Simple retrofit to make your earthen oven burn cleaner now.

My “rocket door” retrofit is simple, made of earthen materials, will make for cleaner burns, and

-> You can add it to any earthen oven without modifying the oven!
-> I have been using this with my earthen/cob oven with great results, tested and working!
-> You can make it yourself with materials you have around in a few hours spread over several mornings.

You are just adding 3 removable parts to the door opening.

Ultimately and ideally, we will all be making biochar via clean-burning approaches when we use our ovens. At some point, I’m going to be figuring out how to heat my tiny cob house (in the making), heat water, and cook with home-grown biomass, making biochar as I go. But for now the “rocket door” is quick and easy and it gets me to better burning in my standard-format earthen oven with minimal fuss.

Here is how you do it.

There are three parts to the: Tube, Arch, and Plug.

The earthen mix is whatever sticky clay you have locally available and some fine sifted sand, mostly clay. Make a few small test batches to see what approximate mix ratio of clay to sand will dry up without shrinking a lot or cracking. Don’t worry, it is not fussy, just find your own ratio. You will need a few days to a week of down-time for your oven as each piece needs to dry in place before the next is made.

Make the Tube first. For a mould, use a split branch, a bit of dimensioned lumber, a square-sided bottle, or anything else that is elongate and will allow you to shape clay over the top of it to form a tunnel that extends about 1/3 to 2/5 ths of the way into the center of the circle defined by your oven cavity from the outside edge of your door. Photos show this piece. Note the floor of the tunnel is just the floor of your oven, so the tunnel is really just a deep groove in the tube piece. The tunnel opening is a few inches wide and tall, depending on the width of your door, but this is not fussy either, just use the images as a rough guide to proportions. Remember to put some waxy leaves, wax paper, etc. over the mould and on the oven floor so your tunnel mix won’t stick. Put the mould down, then shape the clay around it, and let dry in place. It is good if the outside end of the tunnel flares a bit to allow you to grip it, to allow it to seal against the arch even if it shrinks a bit when drying and to keep you from pushing it in too far when in use. There are no exact dimensions to share here, just use the images as a rough guide to proportions.

When the Tube is dry (might be a few days), leave it in place, and cover its outer upper surface for a few inches of its length with waxy leaves, wax paper, etc. so the Arch will not stick to it or to the oven floor. Build the arch on top of the tube to exactly fit the outside of your oven opening (to the outside of the chimney opening if you have one). Yes, the Arch will shrink as it dries, pulling away from the opening. Don’t panic, it’s not fussy, a perfect seal is not needed, let it do its thing. Note as you build the arch that you need to leave an opening at the top to reach through to the inside to shape the back. Great! The third piece will fill this hole as needed, or if your oven does not have a chimney opening, this hole becomes the outlet for exhaust/smoke. As you are shaping the Arch, it is good to form into it some ridges or pockets on the outside face to use to grab onto it. One of the photos shows what looks like “nostril” openings in my arch, which do not go through, they are just two folds/pockets to allow me to grab the arch with both hands while wearing oven mitts or welding gloves etc.

When the Arch is dry, cover the top hole surface in the Arch with a no-stick protector, as before, and form the Plug to fill the hole. It is good for the Plug to have a bit of a “tongue” or “tail” sticking inward as a counterweight to keep it in place. I shaped a heart into mine to make it easy to grab and remove with thumb and forefingers of both hands. Let dry in place.

Be decorative and sculptural with all of them! I like to leave mine in the oven as they look really cool, storing the traditional wooden door plug to one side.

When you fire the oven, load the oven with fuel as you usually do, but leave a little channel of space from the oven opening in towards the oven center to allow the tube to extend inward. You can also just load fuel as you usually do, then move burning materials a bit to make this space when the fire is going. When the fire has taken, put the Arch in first, then insert the tube. Add the plug if your oven has a chimney, leave the top hole gap open if not. You can now adjust the burn by adjusting how far in the tube extends, or removing it completely. To add fuel, just pull out the tube and load through the lower gap of the Arch. When the Arch’s surface is too hot to touch with a bare hand, your oven walls are fully charged! Now you remove all three pieces WITH OVEN MITS OR FIRE GLOVES as the pieces will be hot hot hot! Clean out the fire as you usually do and proceed with cooking using your traditional door OR you can put the three parts of the “rocket door” back in and let them radiate heat also for your pizzas. Note that with each firing, you are hardening the “rocket door” pieces, making them very tough!

Disadvantages of this system are that the “rocket door” parts are heavy and a bit awkward to handle, and these door parts are added thermal mass to heat with your fuel. I think the improved burn offsets the latter, and the former is no big deal really.

Note I use this every time I fire the oven, quickly getting past black smoke to smokeless burning. Last time I fired, I decided to “acid test” the system with a very nasty piece of wood. It was the size of a pineapple, had some dirt and punky wood in a small tree hole portion of it, and part of it was not completely seasoned. This should have been a smoke bomb! I added it to the fire after the first set of coals was going inside, getting it nice and centered where the Tube opening would be close to it, let it just catch (yeah, is smoked a lot just before catching), then put the “rocket door” together with the Tube all the way in. You could look in and see the blast of air drawn in getting that sucker glowing right away. Smokeless burn in about 30 seconds! Great roaring sound and sunlight-orange glow down the Tube, lots of fun! Try it yourself, report your discoveries and results, thanks, and Enjoy!