וְ֠כָתַב אֶת־הָאָלֹ֥ת הָאֵ֛לֶּה הַכֹּהֵ֖ן בַּסֵּ֑פֶר וּמָחָ֖ה אֶל־מֵ֥י הַמָּרִֽים׃
The priest shall put these curses down in writing and rub it off into the water of bitterness. (Bamidbar 5:23)
I want to know what material was most likely used to "write down" the prescribed writings of the Sotah on? "Be'sefer" I think translates as "in a book", but my Hebrew isn't the best. Was the material from an animal (like how a sefer Torah is made) or was it paper like from a tree?