Hi guys! Hate to disagree with you, but a low fire white potters clay is very pure. It doesn't have any additives per se. Mold grows on it easily, and the best clay is aged and smells of organic decomposition. (high fire clay, like stoneware, does however have minerals, like iron oxide) You can eat it, although you shouldn't. My old dog used to eat it all the time, and she lived to be ancient.
However, potters clay is probably bad for another reason. Once it dries it, it would be very difficult to rewet. It would dry up and crack and that would make it hard to use, even mixed with spaghnum moss.
Joanie