Grog Black Magic Ink 70 is an invisible ink that reacts under UV lights until its complete dulling, with a deep dark brown finish. Its special formulation allows to obtain different shades of color, which are determined by the intensity of light to which it is exposed the artwork. If Black Magic Ink is used indoors, the darkening reaction will be much longer, compared to the outdoor usage, due to the different intensity between the UV artificial lights and the UV sun rays. Black Magic Ink is suitable for writing on all kinds of porous surfaces such as paper, cardboard, marble, concrete, plastic, metal and painted surfaces. It's not suitable to write on glass or other non-porous surfaces. Available in 1 color. Warning: The color intensity is determined by the amount of light absorbed by the ink and can vary depending on which is used outdoors or indoors.