These Ball point gel pens have a ThermoSensitive ink formula that disappears with erasing friction on paper but what makes them great to use on fabric Well not only do they write smoothly, the ink will not smear, will not wash off or rub off while you need it, but when you're ready the ink will magically disappear with the touch of a hot iron. .07mm. Blue ink. Always test pens before use to be sure that the marks will completely disappear. May not be suitable for use with some dark fabrics or dyes, fiber contents, or fabric finishes.