Earthenware utensils do not work as well. The container absorbs energy while the food is heating. In fact, some earthenware utensils become very hot before the food gets warm. Plastic utensils transmit energy well and absorb little heat, but some plastic melts at low temperatures and is damaged by hot food. Covering utensils prevents heat loss and evaporation and decreases cooking time.
Wax paper can serve as a cover.
If there is an air intake filter, clean at least weekly. Check the manual before using foil or any metal utensil. Check manufacturer's manual for the oven wattage. Periodically check to insure the output wattage does not change. To check output wattage, place a pint of water in a glass measuring cup.