After the first rock boiling, I could still drink the water. Sure the water still becomes a muddy mess, but the finished boiled water looks a lot more like water, and a lot less like chocolate pudding. Just burn a hot fire from small sticks in your clay lined hole for 20 minutes. Let the fire die, and scoop out all the charcoal and ash (ash and water make lye, you don't need that). Then improvise something to carry water from your source to the fired clay hole. Heat some rocks for 40 minutes in a fire, drop them in the hole, and boil away.