I have a black cast iron kettle I got from tractor supply that I keep water in to help humidify the house. But, I have gotton rust spots on the top of my woodstove from this kettle. Am I using the wrong type of kettle or should I put something between the woodburner and kettle. I saw a post about putting pennies between the woodburner and kettle. I also see a lot of people are using copper kettle's. All advice is appreciated.