It depends on what kind of pen it is. Some if you iron it will set the ink, some you get out with water and some gradually disappear in the air. OR if you are talking about the Frixion pens that disappear when ironed over there are several discussions on QB on Frixion pens(type in Frixion Pens in search box). One word of caution for the frixion pens is that on dark fabrics they can sometimes leave a faded or bleached line? The marks do reappear if the fabric gets cold enough. I do use them for my small sewing projects.