Besides soot, the other ingredient of traditional Japanese ink is glue (nikawa). This is extracted by boiling the dermis (the layer of skin below the epidermis) of an animal such as a cow or a deer; it is highly adhesive and was widely used as a bonding agent. The ink is produced by mixing the soot with this glue and drying it. In addition, in order to ameliorate the unpleasant smell of the glue, fragrant substances such as musk, borneol or Japanese apricot blossom essence are added. This gives rise to a clean smell when the ink stick is rubbed down.