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