Are all fountain pen inks waterproof?

No, most fountain pen inks are not waterproof. The majority of well-known fountain pen inks are formulated for smooth writing and vivid colors but are designed with water solubility in mind, meaning they can run or fade if exposed to liquid. If waterproof or water-resistant inks are necessary—for example, for important documents or artwork—look for inks that are explicitly labeled as “waterproof” or “permanent.” These inks often use a pigment-based formulation or include specific resins that help them adhere to paper fibers and resist water exposure. Keep in mind that even waterproof inks may still require careful cleaning to prevent build-up within your pen.