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Fire: The Magic Element


Without fire, man would have remained a rather unremarkable animal in the African landscape. It was fire which extended our waking hours beyond sunset, sterilized food that might have been contaminated or spoiled, and, perhaps most importantly, allowed us to change the properties of the materials around us. Wood became hardened, bone was more easily broken to expose the nutritious marrow within, stone became more easily fractured. Fire's heat allowed us to venture north into climates that would otherwise have been inhospitable. But as important as these initial applications were, even they pale in comparison to the new materials which came out of the fire. Clay became pottery, ash became soap, sand became glass, and various minerals became metals. We owe more of our material culture to fire than to any other single phenomenon.

Despite its importance, most modern people know little about fire. Three things are needed to make fire: a fuel, air, and enough heat to get things going. Let us consider the combustion of wood as our first chemical reaction:

To fully understand this process, we need some basic chemistry.

The Chemistry of Wood and Air

There are very few pure substances in nature. Dirt, rocks, sticks, bones, and even air are complex mixtures. In order to fully understand chemical reactions (like the combustion of wood), we need to be able to classify materials according to their purity. Consider the following figure:

All Substances
Heterogeneous SubstancesHomogeneous Substances
SolutionsPure Substances

Heterogeneous vs Homogeneous Substances

The first distinction to be made is between heterogeneous and homogeneous substances. Almost everything in nature is heterogeneous, that is, its composition is not uniform. Wood is a good example. If you look at a cross section of a piece of wood, the rings are immediately apparent. These rings are alternating bands of light and dark colored wood which were laid down as the tree grew. This variation in the composition of the wood from one place in the sample to another is what makes it heterogeneous. If you further examine the wood under a microscope you will find that even within a single band the wood is non-uniform, with fibers composed of chains of tiny boxes; the remains of living plant cells. Wood, then, is heterogeous through and through as is typical for natural substances.

A few natural substances are homogeneous, that is, their composition is uniform. Air is an example of such a substance. If you look at a sample of air, its composition is the same from place to place within the sample. There are no areas of different color or density and this uniformity extends even to the microscopic level. Of course, if there is a fly in the sample, it is technically heterogeneous since the fly is in one part of the sample and not in another. And we could have heterogeneity at the microscopic level if there is dust suspended in the air. So the distinction between heterogeneous and homogeneous substances is a relative one, with wood and dirt and flies at one end and clean air at the other.

Is it possible to "purify" heterogeneous substances to render them homogeneous? It is with dusty, fly infested air. All we need to do is to pass it through a filter. The flies and dust (still heterogeneous) are left on the filter while the now-homogeneous air passes through. The task is much more difficult with wood, however. We could grind it to a pulp and stir the pulp until all the non-uniformities are blended out. But we are not likely to be able to break up all the microscopic structures and so wood pulp, while more homogeneous than raw wood, is not quite as homogeneous as air.

Now is a good time to test your understanding of this distincion. Classify each of these substances as heterogeneous or homogeneous:

  1. soil
  2. charcoal
  3. seawater
  4. wine
  5. glass
  6. quartz
  7. blood
  8. bone

Solution vs Pure Substance

Among homogeneous substances, there is a further distinction to be made between homogenous mixtures (or solutions) and pure substances. While both have uniform composions, the composition of a solution is variable while that of a pure substance is fixed. Wine, for example, is a solution of water, alcohol, and a few other substances. The composition is uniform throughout any particular sample, but variable from one kind of wine to another. Air is another example of a solution, being composed of nitrogen, oxygen, argon, water, carbon dioxide, and tiny amounts of other gases. While the composition is uniform, the relative amounts of each component vary from one sample to another. Air going into a campfire, for example, contains more oxygen than (filtered) air coming out.

By contrast, the composition of pure substances is both uniform and fixed. Pure water, for example, has the same composition whether it was isolated from fresh water, seawater, wine, or even campfire smoke. Even "pure" water is a relative term. No sample of water (or anything else) is 100% pure. For our purposes, 99% (99 parts water, 1 part something else) is pretty good, but for some uses 99.999% (99,999 parts water, 1 part something else) is required. Wood pulp is "mostly" cellulose with minor amounts of minerals and other compounds.

