What happens is that the watery carrier fluid evaporates much faster and the solids, e.g., the soot, sediment in the fountain pen. Consequently, the ink becomes thicker and gathers in the capillary grooves of the ink regulator and the fine capillaries of the piston filling mechanism. To make a long story short: it will stop up your fountain pen. Therefore, if you only need your fountain pen every once in a while, we recommend you to only partly fill up the ink reservoir and best rinse it under lukewarm water every 3 to 5 months (leaving away additives).