Editing Microwave oven (section)

=== Metal objects ===

Contrary to popular assumptions, metal objects can be safely used in a microwave oven, but with some restrictions.<ref>{{Cite web|url=https://www.goodhousekeeping.com/food-recipes/cooking/tips/a19797/microwave-metal/|title = Yes, You Can Microwave Metal|date = January 31, 2014}}</ref><ref>{{cite web |url=https://www.fsis.usda.gov/food-safety/safe-food-handling-and-preparation/food-safety-basics/cooking-microwave-ovens#11 |title=Cooking with Microwave Ovens: What containers and wraps are safe to use in the microwave oven? |website=[[Food Safety and Inspection Service]]}}</ref> Any metal or conductive object placed into the microwave oven acts as an [[antenna (radio)|antenna]] to some degree, resulting in an electric [[alternating current|current]]. This causes the object to act as a [[ohmic heating|heating]] element. This effect varies with the object's shape and composition, and is sometimes utilized for cooking.

Any object containing pointed metal can create an [[electric arc]] (sparks) when microwaved. This includes [[cutlery]], crumpled [[aluminium foil]] (though some foil used in microwave ovens is safe, see below), twist-ties containing metal wire, the metal wire carry-handles in [[oyster pail]]s, or almost any metal formed into a poorly conductive foil or thin wire, or into a pointed shape.<ref>{{cite web|url=http://www.conagrafoods.com/utilities/mwbasics.jsp?cookietest=true|title=Microwave cooking|at=sec. "Q: What is a microwave-safe plate or container?"|website=ConagraFoods.com|archive-url=https://web.archive.org/web/20120330102812/http://www.conagrafoods.com/utilities/mwbasics.jsp?cookietest=true|archive-date=March 30, 2012|access-date=October 25, 2009}}</ref> Forks are a good example: the [[Tine (structural)|tine]]s of the fork respond to the electric field by producing high concentrations of electric charge at the tips. This has the effect of exceeding the [[dielectric breakdown]] of air, about 3 [[volt|megavolt]]s per meter (3×10<sup>6</sup> V/m). The air forms a conductive [[plasma (physics)|plasma]], which is visible as a spark. The plasma and the tines may then form a conductive loop, which may be a more effective antenna, resulting in a longer lived spark. When dielectric breakdown occurs in air, some [[ozone]] and [[nitrogen oxide]]s are formed, both of which are unhealthy in large quantities.

[[File:Microwave metal shelf.JPG|thumb|A microwave oven with a metal shelf]]

Microwaving an individual smooth metal object without pointed ends, for example, a spoon or shallow metal pan, usually does not produce sparking. Thick metal wire racks can be part of the interior design in microwave ovens (see illustration). In a similar way, the interior wall plates with perforating holes which allow light and air into the oven, and allow interior-viewing through the oven door, are all made of conductive metal formed in a safe shape.

[[File:RainbowDVD.jpg|thumb|A microwaved [[DVD-R]] disc showing the effects of electrical discharge through its metal film]]
The effect of microwaving thin metal films can be seen clearly on a [[Compact Disc]] or [[DVD]] (particularly the factory pressed type). The microwaves induce electric currents in the metal film, which heats up, melting the plastic in the disc and leaving a visible pattern of concentric and radial scars. Similarly, [[porcelain]] with thin metal films can also be destroyed or damaged by microwaving. Aluminium foil is thick enough to be used in microwave ovens as a shield against heating parts of food items, if the foil is not badly warped. When wrinkled, aluminium foil is generally unsafe in microwaves, as manipulation of the foil causes sharp bends and gaps that invite sparking. The [[USDA]] recommends that aluminium foil used as a partial food shield in microwave oven cooking cover no more than one quarter of a food object, and be carefully smoothed to eliminate sparking hazards.<ref>{{cite web|title=Microwave Ovens and Food Safety |url=http://www.fsis.usda.gov/PDF/Microwave_Ovens_and_Food_Safety.pdf |work=[[Food Safety and Inspection Service]] |publisher=[[United States Department of Agriculture]] |access-date=August 10, 2011 |date=October 2011 |archive-url=https://web.archive.org/web/20110108064919/http://www.fsis.usda.gov/PDF/Microwave_Ovens_and_Food_Safety.pdf |archive-date=January 8, 2011 }}</ref>

Another hazard is the resonance of the magnetron tube itself. If the microwave oven is run without an object to absorb the radiation, a [[standing wave]] forms. The energy is reflected back and forth between the tube and the cooking chamber. This may cause the tube to overload and burn out. High reflected power may also cause magnetron arcing, possibly resulting in primary power fuse failure, though such a causal relationship is not easily established. Thus, [[dehydrated food]], or food wrapped in metal which does not arc, is problematic for overload reasons, without necessarily being a fire hazard.

Certain foods such as grapes, if properly arranged, can produce an [[electric arc]].<ref>{{cite web|last=Popa|first=Adrian|title=Re: Why do grapes spark in the microwave?|url=http://madsci.org/posts/archives/dec97/882909591.Ph.r.html|archive-url=https://web.archive.org/web/20010305093727/http://www.madsci.org/posts/archives/dec97/882909591.Ph.r.html|url-status=dead|archive-date=March 5, 2001|publisher=[[MadSci Network]]|access-date=February 23, 2006|date=December 23, 1997}}</ref> Prolonged arcing from food carries similar risks to arcing from other sources as noted above.

Some other objects that may conduct sparks are plastic/holographic print [[Thermos flasks]] and other heat-retaining containers (such as [[Starbucks]] novelty cups) or cups with metal lining. If any bit of the metal is exposed, all the outer shell can burst off the object or melt.{{citation needed|date=March 2013}}

The high electrical fields generated inside a microwave oven often can be illustrated by placing a [[radiometer]] or neon glow-bulb inside the cooking chamber, creating glowing plasma inside the low-pressure bulb of the device.