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The Science of Fire

Our little UK group is in the process of writing a book of essential skills for our kids etc. I know whats in here is probably old news to a lot of you guys but we strongly believe in sharing what we have found, so I hope this may be useful to someone.

(1) Making Fire
Ever wondered why if you blow on a candle it goes out, but when a blacksmith blows on a forge it gets hotter? Why when you light birch bark with a spark it flames but a cramp ball just glows? Wood shavings catch fire more easily than branches? Why a chimney makes a fire burn better?

I have been making fires for decades and, whilst I had discovered that these things were true, I had no idea why they were true. I decided that an examination of the fundamentals of how fires actually work would help me in my fire lighting efforts. This is the result.

What is Fire?

The Oxford English Dictionary defines fire as “the state of burning, in which substances combine chemically with oxygen from the air and give out bright light, heat, and smoke.

There are some key words in that definition:
Substances (fuel) – the thing that burns

These things form the fire triangle.

In order to be able to make fire well, we need to understand the properties of each of these three elements and learn to manipulate them well. Then we bring all these three elements together and apply some fire making technique. The result? The ability to make fire under challenging conditions by manipulating the tools available.

(2) The Elements of Fire
This bit is dull, no I mean really dull. But to get good at firelighting, it really helps to understand it. If you can stick with section 2 (or at least read the bluffers guides) it will get easier (promise)

2.1 Understanding fuel
It is worth noting that wood contains three main components that we are interested in:

All wood contains water. Green wood contains a lot, dry wood less. But it all has some.

Cellulose – Tree cells are made of cellulose. When cellulose is heated it changes. Up to about 200 centigrade, carbon dioxide and water vapour are given off. Above this temperature, pyrolysis takes place. In this process, volatile gases are given off that catch fire. These burning gases give off heat causing a chain reaction and the fire gets hotter. The fire gets hotter and at 450 centigrade, the carbon left behind by the pyrolysis starts to burn. The wood goes through three stages burning

1. “Smoking” stage as the water and CO2 are driven off

2. “Flame” in which the volatile pyrolysis gases are driven off, burn, and raise the temperature

3. “Carbon burn” in which the fire reaches a high enough temperature to generate glowing embers. Wood can be converted to carbon (charcoal) by heating it without oxygen. This prevents the pyrolysis gases igniting

Minerals - this is the non-burnable stuff like calcium, and potassium – this makes ash.

(Bluffers Guide)
To start a fire, heat wood up, it gets rid of water and (non flammable) CO2 at the lower temperatures. Once you have got rid of this, it starts to give off burnable gases. Once you get it hot enough (around 200 centigrade) these gases will ignite. When they ignite, you get heat given off for the first time. This is when the fire “takes hold”. The heat given off by the gases gives off more gas, which catches fire in turn giving off more heat. When the fire reaches 450 centigrade, the carbon left from the pyrolysis will catch fire and glow as it burns without smoke or flame.

2.2 Understanding Heat
It is worth explaining that in the process of combustion, heat is both an input and an output. Heat exists in combustible material (fuel) in the form of chemical potential energy. That is to say that energy is stored in the chemical components of the fuel and can, given the right circumstances, be released. We all know this – you burn wood to generate heat. Heat is also an input in that for the process to take place, the right materials must be present and the temperature raised to the point at which the reaction begins. This is known as the process of ignition. In burning wood, we raise the temperature of the wood to initially drive out water and then to the point at which the gases being to burn. This releases more energy that causes the carbon to burn.

This explains why it is in some ways easier to light dry wood than charcoal – there are no pyrolysis gases in charcoal to burn. However since charcoal also doesn’t need to have the water driven off, whilst the temperature needs to be higher, the actual amount of energy that needs to be expended may be lower. We’ll come to that.
Now its important to understand two things here:
The amount of input energy required to achieve is proportional to the amount of material being heated. It takes more energy to raise the temperature of a larger object. If the same amount of energy is input to a large object as a small one, it will raise the temperature less – possibly below the point of combustion. So its easier to light small things than large ones!
When an object is heated to a point beyond its surroundings, it will begin to lose heat to its surroundings (through a variety of mechanisms such as convection and conduction). If an object is heated too slowly, it will lose heat at an equivalent rate and never achieve ignition.

