Friday, August 28, 2009

Twig Stove (aka rocket stove): how to build one in an afternoon

by Emma Holister

If the price of gas and electricity is getting you down, build a simple rocket stove and cook with twigs! It's free and it's sustainable. Cooking with a rocket stove is another big step towards freedom from extortionate energy bills and the multinational giants ransacking our planet and our bank accounts.

A rocket stove can be used outside in the summer, on your terrace or balcony, or indoors in the winter, hooked up to a chimney with a hood. Because of the insulated interior, this stove gives off far less smoke and carbon monoxide than a normal fire. Only when igniting and putting out the fire is there really any smoke worth speaking of. This simple design creates a strong air suction that makes the cooking flame very intense with just a few twigs. Using bigger and more twigs gives a stronger flame which will heat your pans faster than a gas hob. You can reduce the flame by reducing the amount or size of the twigs.

My rocket stove is very rudimentary. I've never done metal work before so I was as new to this as the next person. You'll need to forgive the unintentional advertising for the sunflower cooking oil...the 25 litre / 5 gallon tin was perfect for my stove! In Europe it's quite difficult to find this size barrel and you may need to go begging at the back door of your local restaurant for their empties. If you are worried about the paint on the barrel then put it in a fire to burn away the thin layer of paint. However, once I got my stove going, the main part of the barrel, being well insulated with ash, doesn't get hot at all, only the top edge gets charred.

In the photo below you can see the 25 litre tin of sunflower oil I found dumped next to the public garbage bins in my neighbourhood. It needed a bit of a scrub to remove the excess oil and a bit of rust that had started to build up. Inside this barrel will go my old iron stove pipe, one straight piece and one elbow piece, that I got out of my cellar. It has a diameter of 13cm (approx 4 inches). In the video the chap uses a thin tin stove pipe that he then changed his mind about as it can release toxins. My stove pipe is a tough old iron thing that had to be cut with an electric metal disk saw. However, cutting it with a metal handsaw would have been possible, had I been brave...

The tools you'll need:

Leather gloves to protect your hands - without them you will gash yourself!
A felt tip pen for marking where you'll cut
A chisel and hammer to make the first hole in the tin before cutting
Tin snips to cut the tin barrel
A metal file to smooth off the rough edges of the cut tin
Either an electric disk saw for cutting the iron stove pipe or a good metal hand saw (and plenty of patience)
Pliers to grip, bend and squeeze the tin into shape
A ruler, tape measure or piece of wood to measure the length of stove pipe

The first thing to cut in the barrel is the bottom hole where the wood will be fed into the stove pipe. This is done easily by using your pen to trace around the diametre of your stove pipe placed 5cm (approx 2 inches) above the bottom of the barrel. This gap is to prevent the base of the rocket stove overheating. Next, take the hammer and chisel, hold the barrel so it doesn't slip, give the chisel a good whack in the centre of the traced circle to create a hole in the tin, then work the tin snips into that hole, cut towards the edge, then all around the edge till the hole is complete. Cutting slightly inside the traced line will give you a neater, tighter fit with the stove pipe, if it's too small you can always make it bigger, but not vice versa.

After cutting the bottom hole, it's time to cut the top of the tin off...If you're not sure of cutting a straight line all the way around, trace one first with your pen using a stick or measure to guide your pen evenly.

The edges of the tin will be very rough and must be filed down carefully so as not to cut your hands.

The tin snips can sometimes get a bit stuck so I'd occasionally pull the tin apart with my pliers so as to make cutting easier.

And back to filing off those nasty rough edges...

Once the edges were filed, I wanted to create a rounded-off top edge and so I set about bending the edge of the cut tin inwards, gripping only about four millimetres of tin for the inward folding flap.

I then squeezed that narrow flap down, creating a fold in the tin that will not cut the hands. This top ridge needs to be pretty even so check it with a level if you're a perfectionist - which I'm not - I just gave it a whack with a hammer here and there where it went up too high and checked by eye that the cooking grill sat evenly on it.

And here below, an exciting moment....slotting in the stove pipe juts out of the tin's exterior by about 4 to 5cm (measuring at the top edge).

Here's the view of that exciting moment from above...

Then I slotted in my straight length of stove pipe onto the elbow. I wanted the central pipe to sit about two or three cm below the rim of the outer tin, so I rested a thin length of wood from the outer edge to the central column, inclined it down the pipe a couple of cm and marked that point with a pen.

Then I took that same thin strip of wood, drew a little line on it the same length for the stove pipe I'd marked, and then continued drawing the line around the pipe using the length of wood and its marker to keep the line even all the way round. A tape measure could also have been used, but I was too lazy to go and get it out of my tool box....

True to my usual methods, this was all guess-work. When fitted in place the entire vertical length of the inner stove pipe, from its top edge to the base inside the elbow piece is 32cm.

The line I drew was rough but accurate enough, thanks to guiding my hand with the length of wood... I then got David to cut it with the electric disk saw while I held it in place. The disk saw gives off a lot of sparks and you must protect your eyes with goggles as well as using gloves and keeping your hands well away from the cutting area when you hold or clamp the pipe down so it won't move during the cutting.

