Demonstration RMH made of ... glass?

Lead in: I have always wanted to have a demo RMH to show people how they work. My plan is to build a pebble bench style onto a trailer that I can tote around. That got me thinking of viewing windows, and making a "half" RMH that was all flat glass on one side, and whether it would fire enough to be worthwhile and explain concepts (think a 10" system cut in half). That led me to thinking of a glass bell, which would be theoretically possible, if you could find a borosilicate (think Pyrex) "drum" of large enough size. 20L seems to be readily available, albeit spendy. Can you make a demo RMH that size? With a 4 gallon bell? Then I got to thinking, probably you could make the riser inside the bell out of borosilicate glass. And if so, why couldn't you make the whole engine of the thing out of glass?

Theoretical question: Since borosilicate glass performs well under flame, and has comparable temperature tolerances to good steel and fire brick, could you make the whole burn chamber, riser, and bell out of BS glass? The idea here is a demonstration RMH where people can see immediately what's going on inside, and experiment with changes when other parts of the RMH are adjusted. I am not suggesting this as a household functional RMH, but that may or may not be possible. I am suggesting a somewhat expensive experiment ($2-4k) to get the glass made by a local artisan, and create something worthy of an RMH Laboratory.

What do you think? A thought experiment if nothing else. I don't know if people have experimented with BS glass in any structural or functional part of an RMH, and that might be something worth pondering for the tinkers out there.

Hi Kevin;
I do not remember the details but glass has been used as a sidewall on a J tube, and as a roof on a batchbox.
Neither was in use for very long.
Larger pieces of high-temp stove glass are expensive to buy.
Free stovetop glass will work, but it is a dark brown color.

The main difference I see between a glass bell and a brick bell is how easily any impact could shatter glass, exposing the home to instant fire danger.
Bricks are a whole lot tougher.

suggest changing the thinking several way.  Look up maximum working temperature for borosilicate glass at 400 degrees C.  Notice how low that temperature is.  Now look up mica furnace windows and find 1000 degrees C(1832 F) working temperature.  Not totally clear but you can typically see individual flames thru them.  Notice they are way cheaper too.  So what if your bell was rows of mica windows down the sides in several places with built jumps so the air currents jump past the windows with very little direct impingement on the windows.  You still won't be able to see into the burn chamber or heat riser very well as the heat riser can run as hot as 2400+ degrees F

Mica windows are typically limited to 4 inches by 6 inches or smaller but can be gently bent to form a slightly curved surface.

Maybe make a demo of glass to show interior though not for heating purposes, just a demo ...

Let us know if you do this experiment ...

Hey Kevin.
I like the idea. That is a movable rocket mass heater to explain the system.
But what do you actually want to show?
After the riser there should not be flames anymore, and if the rocket works correctly also no smoke. So actually nothing to see.
I also imagine that a glass would either fog up from vapor and/or from soot.
In the burn tunnel and at the bottom of the riser a window might be very interesting to show the flame path and turbulence. Later I think putting in a couple of thermo probes might „show“ more than windows.

I really like the idea, but I was wondering if the glass would change the amount of insulation which might change how it functions?

Anne has a great idea to make one that is purely for viewing, and a different one for burning.

The glass would be completely non-insulating.
It will affect performance.
It will also soot up reasonably quickly.

A demo J-Tube with no mass would be your best option for visibility.
With no resistance to impede exhaust flow, your glass might stay clear.

Matt Walker put a window into the firebox of his outdoor rocket stove. I don't think it was pushing the limits of RMH temperatures, though.

I use ceramic glass quite a lot in y builds, it is good for around 700c but eventually goes opaque.
My latest designs use glass as the top of the burn tunnel but there is a cooling  secondary air supply above the glass.
The main issue when using ceramic glass is sealing or joining the stuff, it is ok in a frame but trying to join two bits together doesn't work so far,  not for me anyway….

Lots of great feedback, and I appreciate every one of the previous posts with insight or caution or encouragement. I won't reply and quote each person because I don't have the time right now.

The reason for doing this is not to install in a home, it is to see inside an active bell. As far as I know, all we have is temperature readings, and not a full picture, let alone a visual of a RMH in action. I also saw pictures and videos of people who put windows into their bell, and it is interesting, but again, not the full picture. If what it shows is "nothing" i.e. no smoke, that would be a HUGE thing to be able to show people! Obviously we can see it out the back end once it is running, and, as mentioned, it may be that glass keeps it from running efficiently enough to create the secondary burn, but how can we know without testing?

The concept of the glass not being insulative doesn't seem all that relevant, as the Steel Bell isn't insulative either. Of course the properties are different, and BS glass very well may malfunction as a bell, but again, I would not--nor should anyone--experiment with any part of an RMH inside a home.

The glass not being insulative would definitely affect the performance if put in as a riser or burn chamber, and I hadn't considered that, so I'll scrap that idea for the time being. I may come back to that someday for other reasons.

For a longer lasting "see the inner workings" experiment, I like my idea of a 10" system cut in half with glass on the viewing side, and the normal materials, albeit halved and oversized, on the other side. I got the idea from watching Steve Mould videos, where he does this kind of thing with plastic all the time. Here is a link to one particularly relevant to permaculture:  

What I want and expect this (the BS Glass Bell experiment, specifically) to show is the EXACT moment when the smoke catches on fire and the stove starts to function at "phase 2" efficiency. Seeing that happen ... might help with progressing RMH technology in some way. It may not be all that relevant or worth the cost, but I had an idea and I wanted to put it in a few other brains, and it definitely got to some of the intended brains, so again, thank you for reading and commenting.

The side view experiment is probably a bit more obvious, but I would like a better way to "show" people what an RMH is and the current offerings of this site and youtube do not satisfy the person who wants to actually see what is going on inside and help them to understand the inner workings in a meaningful way. The drawings and diagrams and infographics are nice, but ultimately, are not something one can show a building inspector and expect them to understand.

I was at the RMH Jamboree in 2021(?) (I think the first one), and Paul wanted to experiment with his Juice Box Straw design. Things weren't working out like we expected after a week of experiments, and I think some "insight" into the inner workings of a RMH would have made the experimentation MUCH simpler and understandable. I don't know what happened with the Juice Box idea, it sounded good on paper, but we couldn't get the air flow right at the time. Being able to SEE the air flow inside likely could have given us enough information at the time to adjust appropriately and possibly give the idea some legs.

A lot of people have held a camera over the riser to show what is happening at the bottom. Apparently that doesn't turn their camera and hands into a molten mess. But Matt Walker's first continental stove had a window at the back right at the exit of the firebox. So you can see the smoke burning there.

Well you wont see much action inside a bell but this video I made does show what happens to any smoke!