Our electric clothes iron (steam) develops a black...

Question

Our electric clothes iron (steam) develops a black substance on the ironing surface that inhibits the sliding ability of the iron. Sometimes some of the black substance rubs off on the clothing being ironed. The iron is a couple of years old. What is the best method and product for cleaning the bottom of the iron - Could our problem be a function of the steam mechanism and does this require some type of cleaning? Is their a maintenance procedure that would reduce this problem in the future?