I didn't say it was IMPOSSIBLE - far from it - it's just a task that takes - at best -a full day with the right materials and the right treatment of the materials. You can make a pot that will hold water and you can make a pot that can withstand fire. But these are very often two different needs, with two different techniques. My point is that you can't reasonably expect to make, fire and use a clay pot with natural materials in the 8 hour scenariod described. It's physics - the clay takes time to fire, to change chemically to become something that will work. Further, to build a pot that won't explode in the firing, you really need to "wedge" the clay for a while to get all the air bubbles out. This can take about an hour even with store-bought clay, it takes longer if your clay is full of rocks, weeds and insects.