(Bluffers Guide)
You have to heat up wood to get it to burn. The bigger it is, the more heat this will take. The wetter the wood is, the more heat this will take. You have to heat carbon fuel (charcoal etc.) to a higher temperature than wood to get it to burn (which takes more heat) but you usually don’t have to drive the water off (which takes less heat)

2.3 Understanding Air
It is worth understanding that the available oxygen usually limits the process of combustion. Adding oxygen will often cause a fire to burn hotter and faster – the process that makes blacksmiths bellows work. This must be tempered with understanding that you can blow the fuel away. In blowing out a candle, the gas (vaporised wax) is removed from the heat source and the candle goes out. Blowing hard on glowing charcoal will make it burn faster since the charcoal is unlikely to blow away. Blowing too hard when fuel is flaming (burning vaporised hydrocarbons) may extinguish it, blowing on glowing coals probably wont.
Now air can be added to the mix in a variety of ways – by blowing, through mechanical means (bellows or fanning) or via scientific principles. Let’s take one of the most useful. The chimney. Heat rises. We know this right? So as a fire burns, the hot gases and vapours will rise up a chimney. This draws more oxygen into the fire at the bottom. In addition, if a wind is passing over a chimney, it lowers pressure. This lower pressure makes the chimney draw still more gas up through it and hence oxygen into the fire.

(Bluffers Guide)
All fires need oxygen. Forcing oxygen onto a fire based on gas risks blowing it out. Forcing oxygen into a carbon-based fire will probably increase it temperature. The best way to feed oxygen into a fire is to draw oxygen up through it (in the same way as a chimney does)
A hollow elder tube used to blow air into a fire

The effects of a chimney

(3) The Process of Firelighting
We have discussed at some length the elements of fire and how they interact. The next step is to describe a step by step process for lighting fires. All wood fires we make will become a variation on this theme. Having looked at the process in total, we can then examine each step in some detail.

The steps to lighting a fire:
1. Generate heat – there are many ways to do this, from friction to electricity but all rely on generating a relatively intense heat in a fairly small area.
2. Tinder ignition. A small, relatively volatile item is ignited from the heat source. The purpose of igniting tinder is to sustain the heat output from the heat source and increase the heat available by burning the tinder.
3. Kindling ignition. Kindling is perhaps best described as “small fuel”. Your tinder will not produce enough heat to ignite large fuel since its heat output will be low and relatively short lived. Kindling is therefore ignited (twigs perhaps) to built a slightly larger fire. Larger pieces of kindling can be added until enough heat is generated to ignite the main fuel.
4. Fuel ignition. When sufficient kindling has been ignited, enough heat will be generated to light the main fuel source. When this is achieved, the fire will become self sustaining and only require fuel to be added periodically rather than tended closely.

(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:

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.

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.

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

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.

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.

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.

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.

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.

(5) Tinder Ignition
There are many types of tinder and we’ll discuss a few of them. In general terms, tinder must be very easy to ignite with a minimal amount of heat. In order for this to be true, we should try to optimise certain characteristics. Tinder should be:
1. Bone dry. There is often not enough heat generated in fire lighting to first dry out tinder and then to ignite it.
2. Very fine. We discussed earlier that it takes twice as much energy to ignite an object twice as heavy. Having tinder in large lumps makes it hard to light. Even flat sheets are more difficult as the bulk of the tinder is not mixed with air. Thin fibres of tinder with air pockets interspersed with the fibres work best.
3. Highly exothermic, In other words, once ignited it should generate large amounts of heat. However the nature of making tinder fine, means there isn’t a huge amount of it, so ones fire should be built up lighting small twigs first.

Tinder can come in many forms. It can be
1. Naturally occurring
2. Fine or adapted fuels
3. Procured from readily available household materials
4. Purchased

Top Tip:
Its very important to match your tinder to your heat source. Trying to light a feather stick with a piece of glowing charcloth is very difficult. Igniting a nest of dry grass with it is easy though – and then igniting the feather stick with the dry grass is also easy. Generally tinders that “glow” because they are carbon based (cramp ball, friction coals, charcloth) are best added to another tinder that will flame (dry grass, clematis bark etc.) before attempting to ignite kindling (fine wood).

Naturally occurring tinder:

Birch bark. One of the most widely available and easily recognisable of tinders. Its great as it needs minimal preparation. Many fallen trees have paper thin bark already peeling away. There seems to be plenty of fallen birch logs around too. Pieces of ***** bark can be teased apart with the fingers too tissue paper thin strands or scraped into strands and powder with a knife. A good handful squeezed into a loose ball will generally catch a spark first time. Birch bark contains lots of tarry hydrocarbons (so much that tar can be extracted from it) so it burns very hotly with good flame. Shredded birch bark is very fine though – beware it blowing away!

