This blog has been using the campfire as the central theme, exploring the science surrounding it, however until now I have not explored why the structure of the campfire itself is important. I will look at some of the most common campfire structures, their purpose, and the science behind each. Every campfire follows the same basic fuel progression, first the tinder, then kindling, then logs, increasing in size with each level. As a general rule, tinder is small and fluffy, kindling is sticks no thicker than your fingers, and the logs should be around the thickness of your wrist. When choosing kindling, make sure it is dry all the way through otherwise you will struggle to start your fire. An easy way to check is to try and snap a piece. If it breaks easily, it is dry, if it bends it is too wet.
When making a fire, generally the tinder is placed at the bottom, with the kindling and the logs above it. This is because heat rises, so the most flammable materials at the bottom provide the heat needed to ignite the kindling which in turn provides the heat needed to ignite the logs. With this in mind, it is important that you place your fuel close enough to allow the heat to spread, however you want to leave enough space to encourage airflow into the centre of the fire to provide oxygen.
The most basic, and most important campfire structure is the tipi. I say the most important, because nearly every other type of campfire is built around a tipi, and tipis can be used at any scale, from an indoor fireplace up to a bonfire. As the name suggests, a tipi campfire looks like a native American tent. When making a tipi, you want to start by balancing the driest pieces of wood you can get together in a conical shape, planting a forked stick upright in the centre will help hold it upright. The hollow in the centre is where you will put your tinder when you come to light it, so you will need to leave a gap in the sticks to do this. In the picture above, the ‘door’ is at the front. This fire burns tall, providing plenty of light and heat, however it is not really suitable for cooking as all the heat is concentrated in a relatively small area above the fire. This can be used to quickly boil water or heat a pot suspended above it though. At some point the initial tipi shape will collapse as the wood burns, so care should be takes that this does not spill embers onto anything (or anyone) around the fire.
The Star fire is another simple design, built by placing logs like the spokes of a wheel and building a tipi over the top. This design is mostly used for social fires as it gives off plenty of light and heat, and can be kept fuelled by simply pushing the logs further in as they burn.
The Log Cabin fire, as the name suggests, resembles a traditional log cabin. It is made by building a tipi fire then stacking logs in a square pattern around it, as shown in the picture above. This fire doesn’t burn as high as the tipi, however due to its larger area and flatter design it will create an even bed of embers that is ideal for cooking over. You may need to dig a small trough to allow access to the tinder for lighting.
The Inverse fire breaks all of the rules of fire making. You start by making a layer of logs, then a layer of kindling on top of that, and a tipi on the top. The idea behind this fire is that the fire relies on gravity to pull the bed of embers down, igniting each successive layer as it goes. This fire, if it works properly will create a bed of embers similar to the log cabin, however in the initial stages the fire will be exposed and therefore vulnerable to the wind and rain. There is also the issue that the heat transfer will not be as effective as much of the heat will escape upwards.
Finally, the Lean-to. This is possibly the easiest fire to build, and one of the few that doesn’t require a tipi. A lean-to is built by placing a large log across the direction of the wind. Kindling is then leaned against the sheltered side with tinder placed underneath. This fire is incredibly resistant to wind and as the log is against the heat from the beginning it is an effective way of lighting very large logs. This fire won’t burn as high as the tipi or as evenly as the log cabin, but is still a very useful method to know.
Having looked at the different structures, let’s now look at how they work. Heat is transmitted in three ways, Conduction, Convection and Radiation. Conduction is the transfer of heat through direct physical contact. This is used particularly in the inverse fire which relies on heat being conducted from the embers to the fuel below it, but plays a part in all fires as when fuel is added it will come into contact with embers or heated logs which will transfer heat. Radiation is the transfer of thermal energy through rays, which is how we feel the heat from the sun or a fire. Radiation is used in all fires, as the heat radiating from the fire pre-heats the surrounding fuel, allowing it to catch fire more easily.
Convection is the transfer of heat through the flow of liquids or gases. It is from this that the saying ‘heat rises, cool sinks’ comes from. The heat energy excites the molecules of the liquid or gas, causing them to move faster and reducing the density of the heated area. Due to the lower density the heated area will begin to rise, drawing in cooler liquid or gas to fill the gap it leaves behind. This creates a chimney effect and is what causes the draught often felt near bonfires. Convection is also the reason the air is hotter above a flame than next to it. This can be safely seen in candles. If you hold your hand above it, the air is hot but if you hold your hand next to it there is little change in temperature. The diagram above illustrates the convection cycle. Most stoves and fire places have been designed with this effect in mind, designing chimneys to effectively draw the hot air away from the fire and increase oxygen flow. The portable storm kettles have also been designed to take advantage of this effect. The kettle base is placed with the hole facing into the wind to encourage the cool air to flow in, while the opening at the top allows the smoke and hot air to escape, heating the water in the surrounding tank on the way. These kettles are also known as volcano kettles as the fire inside can occasionally become intense enough to escape out of the top!