Words of wisdom from Charlotte Warr Andersen: "I thought I'd give you all a heads up about Frixion Erasable pens. A student showed me hers on my last road trip. She drew a line, thin but quite black, and then ironed it and the line totally disappeared. It looked like the coolest thing ever!
So I went in search of these pens, which you can get a Staples. I bought the pack that has three colors in it - black, red and blue. Being quite the skeptic about things that are too good to be true, I read the back of the package. In small print it says: "Do not expose to extreme temperatures (<14degrees> 140 degrees F). If pen is exposed to temperature that reaches 140 degrees F the ink will be colorless when writing. To restore color, cool to at least 14 degrees F and the ink will again write in color."
Aha! I said to myself. If you iron the ink it's going to be way over 140 degrees so it's going to disappear. So I took some fabric and scribbled on it with all three colors. Then I ironed it and, sure enough, all the scribbles disappeared. But then I put it in the freezer for about 10 minutes and all the lines reappeared. I left it for about a week and a half. Today I tried to wash the ink out. It didn't come out with soap and hard scrubbing and not even after I put rubbing alcohol on it. So then I ironed the piece dry and all the lines disappeared and then it went back to the freezer and all the lines reappeared (perhaps just a bit fainter).
This experiment was enough to persuade me that these pens are too good to be true and even sort of creepy - the ink is always there even if you can't see it. I wouldn't use them on any fabric or quilt you really cared about."