How to purify water with the hot-rock boiling technique

Editor’s note: This article was originally written by our friends at OFFGRID. Check out their site for more survival-related tips.

Most of us know that boiling is one of the most effective methods to purify drinking water in the wild.

In mere minutes, boiling can destroy any nasty viruses, bacteria, protozoa and other waterborne pathogens that could otherwise wreak havoc in your digestive system or kill you.

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However, boiling water off the grid may not be as easy as it sounds. Unless you have a sturdy single-walled metal water bottle, canteen or pot, you can’t simply toss it into the fire and wait for the water to boil.

Most metal water bottles are insulated, and cannot be exposed to direct flame for boiling water. Photo: Courtesy of OFFGRID
Most metal water bottles are insulated and cannot be exposed to direct flame for boiling water. Photo: Courtesy of OFFGRID
Notice we said “single-walled water bottle”; the popular double-walled or insulated bottles found at many stores must not be placed on an open flame, and may even explode from the heat.

Composite and plastic bottles can’t take the heat either and will begin to melt over the flames. So how do you boil water if you don’t have a metal vessel to boil it in?

Rocks are available almost everywhere on earth, and can be used to boil water through heat transfer.
Rocks are available almost everywhere on Earth and can be used to boil water through heat transfer. Photo: Courtesy of OFFGRID

The answer is both simple and ancient: hot-rock boiling. You can simply heat several rocks over a campfire, then pick them up with sticks and drop them into the water vessel of your choice. The residual heat of the stones will heat the water rapidly and eventually bring it to a boil, purifying it.

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Here’s a video that shows how it works. (You may want to mute the audio if you’re not a fan of smooth jazz.)

Notice that, in the video, hand-carved wooden vessels are used and the rocks are rinsed to remove ash and soot before adding to the drinking water. Both of these are smart choices; although small amounts of ash from the campfire may not be harmful to drink, they certainly won’t taste good.

A few more tips:

• Your rocks should be heated for about 10 to 15 minutes to reach an adequate temperature.
• Never place rocks back on the fire immediately after placing them in the water. They may crack or explode.
• If you don’t have a water vessel, you can boil water in the ditches or pools found near a lake or stream bank. This will require building a fire near the river bank, however.
• To be safe, it’s recommended to boil the water for at least one minute, especially at higher altitudes, since water boils at a lower temperature there.