Cleaning a grimy oven. Pre-heat your oven to 180ºC. Place some citrus peels (orange, lemon or lime etc.) in a roasting pan and just cover them with water. Place the pan in the oven for approximately 20 minutes so that the citrus peels just simmer in the pan. Turn the oven off and once it is cool you will be able to scrub away the grime more easily.