Separating solutions into their pure components is not as simple as filtering. While there are many techniques for purifying substances, we will use only two: recrystalization (discussed under potash) and distillation (discussed under alcohol). For now, try classifying these as solutions or pure substances:

  1. charcoal
  2. seawater
  3. wine
  4. glass
  5. quartz

Compounds vs Elements

The final distinction, among pure substances, into compounds and elements is the most difficult for the beginner to grasp. Indeed, up until the end of the eightenth century it was not entirely clear whether water was a compound or an element. The historical development of this distinction is fascinating but beyond the scope of our work here. We will bypass this historical development and simply define a compound as something whose chemical formula contains more than one kind of element. An element is defined as something which appears in the periodic table. That said, let us consider the chemical formulae of some common pure substances:

What do the subscripts in these formulae mean? They give the relative amounts of the elements in each substance. For each "part" carbon in cellulose, for example, there are two parts hydrogen and one part oxygen. The exact meaning of a "part" in this context will be left until we discuss stoichiometry. For now, we will familiarize ourselves with using these formulae to discuss chemical reactions.

The Chemistry of Fire

Recall our earlier equation for the combustion of wood:

We can now discuss this reaction in more detail. Most of the wood is cellulose, CH2O. The air is a solution of (mostly) nitrogen and oxygen, O2. During combustion, the nitrogen is unchanged and so the air coming out of the fire has the same amount of nitrogen as the air going in. We include in the chemical reaction equation only those substances which were changed during the reaction:

This equation says that when cellulose burns in oxygen, one part oxygen is consumed for each part cellulose, producing one part carbon dioxide and one part water as products. The (s) and (g) designations tell us whether the substance is a solid or gas ((l) will be used for liquid).

What about the ash and smoke? It may surprise you to know that the complete combustion of cellulose produces neither ash nor smoke. But wood is only mostly cellulose. In addition, wood contains small amounts of non-flammable minerals which are left behind as ash. And if there isn't enough oxygen around for complete combustion, fine particles of charcoal will be produced in the form of smoke. In fact, if we heat wood in the complete absense of oxygen, we get a different chemical reaction:

In this reaction, solid charcoal is produced instead of carbon dioxide gas. Please note that charcoal and ashes are two completely different substances. Ashes are a mixture of minerals which will not burn. Charcoal is (almost) pure carbon and can be burned:

It should be apparent that RXN 1 is the sum of RXN 2 and RXN 3.

So fire can produce two different solid products, depending on whether the conditions are oxidizing (oxygen rich) or reducing (oxygen deficient). A fire which has plenty of air will burn to ashes, which are white or gray, non-flammable, and composed of a mixture of minerals. A fire deprived of oxygen will produce charcoal, which is black, flammable, and composed mostly of the element carbon. A fire which has some air may produce a mixture of ashes and charcoal, but these are easily separated. Charcoal is insoluble in water and because the air pockets present in the wood are preserved in the charcoal, lumps of charcoal float to the surface of water. By contrast, some components of ash dissolve in water, while the others sink to the bottom. These properties will be exploited in the potash project.

Fire is an exceedingly complex and fascinating phenomenon. Some chemists' entire careers are devoted to the study of flames. You may still have many questions about fire: "Why does it produce heat?" "Why does it produce light?" "Why do some flames have different colors?" But for now these questions must remain for us deeply mysterious. For now, lets move on to some practical work with fire, our most important technological tool.

Great Fire Links

The Fire Quiz

The fire quiz consist of three questions on any of the following topics discussed in this page.

Project Description

Evidence for the human use of fire goes back at least half a million years. Direct evidence for fire making is much harder to pin down, since the materials used decompose rapidly. But fire making is widely believed to be a relatively recent (10,000 years) innovation. For the vast majority of human prehistory, fire was preserved rather than produced. Fire may have been obtained from time to time from natural forest fires, but survival depended on being able to keep the fire going in the mean time. The phrase "keep the home fires burning" was a literal imperative for prehistoric tribes.

