the science behind the bowdrill fire set.

Tango3 · Jan 2, 2007

The Miracle of Fire-by-Friction

Starting a fire by rubbing two sticks together. Why do I always get a thrill out of doing it? Is it because there are probably less than 500 people in the United States who can consistently start a fire with a hand drill? Is it the entertainer in me? I don't know. I assure you that the thrill is not diminished by knowing more about the scientific events that go on during the process.

The objective of this article is to provide some scientific insight into the events which happen when two sticks are rubbed together to start a fire. In particular, why is it that some woods don't work at all, some work with great effort and others with relative ease. The principals discussed apply equally well to the fire saw, fire plow, hand spun drill or bow drill. Will it help you start a friction fire more easily or quickly? Probably not. Will it give you a deeper appreciation of the process? I hope so.

Basic Principles

You have to get the char, powder that is rubbed off the wood, heated up to about 800 degrees Fahrenheit before it will start glowing (ignite). I measured this by sprinkling char generated with a bow drill on a soldering iron heated up to a known temperature. Below 800 degrees the wood dust would give off a little smoke but that's all. Above 800 it would smoke and then start to glow. Anything that prevents the char from reaching 800 degrees will interfere with fire making.

Composition and Structure

By this I mean what kind of molecules is the material composed of and how are the molecules arranged? If there is any volatile resin or tarry substance in the wood, then as the friction heats the wood the tarry stuff will take heat away from the char (heat of evaporation) or will condense on the char and form it into a coarse gritty substance, preventing ignition. If the correct molecules are present and all the wrong molecules are absent, there is still a problem if the molecules are not arranged properly. Imagine your best hearth board and hand spun spindle which will twirl up an ember with very little effort. The wood will be very light, a very poor thermal conductor (a good insulator). Now put your hearth board and spindle in a vice and compress the wood to 1/2 its original thickness. It will be twice as dense and its thermal conductivity will be doubled. You can still twirl up an ember but you will have to work twice as hard because you have altered the structure of the wood. You have made it a poorer insulator and you have doubled the amount of muscle power needed to reach ignition. For a person with limited muscle power attempting to start a fire by friction, the use of low density wood is critical.

The simplest test for whether a particular piece of wood will twirl up an ember is the most obvious: try it and see if it works. A quicker test is to examine the char that is ground off as you twirl the spindle on the hearth board. The rule of thumb, literally, is to rub the char between thumb and forefinger. If it is coarse and gritty then reject that particular piece of wood. If it is very fine, like face powder, then you have a good chance of twirling up a fire. Both Kochansky and Graves mention this. What is the difference between these two classes of wood? Those that work and those that don't. We know that in the category of "good" woods there are soft woods, such as yucca, which can be easily dented with the thumbnail and hard woods such as sage brush which are much more resistant to the thumbnail test. Could it be that the "good" woods ignite at a lower temperature than the "bad" woods? That should be easy to measure. The straightforward way would be to measure the temperature of each tiny little particle of char as it is ground off the spindle or hearth board. Trouble is that it is very hard to measure the temperature of something that tiny without disturbing what is going on. The next best way is to measure the ignition temperature indirectly. Sprinkle some char on a piece of metal which has been heated to a known temperature. See what temperature the metal has to be heated to in order to ignite the char. As a practical manner I used a thermostatically controlled soldering iron as a source of known temperature. Tips with two different temperatures, 700 degrees F and 800 degrees F were available. I had observed previously that the char ground into the notch in a "good" hearth board would start glowing (ignite) if a pinch of it was placed on the 800 degree soldering iron tip but would not ignite if placed on the 700 degree tip. The conclusion from this was that if friction heats the char above 800 degrees it will ignite.
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The Miracle of Fire-by-Friction

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