Explorations into Candle Stoves

Introduction

Wax is a hydrocarbon fuel with an energy density (by weight) comparable to other fuels like canister gas and white gas. It is therefore very similar in some ways to other fuels used in our little stoves, but it has the advantage that it is a normally solid: it won’t spill. Small tea-tray candles have been used for some time to keep food warm, but they are a bit too small to bring water to the boil in a reasonable time. A problem with just scaling a tea candle up is the soot generated. We explore here what can be done to improve the heating capacity and reduce the soot from a ‘candle stove’.

All the experimental work detailed here was done by Forum Reader Mark Hurd. Backpacking Light Senior Editor Roger Caffin offered some advice and wrote the article text, and accepts responsibility for any errors.

What’s Good

• Robust, won’t spill
• Can be refuelled while running
• Very cheap to make
• Stable on the ground

What’s Not So Good

• Low in power
• Generates considerable soot
• Generates acrid fumes while burning some waxes
• Generates lots of fumes after being extinguished
• Hard to light

Background Discussion

First of all, it is obvious that candle wax burns and makes heat. Small flat candles have long been sold and used to heat ‘chafing dishes’, and are often known as ‘tea candles’. But candle wax has one advantage over most other fuels except for those in the Hexamine class: cold wax is a solid and does not spill or leak in your pack. Compared to Hexamine-class fuels it appears to have extremely low toxicity as well. So for many situations it looks as though it might have some potential.

Just to reinforce this, we list here some properties of some the more common fuels we use, including two forms of wax – beeswax and paraffin. Be assured – there are many others.

Surprise: beeswax and paraffin wax are up there with the best! But in fact that is quite reasonable because they are just higher hydrocarbons, in the same general series as propane, butane, gasoline and kerosene. Incidentally, you should treat many of the values given here with some caution as the composition of many of the fuels can vary. (Just what is in gasoline anyhow?)

But it is also known that candles do have some problems: typical candles have very small flames and make soot. Can these problems be overcome and a more powerful candle stove created?

Flame Design

You might think that this section should be headed ‘wick design’, but that would be to miss the point. What we are concerned about is the flame, not the wick. We want a flame large enough to heat water at an acceptable rate, while not emitting too much soot. Sadly, at this stage it would seem that some soot is going to be inevitable, so we will try to minimise it.

One way of increasing the flame size is to make a bigger wick. In the photo to the right we show a ‘coil wick’ (picture courtesy zenstoves.net). This is a big coil of corrugated carboard embedded in the wax. In fact the coil almost fills the tin, even though it can be hard to see it all under the wax. This design is not going to work very well at all. For a start, getting it lit may prove almost impossible unless you use a small blow-torch to melt enough of the wax that the wick can light. Even if we assume a slight modification so the wick can be lit, it may make a large flame but it will also generate a huge amount of soot and lots of fumes too. This is because the centre region of the wick will be releasing lots of wax vapour but this vapour won’t get to burn very well: the outer wall of flame will effectively prevent any oxygen from reaching it. No oxygen means no combustion, but lots of soot (and maybe a bit of carbon monoxide as well).

The flame needs to have lots of sidewall where oxygen can mix in to support combustion. And both sides of the flame need to be open to the air: a simple ring will still leave the core without oxygen. This makes for poorer combustion and probably more fumes and carbon monoxide. So instead of a ring we look at a cross arrangement. The centre region of a cross is likely to be oxygen poor so we eliminate it. The result looks like the stove shown to the left.

Here we have a new candle stove cooling down from the casting process: the central pool of wax is still hot and clear. There are effectively four wide flat wicks here. The flat design of the wick means that the flame will be flat and will get as much air as possible. Having the four wicks out at the edge means there is minimal oxygen starvation in the centre. The four-wicks design is simple and possibly adequate in a can of this size: a larger can could have more wicks – perhaps.

How big a gap should be left in the centre of the wick? That is hard to say, and has to be judged from the flame pattern. The flames from this example may be seen in the first photo. They are perhaps a little high and smokey, but that seems inevitable with candle stoves.

Other designs are also possible: a tubular wick as shown below to the left has been tried and is called a Circle Wick. It is a simple circle of wick, as opposed to the coil wick shown before. The wick was made quite high to get good power, but there are costs to this as explained below. A Spiral Wick consisting of round ‘wick’ cord wound around the side of a light tube has also been tried, as shown to the right. This was meant to be similar to some alcohol stoves, but the results were not so good.

Testing the Efficiency

The testing discussed here and in also the Carbon Monoxide series of articles, plus the basic chemistry of combustion, indicate that having enough air supply to the flames is vital to getting the maximum heat from the fuel. So the obvious question is how can the air supply be maximised? In addition, note that maximising the air supply to the flame should minimise the amount of soot and fumes produced. Reducing the amount of soot is clearly desirable just from the convenience point of view: consider the deposit on the bottom of this pot after boiling half a litre (one pint) of water on the stove arrangement shown above.

But remember that the soot deposited on the pot is actually unburnt carbon from the fuel: it is wasted fuel and wasted energy. In fact it is the carbon in the fuel which supplies most of the energy (as opposed to the hydrogen), so the loss of power is quite significant. If all that carbon had been fully burnt in the flame instead landing on the pot as shown the stove would have given out more heat per gram of fuel. So soot is very bad for a number of reasons.

