Well I tried it. Molded and fired a couple clay pots. One cracked badly during firing. I think due to me being a little rough when adding more wood to the fire on one occasion. No way this one would hold water. The other had a slight crack in it and would hold water, but not long enough for boiling. I was also able to do the whole thing within the eight hour time frame. Though smaller pot leaked too much to hold water long enough for a boil, I got close enough that there is no doubt in my mind that if I were a little more careful I could have had pots capable of boiling a liter of water in under eight hours. A couple things I learned (besides being careful not to crack your pots when adding wood to the fire) is that the type of clay you pick is not so important but is helpful. The less gritty/sandy the better. My clay was more like mud and although this would have been fine (if I hadnt cracked them) for boiling a liter or two or three of water, I dont think they would have held up to extended use. The other thing I learned is that unless youre extremely careful (which I was not) it helps a lot to let the clay pots air dry a little before firing. This gives them a little added strength to keep them from colapsing under the weight of firewood. For me on a 90+ degree dry Texas afternoon this didnt take long. However on a cold humid day this might make staying within the eight hour time frame impossible. Also if youre making a large pot you have to build half, let it air dry, then build the second half. Otherwise it will collapse under its own weight. In a survival situation I'd say 8 small pots beats two big ones any day. One more thing. Forget hot rock boiling with these babies. You fill em with water and stick em right on top of your open flame for boiling.... no problems. I was real impressed by this. I read over at BCUK that this was how they worked but seeing it for myself, I was still impressed. A more info plus pictures will follow in a couple days.
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Learn to improvise everything.