From the 2017 rocket mass heater workshop jamboree ...  this could turn out to be the biggest innovation of the event (the event is still going on as I type).  

Batch box rocket mass heaters suffer from needing a good door.   So far, the door has often times tripled or quadrupled the build time, requiring welding skills.  But Uncle Mud and Donkey came up with a freakishly simple solution.    The glass lid from a casserole dish and a slightly sloped entrance to the batch chamber:




It is officially called "the casserole door"

We've heard of CFLs, LEDs, RMH's......and now!....THE CDL (casserole dish lid)-featured batch-box (CDL-BB-RMH).  (For those in the Midwest US, make sure to order yours as a "HDL"---'hot-dish lid'.)!



Great fix!....and easy to replace when broken.

catch a peek of the "casserole door" in action:

A nice elegant but simple solution - Its one of the main reasons why I havn't switched over from the J tube.  l'm wondering though where the secondary air port is? ...and also if that has a simple solution to getting away from welding as well?

My idea of brilliant!!!  Kudos, gentlemen : )

A video about the casserole door:

I hate to be a spoil sport, but there are some comments to make.
A casserole door is really nice for a 4" system and it might be possible for a 6" system given a really large glass lid. The oval shape is ideal for this, round ones are not as easy to implement as I've found out. But I can't picture a casserole lid for say, an 8" system. That would be very heavy and unwieldy, not to mention the higher temperature stress that would be put on the glass. That casserole lid isn't made of the same Neoceram as the ceramic glass on the top of the 8" thingy's firebox in the shop, remember? The largest implementation of a batchrocket to date is a 12.5" system, and development is still going on, who knows what is in the pipeline.

I don't agree with the notion that batch rockets are no good without a proper door. Air flow in a well-running J-tube need to be restricted with a couple of fire bricks. Nobody is objecting to that, so it's as good a solution when used for a batchrocket. Two more bricks and you'll good for to close off about 3/4 of a 6" batch box rocket's front opening. Done that many times, none of my development projects had a door, I used bricks or a combination of a glass piece and bricks.

And last but not least: It might be that Paul personally would need 3 to 6 days to produce a door. But I'm inclined to think that statement is highly exaggerated when applied to heater builders in general. At the time I had at home materials and tools ready it did take one day to build a door, frame and all. I am not a welder, far from it, I loathe steel work and do it only when I have no other choice.

But at the same time, that casserole door is a very nifty solution for a tiny house heater, I fully agree with that.

I would like to add a caution to anyone thinking of using this solution for their own build.  I used casserole lids as doors extensively back in  2012, and they seem perfect for a short while.  And then, they explode violently.  No kidding, every one I tried, after a few weeks of hard burning, would eventually fail dramatically, spraying glass chips all over the room and pelting things with hot glass.  I consider them dangerous, so please use at your own risk.

Thanks Matt

I kept looking for the specs as far as working temps, but no one ever seemed to mention the temperature resistance, So I was assuming it was perfectly cool.

I know pyroceram heat glass is out of sight  price wise, and there was a bit of dissonance in my brain about why something so cheap (casserole lid) could substitute for something so expensive

I did some research in a few places as to temps  the glass tops are rated for, but got lots of conflicting information. This thread had a fair amount of experience and looked at different reasons for the shattering because people were experiencing the shattering at cooking oven temps--normally 450 or so.

There are lots of possible reasons, and although I was planning to replace my glass top after reading Matt's post this morning I went ahead to do one last fire and the funniest thing happened, it shattered.

Although not as violently as Matt described--perhaps because it had a stainless steel rim, nonetheless it is waiting for the batch to cool to be discarded. I'm thinking about maybe lining a cast iron pot lid with refractory cement and a ceramic fiber pad on the inside of it, leaving just enough space to set the lid in the mud frame. I'll miss seeing the flame, but not the danger.

There are obviously different types of glass tops, some of the early corning pots were pyroceram based and those tops may be ok, but many of the newer tops are borosilicate, and some are even soda lime technology.

So all glass lids are not created equal, and maybe someone will have more knowledge to add to this scant review.

I wonder if the door from a toaster or convection oven would work.
Glass from a halogen lamp might be resilient as well

That is, if veiwing the fire is crucial.
If not, a stainless steel stock pot lid could be cobbed in place.
Maybe get a masonry trowl, run some screws into it and set it in cob or refractory,kind of a metal version of a "traditional" cob oven door.

As an aside, I was trying to build a still the other day, and in doing so drilled holes in two glass pot lids.
The first broke violently,and I assumed it was the fact i was drilling outside in the cold,too much thermal shock even though I was  dipping the bit in cool water.
The second I drilled in a pan of water and ir was fine,untill I removed it from the water...
Evidently thermal shock is the Achilles of this otherwise very tough glass.

