Clay Ovens

[Archena]

Re: Clay Ovens

Vermiculite is made of pellets right? My guess is that any mixture have enough vermiculite to actually provide good insulation would also have problems with cohesion. You couldn't use it in the interior it all, or in any part that's likely to be bearing weight, because the vermiculite would seriously compromise the material strength.

The milk paint with linseed oil sounds interesting. Both milk paint and whitewash have to be reapplied periodically no matter what it is you're applying it to. If I'm not mistaken, and it's early in the morning so I very well may be, whitewash is traditional for cob. If you look at pictures of 15th century houses, some of which are still standing and in use, they usually appear to be whitewashed. I know lime is one of the components of the mix so that would be whitewash, correct?

One of the things that I’m intending to do to test some of this stuff out on it to make very small ovens, and try it out that way. I saw a website somewhere. I’m not my desktop so I probably can’t find the link right now but they made one very small oven big enough for a muffin. I figure I can use concrete pavers as bases and see how things work out. I figure it’s a lot easier to find out at small-scale than it is at full scale.

[Tim F]

Re: Clay Ovens

How about if you used a mixture of clay and vermiculite for the insulation? And then for waterproofing, use milk paint with linseed oil. I haven't tried this yet but I have read that milk paint made with linseed oil is effectively waterproof. There are some other recipes here: Homemade Milk Paint Recipe - Real Milk Paint ?

[Frances]

Re: Clay Ovens

I've read that the oven has to "breath,"

I think I would hesitate to put a cement product over a clay dome before doing some serious reading up on it.

I have heard that clay can be a brilliant product for the inside of houses... but only if they are not hermetically sealed (as many more modern buildings are), exactly because the clay can't breathe. Our cellar has wooden boards over clay on the floor - I don't know of any reason why this should be superior to a cement floor, but I do know you have to be careful converting/insulation projects, to take breathability into account.

It also strikes me that if you made a second dome over the first with a gap between the two, it'd be a really cool place for creepy crawlies to get in and virtually impossible to get them out. I think you would want to keep access to the outside of the dome. Maybe .

Sorry, don't want to be overly critical here, just trying to think this through. Maybe a more substantial shack built over the oven would be a better bet after all?

How often does the oven go without firing? Because if you could make sure to fire the oven up every couple of weeks even it you don't really need it, that would keep pests away, wouldn't it?

[Archena]

Re: Clay Ovens

Um, I’m not sure if you mean me or RT. I was the one that mentioned papercrete a while back, which is a concrete product. But I had it more in mind for an enclosure rather than a coating. Obviously, my plans aren't terribly well thought out in the enclosure department…

[luca]

Re: Clay Ovens

Thanks for the idea; I'll definitely look into it. My only hesitancy with any cement product over the clay is that I've read that the oven has to "breath," and it certainly expands/contracts with use, so I'm afraid any inflexible coating may create mechanical problems (e.g. cracking during firing, which will then be inaccessible from the top). Back to the stucco, what would you recommend for the interface between the stucco and clay surface? (I would think the movement issue could be partially remediated if there was some space between the two materials that allowed some minute movements when the oven is running full tilt...what do you think?).

[luca]

Re: Clay Ovens

Hi,

I love the pics. Thanks for sharing! One question though, why two openings? I assume the wooden thing in the background is some kind of roof that you put over it. Are you using a tarp as added protection? (Okay, so that was two questions...).

Note to self: make sure to use oven regularly! Bugs are icky!

The side opening was an experiment which didn't work (the oven's air flow was awful and starting a fire was tough. The air has to make a kind of circular pattern which is encouraged by the domed roof, but this doesn't happen if the air flow is perpendicular to the domed roof rather than in line with the dome). So I simply closed it up with more clay, and drilled some holes in the end as you can see in the pictures, to make a new opening. This works much better!!! As for the roof, it is OK but has blown off and is subject to scorching when the oven is running, so I have since changed to a tarp cover; hence the bug problem.

[RTflorida]

Re: Clay Ovens

Luca, sounds like you are happy with the ovens performance and you state that you have no desire to tear down and start over (I certainly don't blame you), I would give serious consideration to a render or stucco coat; this will give you the weather protection you are looking for and keep bugs from burrowing in from the outside. It really isn't expensive...Louisville has 8 Home Depots and probably half as many Lowes, go check out the prices in the masonry dept. 2 bags should do it. This will also allow you to maintain a similar overall appearance.