Clematis bark: Clematis is a climbing plant with a soft bark that forms vertical lines. Its downy seed pods are instantly recognisable. The seed down will burn but generally absorbs water from the atmosphere and so needs to be dried before use. The clematis bark can be stripped away from the stem and buffed (rubbed between the palms of the hands) into fine fibres. If the plant is not dead, these need to be dried – but they dry easily in a pocket near the skin. They burn well and are great for developing flame from a piece of charcloth or cramp ball.

Dry grass or straw: Sounds great but actually very hard to find dry stuff in the wild. Again if dry(ish) stuff can be found and kept in a warm place for a while it gets much better. Coarser stems should also be buffed up to make finer fibres (especially straw). A handful of hay or straw from the middle of a bail is generally dry even if stored outdoors.

Fatwood: Fatwood is formed when natural resin (pitch) is concentrated in the centre of the stump of certain pine trees. This means that wood emits a large amount of volatile hydrocarbon vapour, ignites easily and burns very hot. Thin fibres carved from a fatwood stick can even be ignited from a spark from a Feroccium rod. Fat sticks are sold under a variety of names including “Maya” sticks.

Cramp ball: Cramp ball is a black vaguely round fungus usually found on dead or dieing ash trees. It looks and behaves like a light charcoal honeycomb. On the outside it is smooth and almost shiny and looks almost like a black animal dropping. On the underside it has concentric silvery rings. When ignited it glows like charcoal and burns for a very long time, although a thinner fibre based tinder or twigs are useful to coax flame.

Horses hoof fungus. The horses hoof (or false tinder fungus) grows on dead Birch trees. It has three distinct layers - a very thin crusty outer layer, a thin (1 or 2 mm) leathery layer of amadou and a thick corky spore layer. The crusty outer and spore layer need to be removed and the leathery layer either dried and roughed up with a knife blade or boiled in wood ash, pounded and fluffed which some claim improves the fire taking qualities.

True tinder fungus or chagga. Chagga fungus looks like a black lumpy burr on the side of a birch tree. Dried and crumbled it makes excellent tinder and is particularly effective in fire pistons.

Punk wood: Punk wood is the soft powdery wood found in the middle of rotten logs that is almost as light as balsa wood. It can often be found dry by knocking or kicking apart rotten logs. Be sure to dry it or dry out damp stuff before needed. It light very easily since its well mixed with air already. If charred like char cloth it will catch and burn from a cool spark.

Tinder made from fine or adapted fuels:

Feather stick: In making a feather stick, fine curls of wood are shaved from a dry stick leaving them attached to the main stick. The finer the curl, the easier they are to ignite – some practice is required to get good and quick at this. I have had excellent results by using an axe to split a wet log to get to the dry wood inside, chopping thing pieces of dry wood and then feathering them. A finely feathered stick can be lit with a match – a very well made one ignited with a spark from a ferrocium rod.

Butane: You may know butane as a stove fuel, but it is of course a vaporised hydrocarbon – its also lighter fuel! The heat source in a lighter is the piezoelectric spark or “flint”. Its butane vapour that provides the “tinder”.

Petrol: Petrol vapour (liquid petrol doesn’t burn – just the vapour) is what powers the trusty Zippo lighter.

Tinder procured from readily available household materials:

Cotton wool: The fire makers friend. Dry cotton wool will catch any old spark and burst into flame. Cotton is of course a natural plant fibre! I carry some in small “ziplock” bags. Its cheap, widely available and foolproof. What’s not to like?

Cotton wool and Vaseline: Like cotton wool – just better! Rubbing a small amount of Vaseline into the cotton wool gives a long lasting burn. A small ball will burn from 5 to 10 minutes! Vaseline is of course “petroleum jelly” – petrol! The cotton starts to burn and vaporises the jelly. It then acts like lots of candle wicks and the result is probably the easiest lighting, best burning, tinder that I know.

Drier Lint: A lot of people recommend this but I have to say, cotton wool is better (and only 99p for a big bag). Drier lint is the stuff from the lint trap in your tumble drier. The quality depends on what was washed though and there can be lots of other (non cotton) fibres mixed in. Stick to cotton wool is my advice!

Charcloth: Another of my personal favourites. Easily made by charring natural fibre cloth in an airtight tin with a small hole. Ensure you use only natural fibres (cotton or linen are great). Some are treated with a flame retardant so be sure you use those that aren’t. Charcloth is my tinder of choice with “proper” flint and steel but its good for lots of uses. Again, it’s a “glowing” tinder so to produce flame, its best dropped into a ball of fibrous tinder and blown into flame.

