We are not allowed to charge a fee. But what I have done in the past is have everyone bring in a spackle bucket with a lid, and that becomes their bucket for their clay. As they use the clay, they put their scraps in their bucket (and add water as needed), and that's where they get their clay from to rework. If kids can't afford spackle buckets, they go to the local bakery, and sometimes they have similar type buckets for free. In this way I monitor how much prewedged clay gets doled out, by monitoring their bucket contents.