As I read more about rocket stoves, I came to realize how important they could be to the people in developing countries. As you may know women and children are spending many hours daily as they travel long dangerous distances trying to collect firewood. Also, millions die from the smoke pollution in their homes. The folks at Aprovecho are working hard to improve this situation around the world. While there are few stoves for sale online and even on amazon, this seems like an area for a lot of home inventors.
I've been thinking, these rocket stoves are cooking for hours using the amount of wood I might use as kindling just to start my oven. Could I redesign bread/ pizza ovens for greater efficiency? I decided to take this on as a project to be cast using castable refractory a material, like portland cement but with the ability to withstand very high temperatures (above 2000°F). This product is pretty expensive by itself (about $1/ pound) but can be mixed with perlite for a less expensive castable material. Many people are using low cost cob style ovens
This is the process I used.
1. Determine the castable ratio to use.
I made several samples of castable material using between 100% refractory and 20% refractory. Here are some findings:
While the 100% and 50% castings were very hard and durable, I found the 33% casting had ideal density (0.6 grams/ cubic cm) and excellent insulation qualities. Castings at 25% and 20% did not appear to have enough strength to be useable. As you can see from the photo, the 20% casting didn’t even hold together when I pulled it out of the plastic cup mold.
2. Determine the housing for the oven.
I started to see rocket oven housings everywhere. Let your imagination be your guide!
A portable barbecue could work great and has been done by Jon and Flip.
A large flower pot and a metal table could do the trick.
But since I was going to place this oven right next to my other oven, I decided to replicate the look and style of that oven.
I was in Cash and Carry, the restaurant supply store and saw a perfect mold for the oven dome: a 24” steel wok.
3. Foundation
Made the base out of 20 standard 8x8x16 cement blocks. Construction of this base was straight forward. If you don’t have experience with masonry, a book on the subject may be all you need. Standard mortar mix.
4. Casting the dome
Casting the dome was surprisingly quick (less than 30 minutes) once all the pieces were in place.
I put plastic over the wok and put a piece of 6” stove pipe in the center, in the event I need a chimney. I’m fairly confident the oven can vent out the door, but it is a lot easier to make the hole now rather than trying to cut the cast refractory later.
This is the plug in case I choose to not have a chimney. It was cast by pressing a little castable material down into a stove pipe and then pulling the pipe off.
The mix was 2 gallons of castable refractory (about half of a 50 pound bag) and 4 gallons of perlite. This photo is prior to pushing the mix into place.
I pushed the mix against the sides of the wok, flattened the top by tapping with a board and then carefully pulled the stove pipe out.
5. Creating the combustion chamber and chimney
The stove pipes and elbow were put into place. (much fiddling needed to put them together inside the blocks)
Mixed up a 33% refractory/ 67% perlite mix to fill the void around the stove pipe. Compacted it around the spaces with a stick and smooth the exterior.
Here is the un-molded dome sitting in the approximate final location. Stovepipe fits well as does the optional cap.
6. Creating turbulence fins
Since we are basically cooking inside a chimney, we must try to stir up the air as much as we can so all the particulates are combusted completely. Jon and Flip shared with me their work on making turbulence fins. While these can be made from broken pottery or formed from clay, I chose to cast them from 100% refractory. Easy to mold, very dense and strong while also thin. I cast them on a plastic covered plate and cut them into shape after about an hour of setting up.
These fins are glued into place with masonry adhesive in a circular pattern. The flue gas will spiral up the chimney, being forced to burn more completely.
7. Creating the oven floor
I chose to cover the cinder blocks with 1/4 inch HardiBaker cement board. It is thin, durable, and heat resistant. Since it can lose strength over 800°F, I cover it with a 40% cement/ 60% perlite mix about 1 inch thick and then smooth and level it. Notice the plywood form to ensure even thickness and level.
8. Walls
For the walls, I used 10 common red brick laid flat with three in front and rear and then two in the middle. On top of these flat brick, I put five fire brick on edge. While I’m glad to be using firebrick here because they are 4” tall instead of 3.5 inches, the main reason I’m using firebrick instead of common brick is they are what I had on hand.
I narrowed the opening with two more cut brick so there is room for the peel while minimizing heat loss through the front.
Here is the oven so far.
9. Small pieces
I need a deflector plate for under the hearth (so it does not overheat in relation to the rest of the oven) and another deflector plate for just below the chimney exhaust. These were cast with 100% refractory using some plastic circle I found on top of a sheet of plastic. Third picture shows deflection plate mounted with coat hanger in the chimney pipe area of the dome.
For the grate under the fire, I cut up an old barbecue grate using a saw-z-all.
10. Fill corners, test firing
The corners were filled with a mixture of 1 part portland cement to 6 parts perlite.
The test firing went pretty well once I got the fire going. I had an easier time once I used cardboard instead of just paper and sticks. The “rocket” sound is pretty cool when the draft gets going. The oven reached 450° in about 20 minutes which was so quick the pizza stone broke. I will cement it back together using masonry cement and also attach a refractory deflector plate to the bottom to take the brunt of the flame’s intensity.
11. Door
Since I wanted the door to curve, I had to create a mold to cast the shape. I used an old piece of corrugated plastic and drywall screws to construct the mold. Imbedded in the form is an old masonry trowel to form the handle.
12. Stucco and tile
Two layers of Type B stucco (rough textured) were applied along with some decorative elements.
Layer 1 of stucco and tile
Thank you
Thank you to those with information about rocket style ovens and stoves. One of the best was a lead I got from Kiko to Jon and Flip and their Haiti Rocket Stove Project. Great introductory video and useful photos. Thanks also to Dave at the Green Rocket Pizza Oven and of course Dr. Larry Winiarski.