The winter of 2015 I went to the desert to cleanse, to observe the time of rest and reflection that Nature so steadfastly provides. It was a time to give thanks for the previous year and usher in some excitement for the year at hand. Well, on the first morning after arriving at my peaceful spot in a coyote-laden canyon beneath a snow-capped peak, my cooking propane ran out! As I considered my options, I decided to use the materials available on the land to build a rocket stove. This is the story of how I proceeded and the lessons that I learned about rocket stoves.
The spot provided all of the materials to build a stove from cob, which is sometimes called adobe. I like to call this building material MUD, glorious MUD! Pre-contemporary people in many places around the globe figured out that they could build shelters with a mixture of clay, sand, and a fibrous material like straw. The proportions of ingredients used to make the mud building material, the method for assembling it, and the additives used to tweak its adhesiveness, hardness, color, etc… vary greatly. Lots of combinations work! It’s fun to play with the materials and just get a feel for what works and what doesn’t.
First, lets talk about the materials that I found to make my mud.
MATERIALS
Clay
Clay is a dense, smooth, sticky substance that comes in many colors. Geologists have figured out the compounds and minerals that comprise clay and have studied the microscopic interactions of clay components that explain why clay hydrates with water and then becomes brittle when fired. For someone out in the desert, looking for clay, you don’t have access to microscopes or electron spectrometers or fancy shit like that. So instead, it helps to know that when water is mixed with soil that contains sand, clay and silt, the sand will settle to the bottom, the clay works its way to the surface, and the organic silt will separate the two layers.
At my camping spot, I had observed that there were puddles of water in the parking lot where the tires had repeatedly mixed the soil with the standing water. Since the parking lot is not busy, the puddles sat and the clay in the soil had risen to the top where I could just scoop it off. I collected a nice bucket of this buttery, sticky clay.
Buttery clay
Dry clay flakes
Clay cracks
Clay has very fine particle sizes and will seem to dissolve readily in water and will feel smooth on your tongue.
I have also found clay in other locations where there is seasonal water and where animals have congregated. The animals tromping in the water holes mixed up the soil in the water and allowed the clay to separate.
Sand
Sand is tiny rocks. The best sand for making mud for building is jagged rather than smooth. Sand that has been formed by water erosion is often quite round. It is not ideal, but it will work. In my situation, there was an abundance of colorful and beautiful canyon sand. I could see how the bends in the creek acted to separate the different granularities of sand. The higher and outer edges of the dry creek had coarser sand, and finer sand was below the rocks in the dry creek bed. It was easy to collect some sand that had a nice variation from fine to coarse.
Fibers
The fibers in a mud mix perform some important functions. They give the mix tensile strength so that when the dry mud has external forces applied to it it is less likely to break. Also, the fibers give body and thickness to the mix to make it easier to build up because it retains its shape better when wet. It also provides a texture to the mix that acts as a rough surface that is ready to receive subsequent layers of mud, particularly thin mud mixes that are used as plaster.
I have heard that not all plant fibers and plant material are created equal or equally desirable in the mud. Leaves, sticks, and seeds don’t really work. I have been told that they rot, but I don’t have direct experience with this. I was told that fibers that are hollow, like straw, are best.
Lucky for me, there was an abundance of cow manure at the site, as it is a free-range cattle ranch. Manure contains fine, pre-shredded stalks of grass. The poo also gives the mix a nice stickiness.
Next, let’s talk about mixing the materials…
MIXING
To make the mix, just experiment. Typical mixes contain about 1 part clay to 3 parts sand with a decent amount of straw and enough water to incorporate it all together. Some guidelines to follow are:
- Don’t use too much clay. Clay will shrink as it dries and can cause cracks.
- Use as much sand as you can. When a baseball-sized handful of mix compressed in your hand sticks together and feels very solid but not crumbly, you have a good mix! You can also drop your baseball-sized sample onto a soft surface like a lawn. The sample should hold its shape.
