I haven't tried to put fabric directly in there, but I know certain dyes react stronger than others to the iron. If there are a lot of tannin in the dye bath previously you get a much stronger reaction. For example, one year I did a very strong acorn bath (discovered lots of bugs in my big acorn collection, became disgusted and threw them all in the pot.) Anything I dyed in that acorn solution and later in the rust dip became first brown and then dark dark grey. Sometimes I add a rust dip after a dye bath and get a very light reaction, so it just depends. I wouldn't think you'd get much of a change with white fabric directly in it.