They used to mine a mineral called Mica in the area I live in. It's almost sheet like and can be peeled to different thicknesses.It was used at one time for " windows" in wood stoves. While not glass like clear, it is translucent. Again, we are talking traditional parlor style wood and coal stoves and not the very high temps of a RMH. I'm not sure where one would find some. Just thinking out of the box. In this day and age there has to be some modern material out there.  I hade a large Pyrex casserole dish violently shatter on me one. That gave me a good scare.    Larry

I hade a large Pyrex casserole dish violently shatter on me one. That gave me a good scare.  

I recently read a good article comparing recent pyrex to the older stuff (and didn't save it of course ).
It does sound like the more recently made pyrex will explode more readily.

f you’ve ever poured hot water into a Pyrex glass dish and been shocked to see it fracture before your eyes, a new report may give you some insight into what’s going on. Pyrex glassware, which came out in 1915 and was long marketed as “icebox to oven” cookware that did not expand or compress when exposed to high heat or low temperatures, is no longer made of that hardy borosilicate glass. And the new stuff, scientists publishing in the American Ceramics Society Bulletin have found, doesn’t stand up well to some of the temperature changes involved in cooking.

more in this link  http://blogs.discovermagazine.com/80beats/2012/09/13/pyrex-glass-isnt-as-shatterproof-as-it-once-was-report-finds/#.WhVy4ZdrxZQ

It might be that Paul personally would need 3 to 6 days to produce a door. But I'm inclined to think that statement is highly exaggerated when applied to heater builders in general.

I have never built a door personally.  However, we have had two different people build batch box style rocket mass heaters here, and the builder left it without a door.  So we had to cobble together a temporary solution to get heat.   One took a year and a half until we could get a proper door, and while it was rough, it took several days to build.  And then was overhauled to make for a better door.  The second NEVER got a door, and that installation was so riddled with issues that it now disconnected and we don't use it.  

On top of that, I have heard from people that their only experience with a rocket mass heater was terrible and that they would NEVER use one.  And they are adamant to tell as many people as they can that all rocket mass heaters are shit and nobody should ever use one.  They are far too smoky and should never be inside of any structure - even outside they are annoying and don't live up to "the hype".    When I asked more about the rocket mass heater that they had a bad experience with, I found out that they used a batch box rocket mass heater that had no door.  So they were operating it like an open fireplace with smoke pouring out the front.

So, one could argue:  well, that is not a fair analysis of rocket mass heaters.   The builder should have finished it, or left instructions or _________.   And the bottom line is that we don't have that problem with the J-tube systems.  

And one could further argue that this is more the exception than the rule.   So far, my impression is that for every batch box rocket mass heater that has a proper door, there are two that don't.  

So, I wish to be extremely emphatic when I say:   a batch box rocket mass heater without a door is not a rocket mass heater and should not be used.   I feel if I don't say that frequently and loudly, then people come to stupid and awful conclusions about rocket mass heaters.  

Matt Walker wrote:I would like to add a caution to anyone thinking of using this solution for their own build.  I used casserole lids as doors extensively back in  2012, and they seem perfect for a short while.  And then, they explode violently.  No kidding, every one I tried, after a few weeks of hard burning, would eventually fail dramatically, spraying glass chips all over the room and pelting things with hot glass.  I consider them dangerous, so please use at your own risk.

We had one explode with a new, experimental design here.   We are exploring the idea of adding more cob to the front so the glass lid is moved further away from the fire.

we have had some soda glass bakeware explode, just using it as bakeware.   The borosilicate bakeware does much better - and is generally considered to be better for high temp use.

I wonder ....    is that amber glass that was used for pots and pans a few decades ago ....   might that be "pyroceram"?   YES!   I found some on ebay

Why is Visions so light and clear and yet able to handle such extreme changes in temperature? Because it’s made of a revolutionary glass ceramic material called Pyroceram, developed by scientists to protect the nose cones of space shuttles from incredible and extreme temperature changes.

Use this link to see polyceram glass lids.

I have a shallow fry  pan made of the stuff, just about the right size, of course I have already demolished the mud front I had mostly built for a cdl, oh well, it's cob so it's  back in the slurry for phase three, batch burner door.

I figure the long handle on the fry pan will be cooler to the touch, even at the end of the burn.

William, I am wondering if you weren't trying to drill a tempered glass product. Those, as you probably know, are incredibly resilient until the surface is damaged, and then they have this unfortunate tendency to explode violently.

The new Pyrex is apparently being produced overseas, and that is the speculative cause of the decrease in glass quality.

