There were a couple of standard "invisible inks." [0] I assume Invisiclues used one of them--most likely baking soda given they didn't use heat to reveal. No id

There were a couple of standard "invisible inks." [0] I assume Invisiclues used one of them--most likely baking soda given they didn't use heat to reveal. No idea how stable either of those were as inks. [0] https://www.chem.ox.ac.uk/files/secretstoppers1pdf https://www.chem.ox.ac.uk/files/secretstoppers1pdf