This is a marvelous Sorcery from the Greek Magical Papyri. It is a form of exorcism of the outside that surrounds us. What is close to you and serve no longer, is commanded to be gone.
* I just performed the Sherd Shield and Sherd Sword. *
I broke a ceramic plate that I used under an empty planter. I dropped the Sherd on the ground to the left of the forked path that goes into three different routes.
I burned Myrrh incense and also passed the Sherd through the incense smoke. I wrote the spell and the symbols on both sides of the Sherd and spoke the spells on each side close to my mouth so my breath would hit the words and symbols on the Sherd .
I buried it there and left.
(Excerpts below from PGM, Betz)