CRAZY IDEA ALERT: Blow torch fired earth plaster exterior?

Nader Khalili fired the insides of raw mudbrick buildings in Iran, he called them Ceramic Houses. They would set up a large oil burner inside and let it go for 24 hours or something like that. The result hardened the interior to a kind baked-in-place finish. He doesn't seem to have pursued the idea, I'm not sure why but I think cheap Iranian oil was part of the scheme. He went on to found Cal-Earth in the US and develop superadobe.

Could the firing idea be applied to exteriors? I am wondering what would happen if you ran a blowtorch over a raw earthen plaster. Could you get it fired enough for good water repelling and stiffening? Is a blowtorch even hot enough? Would the plaster crack or fracture? How much would it cost? How much labor? How crazy is this?

Pretty crazy, 9/10
All the other questions will be answered by trying.

Why do you see a need to do it?

I don't have any need, I'm stuck for the time being in an apartment with a drawing pad. I'm hoping to have land to actually, you know, do something later this year. And test it I will! And much else, at least if it seems worth doing.

We get 7" a year of rain where I am, I'm thinking of different "natural" materials to make a mud brick vaulted roof durable here, no Portland. In Iran they cover such roofs with square ceramic tiles that look a lot like the local Mexican tile. They also do a kind of trencadis. There's that possibility, like a patio floor on the roof. Lime plasters work for some. Seems like the raw earth roofs last maybe a year or two in Iran, with similar rainfall although I don't get any ice here in the low desert. So the thought came to me, hey what would it do if you could fire it as a whole, make one big tile, or like a big inverted teacup of it?

Have you heard of architect Leon Krier? He contrasts modern buildings with traditional by initial time investment vs building lifetime maintenance.  So modern buildings go up quickly but long term cost of maintenance is high. Traditional buildings are slow to put up, but cost a lot less over their lifetimes.

Now take this idea over to typical Arizona suburban home construction vs some natural building type for the climate. The additional time and labor to build an adobe vault roof vs the current prefab frame strut sandwich things has to be offset but much longer lifetime, to be a real alternative.

In sum, I'm wondering if that's possible, and if so what does it. For example, the method should provide at least 25 years before needing any real attention, that seems like a reasonable span. And at that point it shouldn't need a total replacement.

I like the cut of your jib.
I have considered such a thing , adobe brick dwellings, fired from the inside, but with wood, not propane or oil.
I think the idea has merit.
I wonder would the different heat expansion rates between the fired and unfired portions be a problem?

You might be interested in Tadelakt.
It uses lime plaster and olive oil soap to form  Soap scum or lime soap, which is water proof.
This is labor intensive but low tech.

I have played around a little bit with pottery, and the fundamental problem with firing just a surface is the thermal expansion. Pottery needs to be heated evenly or else it will crack. The material even undergoes a fairly sudden change in size as it passes a certain threshold - I forget what temperature it is, but it means that the piece needs to cool slowly and evenly, or else even that will cause cracks.

I think what you would find is that you could bake a sort of "skin" on the outside, but it would consist of small mosaic-tile sized pieces that were surrounded by cracks, and no longer firmly bonded to the underlying structure. They might more or less stay put because they were surrounded on all sides by other tiles, but it would likely not be waterproof or even weather very well.

I suspect traditional buildings required plenty of maintenance, but it was probably in the form of low-skill labor and using free and local materials that would have been easy for the owner to do.

I think making a tile roof would be a very fun project, although if you only get 7 inches of rain, I am going to guess that wood for fuel is going to be pretty scarce.

Carl is spot on about the issues. You would need to cover the entire building in logs and branches, fire it slowly at first, get the whole thing glowing orange-hot, then let it cool slowly. This would only work with coarse cob containing sand and straw, not fine smooth dense clay which would explode in spots unless fired perfectly.

I think a lime plaster or maybe tadelakt and possibly tile on roof surfaces could make the exterior durable in your dry climate. The quicker you can shed any rain off the better.

Hello Friend! I have thought of this exact thing in different terms and ran it by the guy who taught me cob and he made a really good point.

One of the biggest advantages of cob is its ability to "live". You give a lot of little trade offs for one big return. Having it be more sturdy is amazing, but you are giving away, the flexibility,
ability to modify easily, ease of repair, breathing effect and air exchange of the material not to mention many more. Some things to ponder over, but I think the idea is amazing, the thing that
stopped me was wanting all those other things.

Good luck!

It occurs to me that you might be able to use solar to heat the cob.
I wonder if a variation of ferrocement where clay slip/ sand  is fired onto an armature in layers would be possible.
There is a slip like substance that is sometimes  applied to pottery after the first firing: Engobe

Great responses, Tadelakt looks like the thing to pursue first.

permies.com must be the only place on the internet you can ask a crazy question and get sane replies!

Thanks again.

I forgot to mention one other technique I've encountered, fermented cactus juice.

Apparently the traditional recipe was used to restore the San Xavier Mission in Tucson, in the 1990s I think. In the 1950s it had been surfaced with cement stucco, they removed all that and had artisans from Mexico do the exterior plaster. They let the cactus sit in drums for a few weeks, then remove the slime and add it to the mix. It's said to smell horrible.

It would be interesting to know what their maintenance procedure is. It's a church in active use.

I like your idea, but foresee lots of cracks and popped bubbles in your clay. Due to uneven heating and the sheer sizes you're trying to fire. I was just thinking about building a sandbag house, and thought about doing something like a exterior coating of ceramic fired with a tiger torch. However the more I think the more I realized, naw probably be better to drop a layer of poly and cover the whole in soil make a mound home plant some grass and flowers and keep some bees.

Any kind of firing has to get up to a bright orange or yellow in heat, or it will be very soft and porous, and soak up rain even deeper and faster than sundried clay. Think the middle of a roaring woodstove, now expand that to the size of a house, and you will see how practical it is.