It is likely that fuel consumption will be a good guide to the efficiency of the air supply, so the three wick designs illustrated above were tested a number of times in various configurations. For all the tests results shown here the fuel used was beeswax. The reason for using beeswax is given later. The test conditions were the same for all three designs, thus:

The averaged results from several trials of each design using beeswax are as follows.

Clearly the increased air supply to the Cross-Wick design means that a lot more energy was extracted from the fuel, compared to the two very inefficient round wicks where the air supply to the inside of the flame varied from very poor to non-existent. However, the Cross-Wick design was a lot slower to bring the water to the boil compared with the other two. This happened despite the probable loss of some flame energy up the sides of the pot with the Circle-Wick and the Spiral-Wick designs. We believe that this is an inevitable result of wanting to get the best possible air supply, given a finite stove area. That is, the amount of flame surface area was smaller for the cross-wick design compared to the other two.

The Circle-Wick design allowed the height of the wick to be varied easily – with a pair of scissors. It was found that wicks of 1.5 cm and 2 cm length above the rim of the container gave about the same performance, with average boil times 6:18 min and 6:10 min respectively. A wick height of 2 cm did give a boil time of 5:58 min on one occasion. Reducing the height of the wick to 1 cm above the rim gave an 8+ min boil time. The wick to pot distance was kept constant at 1.5 cm for the rest of these tests.

The performance of each stove is fairly sensitive to the height of the pot above the top of the wick. Reducing the height from 1.5 cm to 1 cm seriously extended the time to boil – in some cases to beyond the patience of the Tester. This is explained by the reduced availability of air for combustion and the much poorer flame which resulted.

Operational Considerations

All of these stoves produce a lot of wax fumes. The fumes are very unpleasant and permeate everything around. They are of course a by-product of the boiling wax. An attempt was made to create a design which would allow the fumes to be burnt, but no suitable design was found during these tests. This may leave opportunities for further experiments by someone else.

The paraffin wax put out acrid eye and throat burning fumes. This does not happen with an ordinary candle, but then, an ordinary candle does not have a flame of the size seen here. The beeswax fumes are somewhat acrid, but mostly cloying. Beeswax candles have always been favoured by the more affluent in times past, and this is also why beeswax rather than paraffin wax was used for the tests reported above. However, beeswax apparently has a reputation of attracting bears: residual organics from the honey vaporising from the wax is one reason. Just the smell of the beeswax stove in your pack might also be enough to attract bears too: they have a very sensitive sense of smell.

As mentioned above, in general the stoves made a sooty mess of the pots, and if the soot included some wax condensate it was very hard to clean the pots afterwards. And of course the stuff makes black marks on anything it touches: hands, clothing, gear …

When making these stoves it is important to make sure the wick is saturated with wax. A dry wick is obviously not going to catch alight at all. But a fully impregnated wick has a fair thermal mass, which means that the stoves are all hard to light, especially the Spiral-Wick design. That one can takes several minutes of flame from a lighter to catch. This is because the flame from the lighter has to melt and then vaporise some of the wax before it can catch alight. The problem can be reduced if a thread or two from the wick is left sticking out from the main wick – provided it too has been impregnated with wax as well. If you do this you have something very similar to an ordinary candle wick.

Extinguishing one of these candle stoves is hard. The Cross-Wick design can be blown out – just, but the other two cannot be blown out. This means they have to be snuffed out. But while this extinguishes the flame (by removing the air supply), it does not stop the emission of vapour/fumes from the hot wax: that still goes on for several minutes. Now there is no flame the amount of fumes is of course even worse than before. The result can be a rather thick fog as shown here, and the fumes get into everything around the stove. In addition, the stove stays very hot for up to 10+ minutes, during which time the liquid wax can be spilt.

Material Sources

Waxes can be bought from most any craft shop and from the web, often by the pound. Sources are not given here.

The first wicks in the experiments were simply corrugated cardboard: as cheap and as simple as you can get. In fact they worked quite well, but they do have the problem that as the wax gets low the wick starts to burn. This makes a bit of extra smoke, which is not so nice. More sophisticated wicks use non-burning synthetics, although one could use cheap cotton tape as well (but it chars like the paper). Sources for synthetic wicks include:

wickstore.com — Lots of different kind of wicks including fiberglass. They sell it by the 100+ yard spool, but more importantly they also sell 10 yard hanks for US$7 each.
Spiral-Wick stoves were made with 1/8 inch and 1/4 inch round braided fiberglass wick, by wrapping the round wick material around a small cat food can, placing the wrapped can in the center of a large cat food can and pouring molten wax in the space between.

www.flammaaeterna.com (suggested by Forum Reader Jason Klass) — This site caters to “fire-oriented performance art”, such as dancing while juggling flaming objects (er – wow!). They sell flat Kevlar/Fiberglass tape wicks in various thicknesses and widths up to 4 inches. (As an aside: these wicks also make good insulators for pots and pot handles, which is what Jason was using it for.)
Two sizes of the K1 Tape Wick, 1/8 inch by 2.5 inches and 1/16 inch by 3 inches, were used for the Circle-Wick stoves. They are stiff enough to stand up by themselves. A length of the tape wick long enough to go around the inside circumference of a small cat food can was placed in position and molten wax was then poured into the can. The wick may be cut to the desired height above the rim, but should be soaked in wax to the top. Cutting the top edge of the wick leaves a raw edge on top of the wick which should be easier to light.