These Ball point gel pens have a Thermo-Sensitive 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. Fine .07mm. Black 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. - Note: Erasable pens are not recommended for use on legal or office documents. Do not expose to extreme temperatures (140 degrees Fahrenheit) If pen is exposed to temperature that reaches 140 degrees Fahrenheit, the ink will be colorless when writing. To restore color, cool to at least 14 degrees Fahrenheit in the freezer and the ink will again write in color.