Once it was cut I slotted it into the elbow piece. So this inner pipe sits a couple of cm below the outer edge of the rocket stove so that the flames don't get blocked by the bottom of your saucepan.

Just in case, I got David to file off with the disk saw the enamel and paint where the pipe comes through the bottom hole because I was in two minds about filling the slightly uneven gaps (I'm no expert with those tin snips!) with some type of soldered metal....but in the end, the ash insulation is tightly packed and doesn't fall out as long as you don't knock the stove around. In this photo below you can see how the final stove pipe looks with elbow and vertical piece slotted together.

Now I filled the gap between the stove pipe and its outer tin casing with ash from last winter's log fire, being sure to pack it down tight and push it firmly under the elbow pipe.

I filled it to a level that left a depth of about 8 or 9cm from the outer tin's top rim to the surface of the ash.

Then I hunted around for the top piece of metal to slot over the stove pipe to keep it snugly in place along with the ash. The original lid of the tin exterior was no I grabbed another lid from an old tin of paint and cut that up instead. Again, if you want to get rid of the layer of paint on the can, burn it first in a fire.

I measured the diameter of the rocket stove's outer tin and cut out a circle from the tin lid with the same diameter. Luckily for me, the interior line of the lid was almost exactly the same diameter so I didn't need to draw a line. Had I not had that stroke of luck, I'd have put the rocket stove onto the lid, traced a line around its base, and cut the circle a few milimetres inside the line to compensate for the difference in diameter between the outside of the tin and the inside.

I left three little flaps to fold down and give the lid a bit of extra anchorage. I filed down the rough edges and with the pliers bent the outer edge slightly downwards so that it would curve down into the ash instead of curving up where it could get caught or scratch.

I then placed a piece of stove pipe in the center of the lid, using my pen as a measure to gauge the length between the outer edge and the pipe, making sure it was even all round and therefore centered properly. Then I held it firmly and traced a line around the pipe for where I was going to cut the hole.

Here below is the final traced line, ready to cut out. I placed it in the rocket stove just to check I hadn't completely bodged the diameter, it's a rough job, but good enough.

Out come the chisel and hammer again. I whacked a hole in the center... my tin snips out again and cut the circle out...

....filed off the rough edges and slotted it into place over the central stove pipe. This keeps the pipe in place and holds the ashes down. I did a rough job (I'm a beginner after all) and there is a little ash around some of the edges still, but this doesn't affect the efficiency of my stove at all.

Now the final touches, putting in the stove's twig shelf. You cannot just put the twigs straight into the stove pipe without a shelf because they need to be raised in order to allow air to flow underneath, as with a vent, to get the fire going. I could have used the tin top but I used another scrap of tougher tin I found lying around. This shelf has to be long enough to hold the twigs but not so long that it intrudes into the bent part of the pipe's elbow, that's where the flames belong.

I cut it with folded-down edges so that it could be wedged into the pipe quite firmly.

I gave it a final tap with the hammer to push it in snugly. I should really have allowed part of the shelf to jut out to give the twigs an extra platform, but as I use bricks to support the ends of the sticks I didn't bother.

And hey presto, the stove is ready to fire up - that is, once you've put the oven/barbecue grill on top and removed the curious cat...

This pot is perfect for two reasons: one, it is black, so when the bottom gets blackened by the flame it doesn't show, and two, it's cast iron, which retains the heat. So once you've got your soup or rice boiling you can remove it from the fire and place it in an insulated box where the heat gets trapped and the soup continues cooking. An insulated box can be made by putting one smaller wooden box on little legs inside a bigger box, filling the gap with ash (cold ash, naturally) or sawdust, boxing it in, and covering with a lid that is also insulated. This way you don't need to stand over your rocket stove for long. You cannot leave a rocket stove to cook the way you can with gas or electric hobs. Rather than seeing this as a disadvantage it might be worth thinking of all the lives that can be saved, as leaving pans on the hob is one of the biggest causes of house fires. If you leave a rocket stove unattended it will simply go out after about five minutes. Therefore, learning a few tricks with insulated passive cooker boxes and keeping your blackened utensils away from other dishes, will make life with the rocket stove a lot simpler.

David had to get cracking because I wanted my dinner...he made me a marvellous curry...

The twigs can be set alight by placing a handful of straw in the pipe's elbow, adding more straw if they are slow to take. With long sticks, bricks can be placed underneath them for support. When you want to put the twigs out, have a little pot with sand handy to plonk them into.

And finally, below is a nice view of the flames from above...

NB . . . be wary of who your vocabulary belongs to . . . a 'Rocket Stove' is nothing more than a Twig Stove and does not belong to a corporation even if said corporation insists it does . . .  This is simply an ancient traditional design used since forever across the globe, for example the Tandoori oven which uses the same J shaped furnace.  Beware of those brandishing ownership (copyright) of words and designs.


RC Hassel said...

Thanks for the step-by-step. I'm about to make a rocket stove myself for an upcoming camping trip.

21-20 said...

Thanks Emma. I'll be trying this soon — and now to look at the rest of your stuff ... looks great!