- If you smash your sample ball into the palm of your hand and invert it, the pancake should not really “defy gravity.” If the mix sticks to your palm too readily, you likely have too much clay or perhaps too much water.
- Add enough water to make it workable, but not so much that it slumps. You want to be able to sculpt it.
- Add enough fibers such that the mix doesn’t want to pull apart too readily, but don’t add so much that it gets crumbly. Be sure to incorporate the fibers evenly.
- If you have enough time, make some bricks with varying recipes, let them dry completely, and test them. Test to see how flakey they are, how much they shrank, whether or not they cracked, how much force it takes to break them, etc… Choose the recipe that was least flakey, did not shrink, and did not crack or break.
Since there is so much regional variation between clay and sand, I have read that anywhere from 15% – 50% clay could work in your mix. It is more important to feel it and test its behavior than to use a someone else’s recipe.
On to the the features of what I built: a rocket stove…
FEATURES OF A ROCKET STOVE
So, there are many resources (e.g. Rocket Mass Heaters by Ianto Evans and Leslie Jackson, or richsoil.com) that describe what a rocket stove is. I will just make a quick summary here. A rocket stove is a particular shape of stove that encourages a fire to initially burn horizontally and turbulently and then to burn increasingly hot in an insulated heat riser. The combination of the turbulence and the insulation means that the fuel for the fire burns completely as do the secondary combustion gases. When constructed well, it does not smoke; in fact, it releases mostly water vapor as exhaust! The materials used to construct the stove or the type of fuel it burns do not make it a “rocket.” It’s really the shape of the fire and the way it burns that make it a rocket. Also note that the heat generated by the “rocket” can be put to use in different ways. The heat can be used to cook food, heat air, heat a dense material like a cob bench, heat a hot tub (my favorite!), etc…
The basic shape of a rocket stove is a horizontal burn tunnel connected to a vertical heat riser. The fuel for the fire is fed into the end of the burn tunnel that is called the feed tube. The ratio between the size of the burn tunnel and the heat riser is important. For a rocket stove to work well, it has to have a strong enough draw to suck the fire horizontally, but not so much draw that it sucks the fire out. Ianto Evans invented this design. He says that a ratio of a burn tunnel that is 1 part wide to 2 parts long to a heat riser that is 3 parts tall works best.
Following are the pictures and observations that I made as I proceeded to make and tweak my rocket stove.
BUILDING MY ROCKET STOVE
I started out by snapping some sticks to help me lay out the shape of the components of the rocket stove.
I decided to make a bit of a foundation for it, even though I knew that the cattle would probably stomp on the stove as soon as I left!
I excavated a little for the foundation, put some gravel down, and locked it in with sand.
My sticks layout
Burn tunnel = 1 wide : 2 long
Layers of my “foundation.”
Completed “foundation” ready to receive insulation in the form of wood ash.
Next, I added a layer of wood ash. Wood ash is insulative. For a rocket stove to attain complete combustion, it needs to reach very high temperatures. Higher temperatures are reached when you insulate the burn tunnel, and especially the heat riser. I had access to wood ash in the fire pits. Awesome! I opted to put a layer of ash across the bottom of the stove to act as a thermal barrier between the fire and the ground. My hope was that more heat would make its way up to my pot of potatoes and parmigiano reggiano than heat the ground and that my fire would be hotter, burn more completely, and be less smokey.
Wood ash
Sand in the cracks
Next I put a base layer of mud and worked on sculpting up the overall shape. I glommed in big pieces of rock to provide structure for the ceiling of the burn tunnel. Note that I could have built the entire stove with mud rather than mud and rocks, but I did not have enough time to apply multiple layers of mud, waiting for it to dry between applications. I needed some warm grub in my belly! It was freezing at night! I also used a little steel in the ceiling of the burn tunnel cause I had it.
