In The Master Magician, Ceony triumphantly gloats that a Blood Magician cannot unbind from blood because the unbinding requires a precursor (wood to paper, oil to plastic), and blood/flesh has no precursor...or at least, one that's not too gross for a YA novel. One would think a Blood Magician could use semen to unbind.
That would still qualify as "flesh." You can get wood without paper, but you can't get semen without flesh.
Then again - the oath mentions "dying and turning into ground/earth" (I've read a translation, not very good one, by the way). So maybe earth is the precursor to flesh? In a mystical, made-of-clay sort of a way?
