Using a Non-Kosher Microwave
At work, there is a microwave in the kitchen which people use to heat up food, much of it not kosher. Could I use it as well, if I cover up my food first?
The Aish Rabbi Replies
Yes, it would be possible to do this. The simplest and most effective means of doing so is to double-wrap your food. Although microwaves themselves do not become hot, food heated in them emits steam. This steam can collect as hot vapor, which might absorb non-kosher taste from the walls of the microwave and then drip down or spread onto your food. Double-wrapping your food is the best way of ensuring this does not occur.
Beyond this you should make sure to place your wrapped food not directly on food particles, but on a clean area. Note also that the outer wrap you use you should consider non-kosher afterwards and discard.
Some recommend against doing this on a long-term basis since plastic wrappings are liable to melt or split apart, leaving the food uncovered.
Note that double-wrapping your food will work equally well for convection microwaves or ones with browning elements (provided the plastic covers remain intact), although they much more closely resemble conventional ovens.
(See this page for further details.)