I've walked into the woods with only a fire starter and a real hankerin for boiled burdock. Natural containers are hard to come by in the natural so I'm gonna have to make one myself with nothing but a fire, nope not even a chainsaw.
Step 1: Locate Materials
Select a log stump or whatever, something not too rotten, and obviously big enough for what you want it for. Don't use a hollow log, you'll do all the work and then realize that all of your water flows out of the end.
A few long straight rocks are very helpful for scraping.
Build a fire. An upside down fire with a nice trench underneath is a great way to make large hot coals. Since I am using a rounded log I found that it is easier to make a separate fire, if you are using a flat stump or something of the sort you can build your fire directly on top.
Step 2: Start Burning
Once your fire has burnt down enough, use a scooper of any sort and pile on as many coals as possible. For maximum time efficiency, blow the coals red hot. It'l go slow at first so don't get discouraged. A reed is a nice touch for keeping the heat from your ugly sweaty face. Replace coals as they are needed.
Caution: coals will roll off so move away leaves from underneath, remember the wise words of smokey the bear, only you can prevent forest fires.
Caution # 2: hyperventilation is bad for the brain cells, don't faint in the middle of the woods and force someone to drag you out.
Step 3: Scrape
When your coals die down, scoop them out and scrape the burnt wood. I wanted nice flat sides, so I only scraped the bottom, and scraped the sides at the end.
Step 4: Finished
Do a final and thorough scraping of ashes to reveal your purdy bowl. Put water in your bowl and rub the sides with your hands, change the water until it is clear.
Step 5: Calipers
The picture above is my version of an already existing tool called calipers. It wasn't necessary with my log to hole ratio, but if you were wanting to know the thickness of the bottom of your bowl here's how it works.
Pinch the two ends of the sticks, put the other two ends together. Make a mark on that little middle jut, where the stick overlaps it, this is zero inches. Then open the jaws to one inch and make a second mark on the middle jut where the stick overlaps it now. Do this repeatedly for the amount of inches you think you might need. Pinch the jaws around your bowl and look to see what mark is overlapped, this is the amount of inches of your bottom.
Step 6: Boil
Collect stones. I had about 10 stones, golf ball sized. I switched five at a time with half buried in the coals and half in the water.
The stones should be visibly red hot ( in the dark), and they should make the water boil. I actually had the stones hot enough at one change, that the water boiled over the top. I found that about five changes of stones was enough to make the burdock soft. Eat um up. And don't forget to slurp down that nice nutritious burdock water.
Native Americans used this method of boiling water for centuries. All the books tell you to stay busy while surviving. This is a useful, non energy expending, time consuming project. Remember, don't learn how to do this stuff when your life is on the balance, learn it now, and you'll be glad you did.