Sotah, Chapter Two, Mishnah Four
This mishnah teaches upon what material the priest writes the curses and with what type of ink.
1) He writes neither on a [wooden] tablet nor on papyrus nor on rough parchment but on a [parchment] scroll, as it is said, In a scroll (Numbers 5:23).
2) Nor does he write with a [preparation of] gum or sulphate of copper or with anything which makes an impression [on the parchment] but with ink, as it is said, And he will blot it out (ibid.) writing which is capable of being blotted out.
Section one: The Torah states sefer which means a scroll made of parchment (animal hide). Hence other materials cannot be used.
Section two: The writing on the scroll will eventually be rubbed out into the water. Hence, he cannot use permanent types of ink, which cant be rubbed out. He also cannot use something which scratches/indents the scroll for then even after being erased, the curses will be left on the scroll.