My wife is a potter. As in she makes pots of many kinds. when I told her of this thread and the suggestion that you could create and fire a pot and then use it both as a water container and as a boilng container made her laugh out loud.

First of all, not all clay is capable of being fired, and if it is, that does not make it water-tight. Burnishing (rubbing) is one technique to make a clay pot that is not water-tight into one that is. We're talking about DAYS of rubbing, not minutes or hours.

Next is the thermal coefficient of the clay. Not many clays you find in nature can withstand the thermal shock that comes from fire on side and water on the other.

Lastly is the amount of time to fire. I've done a wood firing, it's a multi-hour process and it takes a HUGE amount of heat to really fire well.