Matt Todd

John Wilkinson wrote:Hi Glenn,
My desire to have the cob is significant partly due to aesthetics.
If I were to use concrete pavers are you saying I could mask them with a layer of cob?

Masking concrete block is exactly what I'll be doing this week. I also want to use cob, but that will come later in the season when things warm up. In the meantime, I don't want to look at blocks in my dining room so I'll be applying a product called lime wash. It bonds with the concrete but keeps open pores (unlike paint) which should make it still receptive to cob later on. If I like the lime wash look, I might even skip the cob!

There are recipes out there to make your own. I'm cheating and buying a ready made version from Lowes.

Rico Loma wrote:Matt , could you please give details about the 3 different types of wool listed here? Not just origin materials, but do they differ in technique of 'spinning' or manufacturing the blanket?   I feel a bit confused with part of the conversation.

Also you mention satanite as a method to seal or coat the wool surface.  Is this similar to treatment with water glass, and does any treatment have safe lasting effects,  in your opinion?

I couldn't tell you about the manufacturing process. All I know with certainty is that there's two broad categories: ceramic fiber blanket and ceramic fiber alternative. The first containing fibers that are lung hazardous and carry a cancer warning, and the "alternative" (mostly made by Lynn Manufacturing) is safer and does not have a hazard warning.

Waterglass (sodium silicate) is the first step in treatment of ceramic fiber (or alternative) products to "rigidize" them. It also helps to bind the fibers but does not increase overall durability.
THEN step two would be a refractory coating to add a hard outer layer using Satanite or one of the various other refractory coatings like Heat Guard which really seals the fibers in and makes the blanket or board you're coating more durable.

These are products intended for the forge industry but we are able to take advantage of them in rocket stove building. This is my first time using Satanite. It' rated for 3200dF and the highest temps we see in a riser are around 2100dF so I have no doubts that it will last just fine. The only time you might need re coating is if you use it in the firebox where wood abrasion might scratch at it over time.

Scott Weinberg wrote:
Mineral wool or not, I will agree with Fox James 100% about Microscopic fibers.

Right you both are. No wool in the cook box for me. If I want it cooler, I'll add an adjustable air vent.
I did mount a piece of ceramic fiber alternative baffle board to the inside of the oven door for insulation though. I coated it in satanite so that not even "safe" fibers can escape.

I appreciate all the comments on this thread. They definitely steered me in the right direction to a functional oven!

Documented here:
https://permies.com/t/368044/Shorty-Core-Oven-Build-Test

Cristobal Cristo wrote:Good job Matt!
Are you going to cover the slit in the floor (that exposes T-bar part) when you build your heater inside?

Yes, in fact the floor needs to be half an inch higher. I just didn't have the material yet so I built up the back of the firebox with some insulation (ceramic fiber alternative) to smooth out the transition into the port. And stuffed the middle gap too. Only for the test of course.

I'm going to get some ceramic tiles to line the floor for a very low cost bottom wear layer.

Coydon Wallham wrote:Would an oven like that have some sort of active temperature control?

Nothing precise, which is why the goal was to hit an acceptable range around 400ish with materials alone. Just to be in the ballpark of a baking temperature. That said, if it errs on the side of too hot I can add an adjustable vent to the oven door to release heat. To maintain temperature, add more wood. Which is something this core tolerates better than previous designs.

Peter van den Berg wrote:
The deviation from the airframe design might play up when the core is really, really hot.

I appreciate the notes Peter! Knowing this was not built to your tested specs, I was careful to build in a way that could be changed later if needed. I'll keep an eye out for struggles at high temps. While I don't have a testo, I did get a thermocouple and many probes so I can monitor multiple points. My threshold isn't as tall as yours, but I did leave an inch of airframe above the firebox floor to hold the ash in.  

thomas rubino wrote:
In my opinion, this build qualifies you to skip apprentice training and receive full rocket scientist status, with all the outstanding benefits that come with it!

I did build one successful DSR2 and a functioning but abandoned DSR3 core before this, so those may be my apprenticeship. But let me prove I can figure out masonry on the bell before making claims to any titles.  

After many years of study and help from this fabulous community, I am proud to present my Shorty Core + Oven build!

The big test firing was yesterday, and after a smoky start I recalled that all decent core tests need some sort of chimney stub to run properly. Slapping on some old HVAC duct got this thing firing perfectly and burning clean with the door fully closed.

My biggest deviation from Peter Van Den Bergs brilliant air frame design is in the intake. Where he doubles up the bottom square-tube to make an air-box, I risked a change to simplify the design by pointing the air intake downward and skipping the double frame member. The intake hole itself is slightly oversized so it gets plenty of air but could be partially blocked if it seemed like too much air (which appears to be a non-issue with Shorty.) All other size specifications for this batch box design where followed precisely based on the development thread.

I wanted an oven, so I reversed the riser exhaust direction to face backwards (which will be into the open void of the brick bell behind the core.) The 16x18 oxide bonded kiln shelf oven floor/firebox roof cost a lot but conducts heat perfectly, resulting in a floor temp of 500F degrees and an air temp of around 400F. I'll set a metal rack on the bottom to bake on. Oven door is insulated with a piece of ceramic fiber alternative board, also coated in Satanite.

Please see the picture descriptions for all the juicy build details. I'm happy to provide more details if requested.
Here is a video of the test burn: https://youtube.com/shorts/uIy8fhlbHoA?feature=share

Many thanks to Peter, Thomas Rubio, and everyone else who's been willing to post builds and answer questions that lead up to this. I will follow up with the brick bell build in my dining room which will begin soon!

Follow up: The winners for 2025 were Tetsukabuto, Seminole Pumpkin, and Thai Kang Kob!

Thai Kang Kob from the C. Moschata family had great mildew resistance and its solid stem was immune to squash vine borers. It did not need a pollination partner.  These plants had a very sprawling habit, flowing out of the hugle bed and onto the ground around it. Beautiful orange flesh and mild flavor.

Seminole Pumpkin (also C. Moschata) also grew successfully, but only produced ONE big tasty squash on the whole plant. It started another in august but was too late to mature.

Tetsukabuto Squash (Maxima x C. moschata HYBRID) required a pollination partner from a C. Moschata butternut type, which was the Seminole Pumpkin in this case. I did see squash bugs as the squash reached maturity but they didn't seem to do much damage. I occasionally blasted them off with the hose. This was perhaps the best producer and was happy to climb an a-frame trellis (right side of picture.)

The big problem I had with these in 2024 was pill bugs. To overcome this in 2025 I did two things: Grew them bigger/stronger before planting in brown paper lunch bags (plopping the whole thing in the soil to not disturb roots.) And "mulching" around the base of the transplants with gravel to keep it very dry and not conducive to the bugs.

So if you've had issues with squash vine borers, mildew, and pill bugs like I have then give these varieties/techniques a try!

I've had these growing from cuttings for 4 or 5 years now in zone 5b. I hear they are difficult from seed. I think you'll get vegetative growth every year, and maybe your colder zone will keep it more in check! As you've read on some of these other comments, it can very easily get out of hand but it's enthusiasm is limited to early June to frost here. Maybe more limited where you are.

But what I really want to say is: curb your expectations on fruit. Even IF you get it in your zone, this passion fruit is not the same as its southern cousins. The "fruit" is limited to juicy pulpy pouches surrounding the seeds. You can mash out some pulp from lot of fruits to get a little bit of goodies. Not worth much. Generally I just bust one or two open a year to chew on the pulp and spit out the seeds.

While the fruit is nothing special, it's still a fantastic plant for the flowers and medicinal leaves! Even great to grow as a seasonal privacy screen.