Older writing instruments should be filled exclusively with water-soluble ink, because only this allows cleaning to be carried out simply and gently. Whether an ink is water-soluble is normally indicated on the packaging; alternatively it can simply be checked whether the ink can be erased with an ink eraser, which is taken as an indication of water-solubility. Typical examples of water-soluble inks are Pelikan 4001 royal blue and Lamy royal blue.