A new toy & the physics of fire by friction

A new toy & the physics of fire by friction

Well it was back to school this morning for our regular forest school visitors. This week we are looking at fire and this mornings session kicked off with a bout of making fire by friction. Following a demonstration of how to make up the fire sets & how to use them to make an ember, the kids and teachers had a go. Although there were are number of failures there was one really good ember created by the group.

This is where we rolled out our new toy; an infra-red thermometer. For those of you who are familiar with the process of making fire by friction, you may have been told various different temperatures for the point at which a pile of blackened dust begins to coalesce into an ember (or ‘coal’ as they are sometimes called). With an infra-red thermometer it is possible to tell exactly how hot it is.

This morning the group used fire drill sets which consisted of Western Red Cedar hearths boards, Hazel drills & a mixture of Oak & Silver Birch bearing blocks. The ambient air temperature in the woods this morning was a pleasant 15-18 degrees centigrade, but the question was; what temperature would a ‘coal’ be?

Friction fire lighting using the bow drill
Friction fire lighting using the bow drill

Having made & then warmed up the fire sets, the group went for it and made lots of dust. Some drills popped out of the bows at the critical moments, some dust piles just weren’t big enough, some were the wrong types of dust (ie not blackened enough), but one pile of dust had the magic and began to coalesce.

A coal emerging at just 32 DegC
A coal emerging at just 32 DegC. (The red dot is the laser pointer & not the a glowing ember.)

Initially the dust was a fairly cool 32 degrees C & we weren’t sure if it would be successful, but the thin trace of smoke coming from the pile was telling a different story. Within a minute the temperature at the top of the pile of dust had risen to a steady 79 degrees C & with a little wafting could be encouraged to more than 100 degrees C. This was actually a bit of a surprise as we had usually believed that to form a ‘coal’, the dust would be hotter than this; somewhere closer to 100 degrees C to start with.

79.7 DegC & a glowing ember can clearly be seen
79.7 DegC & a glowing ember can clearly be seen. The red laser dot is on the top of the ember.
With the coal smoking away happily, the group then transferred the coal into the waiting tinder bundle (todays’ was straw & a little reed mace). Having gone through the process of igniting a tinder bundle before, the group made short work of this and before long we had a burning tinder bundle sitting in the fireplace just crying out to be turned into a fire.

Blowing a tinder bundle into flame.
Blowing a tinder bundle into flame. Note the smoke at knee height where the tinder bundle has just been while she was breathing in.
One last opportunity for the infra-red thermometer! The burning core of the burning tinder bundle was 540 degrees C, which given that it started out as a 32 degrees C pile of dust on an ember pan just moments before is pretty remarkable.

We’ll be using the infra-red thermometer again during our fire lighting sessions and other activities. If we find that the temperature of ‘coals’ is much different during winter and summer we’ll let you know!

Let us know if you’ve used an infra-red thermometer when making fire by friction. How hot were your ‘coals’ and what woods did you use? We’d love to hear your stories.

Chris & the Forest Skills Team

Let the adventure begin!

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