Scientists have created a new form of invisible ink, but it has one big hitch: it might be toxic.

Invisible inks, or “smart inks,” can be used for everything from preventing counterfeit money to simply storing information. These inks are supposed to be invisible unless activated by light or heat, but in practice, you can usually still see a little bit of what’s written with the ink. In a study published today in Nature Communications, scientists created an ink that is truly invisible — until you add salt.

The new material isn’t perfect: the base compound is lead, which is toxic in large doses. But the scientists are trying to design a safer alternative, study author Liang Li, an engineer at Shanghai Jiao Tong University, told The Verge in an email.

This lead-based compound can’t be seen. But once you add a special type of salt, the lead turns into nanocrystals that glow when you shine a UV lamp on the paper. Once you reapply the salt, the ink becomes invisible again, so the message can be kept secret and passed along again. This particular technique probably won’t be widely used, but it’s a step closer toward better security for important messages — or maybe just passing notes in class.