'm not recommending anyone try it. Arcing can damage the oven and start a fire. ## Video: Microwave Metal: Lightning in a Microwave - Duration: 23.6 s · Published: 2025-06-05 - File: https://detailenjoyer.com/assets/videos/DKiatuvS38M.mp4 ### Transcript DID YOU KNOW? Microwaves cause electric fields to concentrate at sharp edges or points on pieces of metal, leading to a build-up of electrical charge. When the charge becomes strong enough, it ionizes the surrounding air, creating a conductive plasma channel, what we see as sparks or arcing. Smooth metal surfaces like a spoon might reflect microwaves harmlessly, but crumpled foil or utensils with sharp edges drastically amplify local electric fields. NOW YOU KNOW! ## Questions & answers **Q: What does metal do to microwaves?** A: Food absorbs them; metal reflects them. _(Source: Write-up · Exhibit A (Metal reflects) · MIT School of Engineering: Why can't we put metal objects in a microwave?; Scientific American: Why do metal objects create sparks when placed inside a microwave oven? (1999))_ **Q: Where is the electric field on metal strongest?** A: That's where arcs start. _(Source: Write-up · Exhibit B (Points and edges) · MIT School of Engineering: Why can't we put metal objects in a microwave?; Scientific American: Why do metal objects create sparks when placed inside a microwave oven? (1999))_ **Q: What is a microwave arc?** A: It conducts and glows like lightning. _(Source: Write-up · Exhibit C (Ionized air) · MIT School of Engineering: Why can't we put metal objects in a microwave?; Scientific American: Why do metal objects create sparks when placed inside a microwave oven? (1999))_ **Q: What kind of foil does the USDA say can cause arcing?** A: Use new, smooth foil only. _(Source: Write-up · Exhibit D (Wrinkles spark) · USDA FSIS: Cooking with Microwave Ovens (archived 2025))_ **Q: How far from the oven walls should shielding foil be kept?** A: That's the USDA's rule. _(Source: Write-up · Exhibit E (Smooth foil, carefully) · USDA FSIS: Cooking with Microwave Ovens (archived 2025))_ ## Sources - [MIT School of Engineering: Why can't we put metal objects in a microwave?](https://engineering.mit.edu/ask-an-engineer/why-cant-we-put-metal-objects-in-a-microwave) - [Scientific American: Why do metal objects create sparks when placed inside a microwave oven? (1999)](https://www.scientificamerican.com/article/why-do-metal-objects-crea/) - [USDA FSIS: Cooking with Microwave Ovens (archived 2025)](https://web.archive.org/web/20250502014426/https:/www.fsis.usda.gov/food-safety/safe-food-handling-and-preparation/food-safety-basics/cooking-microwave-ovens) ## Cite this case file Easterling, Marcus. "Microwave Metal." The Detail Department (DetailEnjoyer), case file DD-589, last updated Jun 5, 2025. https://detailenjoyer.com/case/microwave-metal/