(Part 4) Generating Heat
In all the many, many ways there are of generating fire there are remarkably few ways in which heat is generated. In fact there are only five that are normally used:

1. Friction. This is where “kinetic” energy (the energy of movement) is converted into heat energy by two or more objects rub against one another. Even friction through the air can generate intense heat – think of a meteor falling through the atmosphere.
2. Chemical. Many chemical reactions are “exothermic” (they give off energy), particularly oxidisation. Some of the commonest methods of generating heat involve rapid oxidisation.
3. Solar. This is the process of collecting and concentrating the heat of the sun. It is normally accomplished with a lens (refraction) although the process can be achieved with a parabolic reflector (a curved mirror shaped a lot like a satellite dish)
4. Pressure. A gas that is rapidly compressed heats up. If combustible material is in the presence of this hot gas, it can catch fire. This is how diesel engines work.
5. Electrical. Stored or generated electrical energy can be used to cause a spark or to heat an electrical conductor to generate heat. A hot torch bulb is electrical energy generating heat.

Lets look at some examples of heat generation and see how they map:

The authors fire drill set

Friction:
Fairly obvious – the fire bow, fire drill, fire saw and fire plough are all examples where rubbing two pieces of wood generate heat. Its worth noting that the process of fire by friction generates wood “dust” that heats up slowly. The volatile gases are gone before enough is generated to ignite, so a small coal of glowing carbon is created. It is often helpful to have this collect on something (a leaf, some leather) to transfer it to your main tinder.

Chemical:

Flint and Steel. A flint and steel is a combination of friction and chemical energy. The sharp hard flint strikes a small shard of carbon steel from the striker which, being heated, burns in the air (oxidises rapidly). Its this oxidising steel that causes the spark. Its worth noting that it doesn’t actually need to be flint – any really hard material will work, jasper, diamond – even a carbide blade.

Ferrocium rods (Swedish Firesteel etc.). A ferrocium rod is an alloy of rare earth metals (predominantly cerium and lanthanum) and some hardening materials (predominantly iron oxide). This soft alloy of metals catches fire easily when struck off (around 200 degrees centigrade) and burns in the air the same way that the true steel spark does. Because its much softer than the steel, far more sparks are developed and a wider variety of strikers can be used (knife spines, steel striker etc.). The nature of the ignition is the same as flint and steel – a shard of metal struck off and heated oxidising rapidly in the air. Where a lighter contains a “flint”, it is in fact a ferrocium rod.

A spark cast by ferrocium rod

Matches. Matches light by rubbing the head of a match on a striking surface. The match head of a “strike anywhere” match contains sulphur, glass powder, an oxidising agent and red phosphorous. When rubbed on a rough surface, the glass powder turns the red phosphorous to white phosphorous. White phosphorous burns in the air and the heat causes the sulphur to burn in the air. Safety matches have the red phosphorous on the striking paper not in the match head.

The heat flare from a match head

Chemical combinations. Various chemicals can be combined to cause fire. I’m not going to cover that here for safety reasons since most of these reactions are highly dangerous and unstable.

Solar:
Clearly heat is already present in sunlight. The task is to concentrate enough of that heat in one place to cause ignition. There are two practical ways of achieving this – refraction and reflection.

Refraction. Refraction is the bending of a wave when it enters a material where it's speed is different. The refraction of light through a shaped lens will focus the light. If a large enough lens is used on a bright enough day, a tinder material can be ignited. Whilst a pocket magnifying glass or fresnel lens can be carried to provide an inexhaustible supply of heat, its worthy of note that a lens can be improvised from many sources – water filled condoms to shaped ice have been successfully used in the past.

A couple of lenses used to make fire

Reflection. Any shiny surface will reflect light, but to achieve ignition, we need to reflect lots of light onto a single spot. To achieve this, the reflective surface needs to be shaped into a dish like form. There are tools on the market that do this, but one of the most interesting ways it can be done is to polish up the indentation on the bottom of a soft drinks can to a high shine. The reflective surface needs to face the light source (the sun) and the tinder put into the light focus that will be in the centre and in front of the curved reflector. Holding the tinder there can block light though so it should be supported on a narrow wire or twig. I have heard of this being achieved using a headlight reflector.

Pressure:
Okay, I’m not going to get in Boyles law (or Charles Law for that matter). Suffice it to say, compress a gas in a small space rapidly and it heats up. The only practical tool for using pressure for fire lighting is the fire piston – a piston that seals tightly into a cylinder using a gasket or “o” ring. A sharp slap on the end of the piston rapidly compresses the gas inside. Include a small piece of tinder on the end of the piston (chagga fungus or charcloth) and it will be heated enough to glow. Difficult to improvise but an interesting device.

A fire piston loaded with chagga

Electrical:

Piezoelectric. There are several ways to use electricity to create heat. Perhaps the most common is the piezoelectric. Crystals acquire a charge when compressed, twisted or distorted are said to be piezoelectric. This effect is used in certain lighters to ignite the butane gas they contain.

Electrical heating. Passing an electric current through a narrow wire will cause it to glow – this is the principle that makes an incandescent light bulb work. If the filament is hot enough and the right material, it will burn. Rubbing both terminals of a battery across very fine (0000 or 00000) steel wool will cause the filaments to heat. The wool will begin to burn like a large number of steel sparks. A square 9v battery has both terminals on the same end which lends itself to this (be careful burning metal is very, very hot). This can also be improvised in a survival situation using a car battery and jump leads or even by cannibalising a torch. Sparks which will catch a volatile tinder will be developed.

"Burning" wire wool