You know, slowing the drying process can sometimes trap moisture unevenly, which I've found creates hidden stresses in the clay body. In my experience, wedging in a bit of grog or sand does more for stability than just covering pieces with plastic. That extended bisque firing might work for your riverbed clay, but I've had batches bloat terribly from organic matter when fired too slowly. It's all about testing small scales first, because every clay source is its own puzzle. The speckled look is a nice bonus, but without screening for larger particles, you're risking weak spots. I spent a summer trying to refine local clay and learned that patience with processing beats patience in firing alone.