The reason you can put certain types of metal in the oven but not in the microwave has to do with how each appliance generates and interacts with heat.
Ovens use radiant heat, which heats food and cookware through the hot air circulating inside. Metal cookware is generally safe to use in ovens because metal is a good conductor of heat. It absorbs the radiant heat and distributes it throughout the cookware, allowing for even cooking. However, it's important to note that some metals may react with certain acidic or highly alkaline foods, so it's advisable to use oven-safe cookware made from appropriate materials like stainless steel or cast iron.
Microwaves, on the other hand, use a different method to heat food. They generate microwave radiation, which excites water molecules in the food, causing them to vibrate and produce heat. Metal objects, such as aluminum foil or utensils, reflect microwaves and can cause electrical arcing or sparking inside the microwave oven. This can lead to damage to the oven and potential fire hazards. The rapid and uneven heating caused by the arcing can also cause the metal object to become extremely hot and potentially cause burns.
It's worth mentioning that there are some microwave-safe metal utensils and cookware available, specifically designed to allow microwaves to pass through them without causing arcing or sparking. These utensils are typically labeled as microwave-safe and have specific instructions for use.
In summary, the different heating mechanisms employed by ovens and microwaves determine the safety of using metal. Metal is generally safe to use in ovens, where radiant heat is used, but caution should be exercised when using metal in microwaves due to the potential for electrical arcing and associated hazards.