Burn tunnel “frame”
Base mud layer
Burn tunnel ceiling
Adding heat riser
Heat riser fin for turbulence
Rather than get too fancy with insulation, I opted to just insulate the bottom and the top of the burn tunnel. I made a moat on top of the burn tunnel and filled it with ash and eventually sealed it in with shingled rocks.
Taller heat riser
From above
Moat from above
Moat from the side
Moat with ash
Fired up!
I was surprised at how well the wood ash worked. When the fire was raging, I could rest my hand on top of the burn tunnel with the wood ash insulation. It felt a bit warm but was not close to burning me. However, the uninsulated sides of the burn tunnel were extremely hot! It was so hot that I heated up rocks to take to bed with me by propping them against the side of the burn tunnel! If I had had enough time to incorporate wood ash on all sides of the burn tunnel and heat riser, I would anticipate quite a boost in efficiency.
As I used my stove, I was quite pleased. It was easy to start, and it boiled water in a reasonable amount of time. I was happy to be eating hot food!
Fired up!
Nevertheless, I started to notice some issues:
- The size of the kindling matters a lot. If I used sticks that were too small, they burned really fast and were hard to move forward. I say forward because the correct way to load wood into a rocket stove is to add wood to the “back” of the fire, furthest from the heat riser. The small sticks also fell down quickly and would fall into the burn tunnel. Eventually the ash would build up, and air intake from the front of the burn tunnel was constricted. When this happened, the fire did not burn as well and tended to want to just burn up the sticks and out the feed tube instead of being sucked into the horizontal burn tunnel. This phenomena is called “back burn.” Also, if I used sticks that were too big, they would have a tendency to burn out. I had to use medium-sized sticks and feed them down and forward constantly.
- When I really got the fire going, the flames of the rocket would lap up out of the top of the heat riser. In retrospect, I think this was probably just fine for my situation, but I decided that I wanted to try a taller heat riser, thinking that a taller riser would give the fuel more time to burn and increase heat before hitting the bottom of the pot.
So, I made some modifications…
- I made a trap door on the bottom of the fire pit so that I could control the vertical level of the fire, keeping it mostly starting at the bottom of the burn tunnel. I really liked this. It was much easier to keep the fire raging at a consistent pace.
- I made the heat riser taller. When I did this, a couple things happened.
- The draw was definitely greater. In fact, it was large enough that it was harder to start the fire. When trying to start the fire, the draw would suck the fire out, something that did not happen with the shorter heat riser. I found myself covering the heat riser to reduce the draw and also moving the fire to the far end of the feed tube/fire pit. The draw was greater right on the edge of the burn tunnel, so I could get the fire started better further from that spot.
- The stove could accommodate a full load of larger sticks and put out a raging amount of heat!
- The stove did not work as well with smaller sticks. This made it a little less flexible because one nice feature of the earlier version of this stove is that it worked better on wood that was more common and easier to collect.
FINISHING TOUCHES
After all that work, I made some alterations that were mostly cosmetic.
- I removed the worn out mud feed tube and just stacked rocks to serve the same purpose.
- I shingled the burn tunnel with some lovely rocks.
- I embedded some rocks on the top of the heat riser on which to rest the pot.
Please keep in mind that mud will not stand up to rain (or cows). For this stove to last, it would need to be protected from the rain by a good roof. To protect cob from sideways wind, a natural water-resistant lime paint or boiled linseed oil could be applied.
FINAL REMARKS
Foraging materials and building this stove was a really fun experience. I’m sure that you will have a lot of fun doing your own Rocket Lab and will get a much fuller understanding of these concepts than I could provide in this blog post. Keep in mind that rocket stoves can be built and insulated with other materials than those that I had available. Steel cans, stove pipe, fire brick, holes in the ground, and other options can be used to form the overall shape. Alternate insulation materials include vermiculite, perlite, pumice, lava rock, and other space-aged shit that I don’t have laying around. 😉
I hope this post inspired you to do a lab, to make your own observations, and to be innovative as you troubleshoot your artful creation!