Thanks. This prompted some quick research. It seems that eggshell is a substitute for sacred earth from Africa (presumably chalk) and that jogged my mind and made me think "Of course! You moron, Ben!"
Of course calcium is important in these things! What is a magic circle and all of the symbols drawn with? Chalk! Calcium! Being surrounded by calcium is part of the Egyptian derived magical tradition as much as the sub-Saharan African tradition! Thanks for prompting me to remember the obvious!
So, sodium or calcium, for some reason are good for creating barriers in magical practice. Chalk would probably do.