Olive oil: I have found that many tinders can be improved by smearing or dripping on Olive or other vegetable oil. I always have some in my Bergan for cooking and have noticed that a piece of cloth or cotton wool with oil rubbed in burns very hot and long – same principle as cotton wool and Vaseline – plenty of easily combustible hydrocarbons in oil – the Romans used it as lamp oil after all.

Charred wick: I use a lot of oil lamps. I’ve noticed that a wick that has been burned before lights very easily. A small piece of charred lamp wick can be used to catch a spark and transfer the heat to another less volatile tinder. It can then be extinguished and re-used. Again it can be improved with a little oil.

Candles: Many survival instructors advise using a candle to start a fire. The one advantage these have is cheapness! “Tealight” type candles can be had for about 2p each and only cotton wool and petroleum jelly can equal that for cheapness. Two top tips.
1) If using tealights, light them at home and blow them out – charred wicks light more easily
2) Try using the “trick” birthday cake candles that don’t blow out!

Candle wax and sawdust: I have heard of people using homemade firelighters made from candle wax and sawdust (with or without embedded wicks). I see no reason why they wouldn’t work, but for me, if going that far, I’ll take a packet of firelighters.

Wire wool: Wire wool works very well as a firelighter and burns very, very hot. Ideally you want the fine stuff (0000 or 00000). It lights from a batter or spark and glows like a bulb filament – literally red hot. It wont flame so have another tinder or fine kindling available to catch the heat – it doesn’t last long. Interestingly a few drops of added oil work well again!.

“Hairy” string: Good old fashioned garden string is just vegetable fibres (usually jute). I keep a small hank in my pouch. Its good for firelighting when teased apart plus shelter building and washing lines!

Prawn Crackers. Really. Want a bet?

Purchased tinder:

Zip firelighters: Yeah I know, hardly “Bushcraft”, very, very handy though when its hammering down, you are cold and wet and so is the wood. I carry them. I can make fire underwater if needs be. So? It takes longer and why be uncomfortable, I don’t carry a first aid kit because I plan to use it, but I use it if I need to. Zip do some really nice individually wrapped ones that save stinking your pack up. Shave pieces off finely and they will light with a spark.

Hexamine block: No real difference to firelighters – esbit and hexamine blocks are solid fuel for stoves. If you are running low though, using a small piece of one to eke your fuel out by starting a fire makes sense.

Wet fire tinder: A commercially made firelighter. Comes individually wrapped in small bags and will burn floating on water. Good stuff but pricey.

Shredded fire logs: Many supermarkets sell self-igniting firelighters and “firelogs”. Small pieces of these broken up burn very easily and make for a very cheap tinder source.

(6) Kindling Ignition
The type of kindling you will need depends upon the type of tinder you are using. If lighting a fire with a small amount of shredded birch bark, you will need very, very fine kindling and plenty of it. If using a couple of firelighters, 1” thick split logs will catch without a problem. If in doubt, use finer stuff than you think you need and more of it than you think you need.

Kindling is really just small wood. In the same way that your tinder should be dry, its important to find dry kindling. Start with twigs about the diameter of a match, you will need more than you think you need – a really good double handful when compressed. The thinner tinders burn up very fast – their purpose is to generate enough heat to light larger and larger types of kindling. People often think of kindling as a single size of fuel – it isn’t on a well made fire – its many sizes in increasing diameter. I use (roughly) these:

1. Long twigs about the diameter of a match
2. Thicker twigs – no thicker than a pencil or bark taken from a dead tree (especially birch)
3. Slim branches – about the thickness of an AA battery and or “feather sticks”
4. Thicker branches – about the thickness of a broom handle
5. Small logs or split logs – about 2” thick
6. Main Fuel

It wont always be possible to find a variety of tinders at your fire site. Many people collect any really good tinder they find along the way stashing it in a large pocket or rucksack pocket. Its also possible to make tinder by splitting larger logs with an axe – logs can be reduced to whatever thickness you require. If the log is old, splitting it will often provide dry kindling even if the outer log is wet.

The best location for tinder is often said to be “standing”. This can either be a dead tree or (more likely) fallen branches caught up in lower branches or shrubs. Living trees often have dead limbs too.
Beware wood on the floor in a wet are – its possible to wring water from a fallen log – this is useless for fire!

Can you see the "standing" wood?

