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Hello! The ink in these markers is not permanent, but more "semi-permanent". It cannot simply be washed off after application. It is designed to stick and be very hard to remove. But over time, depending on the application surface and other variables, it will eventually stop showing. Specifics of this can vary greatly depending on how it's being used. Feel free to contact us directly through our site via message if any other questions.

@@directglow thanks ok

@@directglow I plan to use it on a book, would it last longer? I'm not familiar with the process used to create it so I don't know if it fading over time is just inevitable for all "invisible ink"

@@MemeSama Hello! The biggest factors that reduce longevity on the application surface such as a book would be frequent touching (oils from the skin along with friction in general), direct sunlight (the sun as it does with many other items will dull and fade the color faster with excessive exposure), and general humidity (the marker is water resistant but not waterproof). If these things can avoided or kept at a very bare minimum the ink markings should last about 2-3 months before a re-application might be needed. There are many other factors at play that can affect the quality along with the longevity. Please carefully note these inks are a water/alcohol hybrid mix. Depending on the age, condition and paper type of your book, this may or may not be an issue. If it's an older valuable book, you may want to rethink using the markers for this purpose.

@@directglow Thank you for going into such detail, it'll be really useful in the future when I decide to make a purchase. Appreciate it

Common 395nm blacklights emit a purple light. When purple hits the yellow UV ink, it will slightly alter the color to some degree. This is normal. Comparable to mixing two colors together. The specific type of blacklight used, the wattage of the light, distance of the light from the ink, and the marking surface type/base color are all dynamic factors that influence the look of the finished product. A premium 365nm light will preserve the color better inmost cases. 365nm UV lights emit closer to the blue spectrum and have a softer glow which in turns does not "wash out" or alter the colors as much. We sell both 395nm and 365nm flashlights in our shop:)