Best material to insulate mud oven dome?

[patamos]

i'd go 3" or 4" of insulation. over 1.5" or so of inner dome mass…

IF a pre-fired bake over then you want more mass inside to absorb the bulk of the heat.
With a rocket feeding gradual heat the thinner mass before insulation works better.

I've tried a 6" L-feed with 3.5" of mass in the dome. Late in the build the client added that he wanted something he could long roast a big pan in for 4 or 5 hours. So we changed the plan to add more dome mass after the fire chamber was in. It makes for along flywheel system that requires a lot of feeding to get it up to functional operating temperatures. Not an ideal combination.

[martinpolley]

Thanks Pat. We tried to keep the dome pretty thin, but it ended up at about two inches. The sand we had available was pretty fine, which meant that if we went too fast, the stuff lower down would spluge out under the weight of the material above it. For the final 1/3, we found some different sand (more variety of particle size), which didn't suffer from this problem. (Also, by that stage, most of the weight was pressing on the sand form and not on the lower layers of mud.)

I don't know how it will behave flywheel-effect-wise, because I haven't been able to get it that hot yet. (With the door closed, I can only burn small amounts of fuel simultaneously without overfuelling, which makes loads of stinky black smoke, as I mention in this thread. I'm hunting around for something I can use as a chimney, which I hope will help.)

So, anyway, from what you said it sounds like the insulation I have isn't enough, so I'll add another layer, to bring it up from two inches to four.

[patamos]

2" should be alright

been following your overfueling thread.
chimney is bound to solve the problem
just keep the ISA close to system size, and wider in any elbows

[martinpolley]

We did a test burn with a jury-rigged chimney this afternoon. Despite being less than system CSA, it was MUCH better

[sothiseeker]

this is a regular mud oven and we have 4 inches of perlite slip insulation over 3 inches of Clay dome as heat mass.

[martinpolley]

sothiseeker Nice!

[jeppe]

I wonder why no one is mentioning straw as insulation? It is cheap and works great.
Above the dome add layers of straw dipped in clay-slip, then after your desired insulation thikness, apply a layer of cob or whatever you a working with.

:-) Jeppe

[martinpolley]

Hey jeppe. That's a good point. It sounds like it should work well. And it's definitely cheaper than perlite. Also, it seems like applying it would be faster work than applying perlite.

Has anyone else tried it? Does it work OK? How does it perform compared to other materials?

[jeppe]

Hi Martinpolley, I haven't seen other types than the ones with straw and they have all performed very well. The best one I have seen at a workshop here in Denmark had a very thick insulating layer, perhaps 10-15 cm, and even after a full day of firing It never got hot, even warm on the outside.
So I wouldn't consider anything else than straw for this :-)

jeppe

[patamos]

Any organic fibre in the insulation above the dome will burn out if it is close enough to the heat.
Shorter fibres like horse manure work okay because there is enough of a fine hollow clay matrix to maintain structural integrity. The larger straw fibres leave larger cavities, which tends to make for a weaker clay matrix.
Some of Donkey's early threads on experiments with organic heat riser material speak to these matters.

[martinpolley]

Well, I've already got an inch and a half or so of perlite on there, so the straw shouldn't get super hot. The highest temperature I've seen on the outside of the dome is 70°C/160°F.

So I'm going with the straw, and we'll see what happens