“Attempting to sterilise a dishcloth in this way is extremely dangerous. There are too many variable factors, such as the capacity/power of the microwave oven or the moisture content of the dishcloth or sponge. Shropshire Fire and Rescue Service strongly advises the public not to attempt to sterilise sponges or dishcloths in this way. Attempting to do so may result in your home being destroyed.”
TV advice nearly ends in fire (Shropshire Star)
The washing machine seems to clean them pretty well over here.