Some of his friends were imprisoned by the Inquisition. At the gate of the prison, everything was checked except for eggs. Della Porta wrote messages on the eggshell using a mixture made of plant pigments and alum. The ink penetrated the eggshell which is semi-porous. When the eggshell was dry, he boiled the egg in hot water and the ink on the outside of the egg was washed away.
When the recipient in prison peeled off the shell, the message was revealed once again on the egg white. - Philalethe Reveal'd Vol. 2 B/W
