Indelible Ink

Indelible ink is which cannot be erased or washed away. Many countries use indelible ink during elections by marking the finger of the voter to avoid repeat voting fraud.

An Iraqi citizen, turning his head to protect his identity, proudly displays the indelible ink on his finger as proof he has voted in Iraq’s first free election in over 50 years on Jan. 30, 2005. Everyone voting in the historic election has to mark their finger with the ink to indicate they have already voted as a means to deter voting fraud. DoD photo by Master Sgt. Dave Ahlschwede, U.S. Air Force
Image from http://en.wikipedia.org

The ingredients of indelible ink contains water, dyes, silver nitrate and other chemicals.

Silver nitrate is an oxidizing substance classified under Class 5.1 in transport regulations however indelible ink is generally not a dangerous good.