The Miracle of Fire-by-Friction, Revisited

by Dick Baugh
September 21, 2007


The original article, published in the Bulletin of Primitive Technology, Vol 1, # 5, 1993, pp 69-70, included some data on the ignition temperature required for igniting the pile of wood dust generated by twirling one stick on another. The data was very crude in that only two test temperatures were available, approximately 700 and 800 degrees F. The results of initial ignition temperature measurements were:

a. The significant metric is the temperature of the hottest part of the dust pile just before ignition.

b. The ignition temperature is that minimum temperature which allows oxidation to take place at a sufficient rate to cause the temperature to start increasing.

c. Coarse wood dust requires a higher ignition temperature than finely divided wood dust.

d. It is impossible to measure accurately the ignition temperature of the wood dust with a thermocouple (TC) unless the TC wires are extremely fine and the TC junction is located exactly at the hottest point in the pile. Unless these criteria are met the measured temperature will be incorrectly low because the TC wires will conduct heat away from the location whose temperature is being measured. As a consequence I measure the ignition temperature by exposing dust pile to a known and controllable temperature.

The latest experiments on ignition temperature were done using a thermostatically controlled soldering iron to provide a known constant temperature. A small pile of prepared wood dust was placed on the tip of the iron and its temperature was raised by 25 degrees F increments every sixty seconds. The results are summarized in this table:

Test Sample Ignition Temperature
Mule fat twirled on Mule fat, coarse charred powder: 700 degrees F
Coarse Mule fat dust, not charred: 675 degrees F
Mule fat twirled on Mule fat, fine charred powder: 650 degrees F
Birch tinder fungus (small piece): 625 degrees F
Mystery fungus (small chip): 575 degrees F
Finely powdered Mule fat charcoal: 575 degrees F

Mule fat (Baccharis viminea or Baccharis salicifolia) is a shrub of drying streambeds in the western states. It is an excellent
fire-by-friction material. The Mystery fungus is a pale tan type I found on the base of a Eucalyptus tree.

Radiation heating can be another source of error in measuring ignition temperature. Imagine heating the test specimen inside an electric kitchen oven. The heating element is at a considerably higher temperature than the air inside the oven. As a consequence radiation from the heating element will cause the surface of the test specimen will be at a higher temperature than the air. This effect is greatly exacerbated because the charred wood dust is an excellent absorber of infrared radiation and it has very low heat capacity so little bit of radiation raises its temperature greatly. A similar situation happen when you expose a thermometer to direct sunlight. You are not measuring the air temperature, you are measuring the temperature of the thermometer augmented by solar radiation.

Any thermometer does an excellent job of taking its own temperature. The challenge for the experimentalist is to design a configuration wherein the thermometer temperature is equal to the temperature of what you want to measure. When the nurse takes your temperature he/she sticks it all the way in instead of just touching you with the probe. When, in my earlier career as an engineer, we had to measure and control the temperature of an aluminum cylinder to within 0.01 degree we drilled a long hole in the aluminum and immersed the temperature sensor in the hole and bonded it with thermally conduction epoxy. That way the heat flowing out the leads would have minimal influence on the temperature of the sensor.

Other data on spontaneous ignition temperature of solid wood, for example, "Ignition of Wood, a Review of the State of the Art" by Vytenis Babrauskas in Interflam 2001, span the range from 200 degrees C (392 degrees F) to 510 degrees C (950 degrees F). The wide range of temperatures quoted is because in many of the examples cited the specimens are also heated by radiation. I strongly believe that the most reliable and accurate method of measuring ignition temperature of wood dust produced in friction fire operations is by placing the wood dust on source of known temperature with no additional sources of heat.


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