In order to make fire, you need heat in addition to wood and air. Perhaps in the course of making wooden tools it was observed that two sticks rubbed rapidly together became hot even to the point of ignition. Perhaps in the course of making stone tools it was observed that occasional sparks could be used to set fire to flammable materials. Eventually, both methods were refined and used even into historic times.

You will make fire by friction in this project. Generations of cavemen have found that this is most easily done as a group. Each member of your tribe must have passed the fire quiz prior to attempting the project.

This project consist of making fire by friction. For this you will work in "tribes" of three or four people. You will need a cord, a spindle, a fireboard, and a socket. You may make your own or they may be provided by your instructor.

The cord is simply a five foot length of rope or leather thong. The spindle and fireboard are the most critical elements of the fire kit. They are usually made from the same kind of wood: poplar, tamarack, basswood, yucca, balsam fir, red cedar, white cedar, cypress, cottonwood, elm, linden, or willow. If you are making your own kit you may need to experiment with different woods (see below). The spindle should be about half an inch in diameter, about one foot long, smooth, and straight. It should be rounded at the top and have a concave indentation carved into the bottom. The fireboard should be about an inch thick, two inches wide, and two feet long. It should have several starter holes carved into it and each hole should have a notch cut into the side like an upside down "v." The socket will be used to hold the top of the spindle steady as it is turned. The socket can be made of stone, bone, or wood and should have a smooth hole fitted to the top of the spindle.

Begin by fitting the spindle into the holes in the fireboard and socket. It should turn smoothly and easily. Now wrap the cord a couple of times around the spindle. Give one end of the cord to one persone and the other end to another. Place your foot on the fireboard to hold it steady and hold the socket with one hand. Slowly at first, have the two people holding the cord coordinate with each other as they pull it forward and backward in a sawing motion. The spindle should rotate smoothly. As you get the hang of it, you can increase the speed that the spindle rotates and the pressure you apply with the socket.

After a few minutes, remove the spindle and notice that the bottom end has become hot. You may take turns holding the socket and pulling the cord with other members of your tribe. Once you get the hang of it, you will notice wisps of smoke rising from the fireboard. If you have constructed your notch correctly, hot sawdust (called punk) will begin to tumble from the notch into a little pile. Keep the spindle turning faster and faster until the smoke is thick and red embers appear among the hot punk. Stop now and blow gently on the embers to increase the oxygen available for combustion. These "live" embers can now be used to start a fire. Simply place these embers on a bed of very dry grass and blow gently until a flame is produced.

There are two competing variables with which you will need to experiment as you perfect your technique. The more pressure you apply to the socket, the more friction is produced between the spindle and fireboard. But this also makes the turning of the spindle more erratic. You should strive to apply only as much pressure as is required to keep the spindle turning smooth and steady. Gradually increase the speed as long as it the turning remains smooth and controlled. Long smooth strokes will turn the spindle faster than short erratic ones.

If you are using a fire kit for the first time, you may also need to consider your choice of wood. If you succeed in getting smoke but not embers, examine the punk produced from your fireboard. It should be black and smell of charcoal. Try lighting a bit with a match. It should light easily and produce an ember which stays lit and gets brighter as you blow on it gently. If your punk does not have this quality, you may need to try another kind of wood.

Here are some pictures of Jason Creech, Antonio Bedford, Lou Bryant, and Wes Whitaker re-enacting their firemaking technique on December 16, 1996. They had actually made fire outside a few minutes before this picture was taken. They used an elderwood fireboard, a mullin spindle, and a maple bow with a sinew cord. Each person took a turn at bowing. After about 30 minutes of trial and error they got the technique down and with about 5 minutes of bowing, a pile of embers formed at the side of the hole in the fireboard. These embers were gathered and placed in a wad of tinder (paper and cotton) and blown gently into a flame.

making fire

glowing ember

Criteria for Success

Either you have fire or you don't, right? Well, the production of live embers takes considerable practice but is not beyond the abilities of a dedicated tribe working with a proper fire kit. To pass this project, your tribe must pass the fire quiz and show at least enough firemaking skill to produce smoke. Your instructor will inform you whether live embers are required to pass the project.