RT

[luca]

Re: Clay Ovens

Interesting comments, all! Let me add a few thoughts. First, my clay oven was built almost exclusively for making real Italian pizza. Since I can get the high temp I need with relatively little wood and no insulation, I've never really seen a need to insulate the oven beyond it's 4 inch thick walls. Perhaps if I wanted to cook bread all day or slow cook something, insulation would be more important, but I've never really found the need. Secondly, about damaging the oven, while you are right that the oven is relatively fragile--I have noticed small fragments falling from within from time to time, especially at weak joints from my initial construction--I also find that repairs are quite simple. I simply take some fresh clay and "slap it" where needed, both inside and out. (This summer, I plan to experiment with redoing the outside of the oven by wetting the top 1-2" and adding additional clay to see if I can "renew" the outside layer). After 6 years of playing with my experiment, I don't see a need to completely break it down and rebuild it (the French Quebec ovens were said to last for a generation!). However, as I stated in my initial post, I am trying to make mine relatively maintenance free (if that's even possible), and the only area of concern is the exposure to the elements of the unfired outer layer. Finally, if you are out to cook in the oven for extended periods of time without using additional wood, I would certainly recommend that you build a clay oven with walls at least 8-10" thick as the Quebec French ovens so ably demonstrated.

[RTflorida]

Re: Clay Ovens

Now we have a discussion going, just what I was hoping. Sounds like a viable inexpensive option. Spending a few bucks on insulation and a finish coat would be the route I would take.
Wiley, I too am a klutz around my oven - If I am not scorching hair off my head and arms, I am banging and clanging my tools on the entry, hearth, and dome. I don't get it, I've been using all kinds of tools all of my life and never a mishap....let me build a fire and I become a klutzy, giddy, schoolboy. Sounds like I would beat the hell out of a cob oven. It does give me something to think about if/when I move and want another oven.

RT

[Johnny the oven man]

Re: Clay Ovens

RTflorida, the term Superwool refers to Thermal Ceramics line of Biosluble ceramic fibre range, not to be confused with normal RCF. Superwool is rated to 1000 degrees c, vs 1260 degrees c for normal RCF. Not that this would matter as WFO's operate at far less than these temps. It just means that you are working with a safer product in relation to possible safety issues, which can only be good.

[Wiley]

Re: Clay Ovens

My thinking on it not being worth the expense of time and dollars/materials in rendering and insulating a cob or mud WFO isn't just doing it the first time but the second and third time. Having two neighbors with cob ovens (one of which is quite a devotee of cob) I have a little first hand experience.

Cob WFOs are in my opinion fragile, at least for a klutz like myself. Damage to the entrance is easily done with peel or rake and the hasty and over zealous pitch of a piece of wood that happens to strike an interior wall can cause significant damage. No big deal for when the damage gets bad enough, one simply breaks and crushes up the whole oven and starts over. A couple days work and some curing and they have a new oven. So they wear out fom the inside as well as the outside. Insulating and rendering the outside only protects half the WFO. I suspect that any plus from ease of rebuild would rapidly disappear if one had to separate out the wire and stucco and ceramic insulation, especially if one planned on using the insulation over. True one could pitch the stuff and start over using new materials, but two things on that idea: One) cob is a specific mix of materials not the typical back yard earth; clean organic free sand and clay are required for good construction. So unless one is lucky to have such deposits at hand one will have to travel and or pay to get them. And 2) people who build in cob aren't of the mindset to simply cart the reject stuff off to the landfill. They're more into organics and caring for the earth and environment, not wasting resources etc. etc.

I'm not into cob but I'm not against anyone who thinks it's the medium for them. It's obviously worked for centuries world over. But relative to our brick WFOs, cob ovens are more ephemeral, it's part and parcel to what they are. If one chooose to build in cob at some point one has to expect to have to rebuild it.