I was part of a batch-box RMH workshop here in Scarborough, Ontario. We thankfully had a welder and a ton of scrap metal lying around. There was an old-style cast-iron woodstove door with a window and adjustable air port in it. That's what I will look for the next time I build one, or I will make do without glass. It seems an unnecessary frivolity if it's prone to such spectacular failure, but maybe that's just me.

-CK

In my experience the pyroceram and similar type lids fail even more spectacularly than the others.  Cooking glassware just is not capable of handling the temps and variations we see in a batch rocket, in my opinion.

Doors don't need to be that difficult in my experience. Here is my method for making batch doors and hardware.

I had a conventional oven with a glass window which exploded in the first week, my lasagna turned out really really crunchy as a result... I think any application of glass near high heat has to be approached with care, even if the glass used is "supposed" to work fine.

Unfortunately RMHs are new enough that experimentation is part of it and as long as folks qualify their work with "this is an experimental piece, please be careful if you duplicate this", especially when filming for Youtube where the video could sit for years of repeated views, then the watcher is forewarned.

Well Matt, thanks for the new heads up, I was thinking/hoping that perhaps you had been using borosilicate cookware inadvertently.

I have a visions fry pan as a door now, but I did like the riveted design of your door since i don't weld, is it lined with ceramic fiber or refractory cement? Do you have better details of the door build somewhere?

do you think a cast piece of refractory -perhaps with a ceramic fiber blanket as reinforcement could be used in the same manner as the glass cookware?

If the glass is built out further from the heat will it be prone to black soot accumulating?

The USSC barrel stove kit doors are really easy to apply to 18g steel and use as a rocket stove firebox door. I've done it 3 times now:


There is a side loading barrel door manufactured for Ugly Drum Smokers, which can be used and its easy to add wood stove door glass for a window:



These aren't hard to modify to fit flat sheet metal, but I'm using this as is for my current barrel build (fully contained 6" system in one barrel) and I'll have photos this week. The whole build will use a shippable core kit, and its easy to bolt together and uses a simple unique approach for the exhaust manifold. Photos soon.

As soon as I have enough income from the new business, I'm going to import some of these, and they'll be affordable bolt on doors which will sell for $50-$60.

Inner dimension: 250x240 mm ( 9.8" x 9.45" )
Outer dimension: 310x300 mm ( 12.2" x 11.8")

I know part of the charm of the casserole dish is that the glass makes the fire visible, but if shattering is a risk has anyone just used a nice cast iron lid?

Brian James wrote:As soon as I have enough income from the new business, I'm going to import some of these

Looks like minimum purchase quantity from Alibaba is 50 pieces!

Anyone have a clearer idea of why these pyrex doors shatter? Are they being exposed to more than 500 deg C?  Have they shattered just sitting there, or is it thermal shock?

Has there been progress on creating a longer feed channel to lessen the temperatures the door is being exposed to without lowering the inner burn chamber temps?

Graham Chiu wrote:Anyone have a clearer idea of why these pyrex doors shatter? Are they being exposed to more than 500 deg C?  Have they shattered just sitting there, or is it thermal shock?

When the glass shatters in little crumbles it isn't pyrex but tempered glass instead. When it cracks in a non-straight line it is pyrex which happened to be borosilicate glass, maximum heat resistance is up to 500 ºC. The real stuff is special ceramic glass, heat resistant up to 900 ºC and insensitive up to a certain level to heat shock. This ceramic stuff is sold under the names of Robax or Neoceram, only two producers in the world as far as I am aware of.

Edit: I've never seen a piece of Robax to shatter or even crack. Not even in very extreme circumstances, provided it is able to expand.

I was rounding the charity shops yesterday and bought a Pyrex a12-c casserole dish with lid. So it sounds like it if i use this as my intended door the worse thing it will do is crack. Pyrex say that their products should not be exposed to direct flame https://www.pyrexware.com/use-care-pyrex.html so I guess I need to go back and look for a cast iron lid instead. Trouble is all the Dutch ovens I've seen so far are round.

This is interesting information if true.  This thread on reddit says https://www.reddit.com/r/YouShouldKnow/comments/7h7ehf/ysk_how_to_identify_the_older_safer_pyrex_glass/ that PYREX borosilicate glass has the brand in upper case, but the new tempered soda lime glass uses a lowercase brand name of pyrex.

I think Donkey mentioned that the glass lids could shatter if they are under high heat/in use, and the user then grabs the door with a wet/cold glove and that temp change causes the shock/break.

Borosilicate glass cracks with a temperature differential of 165 C/329 F whereas soda-lime glass shatters at 37 C/99 F.

I guess you should use fire gloves rather than a wet towel.

I wonder if Matt's problems are because he has a highly insulated fire box.  I'm certainly not recording 500 deg C at the opening to mine ( which is just dry stacked ).