Here's a video I made a while back for a series of challenges on BCUSA
[youtube]yP2Ug0VzF5g[/youtube]
Here's the pictures that go along with the video
A Glad plastic bag that stands up right on its own.
Also used a piece of "found" plastic, acting as if it was trash that had been in the area. Dug a small hole and lined it with the plastic.
The pebbles in the container are to keep the hot rocks from coming into contact with the plastic and prevent melting form happening.
The stones were gathered from a high spot and insured they had not been immersed in water for quite sometime, to help avoid any unwanted explosions. I also use quartz type rocks as them seem to fair better to the repeated rapid heating and cooling. At least they are easy to find in my area.
You want to avoid flint like stones, as think what happens to cold glass when you pour hot water on it. Same holds true the other way! And Granite seems to just fall apart, kind of like soft concrete.
You need something to move the stones with, I made a set of tongs by splitting a branch down and putting a wedge in and binding it alltogether. They work very well. But, you can just simply use two sticks, but it doesn't work near as well as the tongs.
Another Method is to use a forked stick as a scoop and another stick to push and hold the stone in the fork. This works very well for larger rocks.
Sometimes I like to clean the rocks of ash and dirt before moving from the fire to the container. A field made brush works well for this, or a second container to quickly dip the rocks in, but that takes alot of the heat out the rock.
Sorry to those of you that can not watch video, but taking pictures while playing with really hot rocks just wasn't going to happen!
By no means am I an expert on boiling with rocks