More obvious - but will need splitting

Beware wood on the floor in a wet are – its possible to wring water from a fallen log is useless for fire!


(7) Burning Fuel

Selecting Fuel

Beechwood fires burn bright and clear
If the logs are kept a year
Store your beech for Christmastide
With new holly laid beside
Chestnuts only good they say
If for years tis stayed away
Birch and firwood burn too fast
Blaze too bright and do not last
Flames from larch will shoot up high
Dangerously the sparks will fly
But Ashwood green and Ashwood brown
Are fit for a Queen with a golden crown

Oaken logs, if dry and old
Keep away the winters cold
Poplar gives a bitter smoke
Fills your eyes and makes you choke
Elmwood burns like churchyard mould
Even the very flames burn cold
Hawthorn bakes the sweetest bread
So it is in Ireland said
Applewood will scent the room
Pears wood smells like a flower in bloom
But Ashwood wet and Ashwood dry
A King may warm his slippers by.

An old poem, but a good one. Ash and Holly burn green, beech and Oak should be seasoned (I quite like birch if well dried), elm is a poor burner. That said, for bushcraft and survival, the dry wood that is to hand is best! Its often more a matter of how you burn it than what you burn.

Preparing Fuel

Wet fuel will not burn, but it can be prepared. A log that is wet on the outside can be split and the dry fuel obtained. The wet outer layer can be discarded or added later when a lot of heat has been generated to slowly dry and burn. If stacked near a long lasting fire, wet fuel can be used as a reflector and dry in the process.

Laying a fire

Arranging the fuel
There are many ways to arrange a fire – here is my chosen way to get a fire going.
(1) Ignite your tinder

(2) Make loose tepee of thin kindling. Add plenty of kindling when the first layer is well and truly alight. Use your best and finest kindling first leaving plenty of air gaps between the twigs and adding more only when the finest are well alight

(3) Add thicker pencil shaped sticks until you develop a good “heart” of coals to your fire

(4) Start to build a “criss cross” of sticks and thicker fuel around the “heart”. Alternate the lay of the sticks to allow plenty of air in. This will eventually collapse into a large pile of coals but the fire will be self-sustaining by then.

Introducing air

A small cooking fire can be built as shown above. If fuel is damp or a longer term fire is needed, consider digging a 3” deep trench pointing towards the prevailing winds and igniting the fire on a layer of sticks laid across it. Build the “criss cross” above the trench and fire base and wind will funnel down the trench and be drawn up through the fire literally “fanning the flames”. If the wind moves a lot dig two trenches in a cross format and one or another will catch the wind.

(8) Fault Finding Guide

Volume of fuel to quantity of heat

When achieving ignition, the volume of fuel needs to be balanced with the amount of heat generated. If too much fuel is used, the amount of heat energy required to cause ignition rises beyond the ability of the heat source to provide it. The smaller the tinder, the easier it is to ignite. However, small tinder will give out small heat by combustion and so only tiny increases in fuel size will be possible without choking the fire
You cannot ignite a log with a match, but you can ignite some wood shavings. These can also not ignite the log but they can ignite twigs, which can in turn ignite sticks etc.

Nature of Fuel

Water content
If fuel is damp, sufficient heat will have to be applied to cause all the water to be removed before ignition can take place. If the heat source is not sufficient to drive off all the water, ignition will be prevented.
Lighting damp wood with a blow torch or firelighter is possible, but not with a spark.

Carbon / Hydrocarbon content
Different fuels contain different amounts of volatile hydrocarbons. Hydrocarbons ignite at a lower temperature than carbon and so can be ignited by a cooler heat source. However the hydrocarbons must be vaporised to ignite so the heat source must either be intense (very hot) or sustained.

The short-lived spark from a flint and steel cannot vaporise and then ignite hydrocarbons. It can however raise the temperature in a few fibres of carbon in charcloth enough to cause combustion. The hotter, longer-lived spark from a ferrocium rod, can ignite either however.

Shape of Fuel
In order to ignite quickly, fuel needs to have a large amount of surface area in proportion to its mass. Put simply it needs to be thin. Sheets (like birch bark) can work but thin fibres are best. This provides a good mix of air and fuel and minimises the mass you are trying to ignite.
It’s a lot easier to heat up a fibre of bark so that it burns than a whole sheet. As one or two fibres ignite, they will burn giving off more heat and igniting more fibres – this sets off a chain reaction.

I hope this guide is useful to some. Long as it is, it barely scratches the surface, however I have found it useful to sometimes take the “craft” from “firecraft” and put a little science in!