The Glass Pen formula is specifically designed for use on glass and mirrors. Its water-based ink comes in ten very opaque colours which are easily removed from the surface with a damp cloth or for more convenience Rainbow Glass Cleaner.Product Specifications? 5mm Bullet Nib Reversible.? Wet wipe erase with a damp cloth or Rainbow Cleaner.? Suitable for indoor and outdoor use.? Resistant to light showers.? Valve activated to control ink flow.? For use on glass, windows, and mirrors.? Water based & non - toxic.