Walnut ink is an ink produced with the green skin that surrounds the seed; usually, the fruit of the species black walnut or black walnut, botanically classified as Juglans nigra, is used .
The ink can be liquid or crystallized and must be mixed with water before use; it is used to paint, speckle and darken the paper, in order to obtain a "vintage" effect.
The walnut ink has good archiving properties; this means that it resists brightly aging and, over time, does not lose its visual characteristics. The hull ink is famous for having been used by many great artists, including Leonardo da Vinci and Rembrandt.
In ancient times, in the Roma communities, it was used to stain the hands of criminals so that they could be uniquely recognized; these, once dyed, could not be discolored and remained on the skin for a long period of time.