Wiley

[luca]

Re: Clay Ovens

Thanks for all of the interesting ideas! As for Whitewash, I tried that about 4 years ago and it slowly disintegrated over time; in part from beneath, since ambient moisture/humidity is always influencing the water content of the outside (unfired part) of the oven. I've also built a wooden cover which worked OK, but got blown off once or twice in storms!. As for heating capacity, I love cooking pizza and can fire my oven up to 800-900 degrees (great pizza needs very high heat) with very little wood!! (I'm talking about 4-6 2x4 size pieces of wood 24 inches long for the whole 2-3 hour firing process). I have also experimented with slow-cook barbecue by placing a pan with meat and barbeque sauce in the oven overnight with a door on the oven with excellent results. My particular oven was started as an experiment and has walls only 4 inches thick, so the heat retention is only several hours, but the classic Quebec bread ovens would have walls from 8-10 inches thick, thus giving a much better heat retention (and in those days they had no insulation on these outdoor ovens). As for construction, I'm attaching a few more pics. My oven floor is only 24 inches by 36 inches, and in my first attempt, I incorrectly placed the door in the side of the oven, thinking I could get more "stuff" in it. That totally ruined the aerodynamic properties, and thus the hot fire/smoke would not circulate properly, causing start-up and cooking problems. This was easily solved by filling in the side entrance with more wet clay, and "drilling" a new hole (I just took a drill bit and punched 50 or so holes through the already dried and fired clay) at the end. This has worked beautifully in that the oven fires easily and I can crank the heat up by adding a small piece of wood a few minutes before before putting my food in. This is definitely the way too go for a no-cost oven, but I will look into the insulation ideas that have been mentioned.

[Archena]

Re: Clay Ovens

Hi,

My own interest is historical, that and they're cool. Cob ovens have no difficulty holding temps - they're made of the same material as brick, the brick is just fired in place instead of being fired in a foundry. As far as the need for insulation, it depends on how thick the oven is made. It's quite possible to make the walls so thick that they require no further insulation.

It's a myth that the dark ages were invariably inefficient. Think about it, if you had to go cut, gather, store, and then fetch all the wood that you would need for an entire winter - not just for the cooking but heating as well - wouldn't you use the most efficient materials that you could? Medieval technology was surprisingly efficient for its time. Properly build cob homes are actually very energy efficient. There's no reason that a cob oven wouldn't also be fairly efficient.

I'm honestly not sure which would be more efficient brick or cob. Firebrick would certainly have better refractory properties, but it would also have mortar. As I understand it the refractory properties of cob would depend largely on the type of clay used and the amount of silica present so there would be some variance there but there would also be no need for mortar.

Cob also has some significant advantages in construction besides cost. Tools required are not particularly specialized and indeed only a very few are actually necessary. Trowels and cement mixers make it easier but they're not essential. There's no need for specialized saws and if you do make a mistake it's much less expensive to tear down and rebuild than a brick oven would be.

At the end of the day I think once one isn't particularly that much better than the other. It depends on your actual needs and intentions as to which one will work out the best for you. For industrial kitchen I suspect brick would be far superior to cob simply because you have better control the refractory properties. For a backyard oven only to be used occasionally cob may well be the better bet just in terms of time and expense.


For me, my choice of cob is severalfold. It's historically accurate for the medieval period (some other time time I'll explain why that matters to me), it's much easier to work with, which means it will match my skill level, it's less expensive to build and I just think it's cool.

Okay, I think maybe I should stop the dissertation now... Me? Get carried away? What makes you think that?

[Frances]

Re: Clay Ovens

Didn't John Fahle and his friends insulate their oven with a sawdust mixture? (And don't ask me how that works, but apparently it did.)

I like the idea of clay ovens because I think it'd just be so cool to dig up the necessary materials out of the garden - it'd be really interesting to compare the performance with a brick oven, too. Benifits/negatives, I just don't know RT. Maybe that's why I'm interested? We've also had a couple of questions from third world countries, where it seems like it'd be a usefull skill to have, building a mud oven I mean.

If I built a mud oven, I think I'd go wth the vermcrete and stucco finish and fire it up once a week or so to keep the bugs out... well actually I'd do a lot of reading first and then decide.

[Tim F]

Re: Clay Ovens

After reading a book about them this week, I had an urge to build a cob oven in a spare corner of the garden, just a little one built as part of a stone wall or bench or something. They do look very cool and I like the idea. However if I do make one I was thinking that vermiculite-concrete would be the insulation of choice. Or if you want to stick with the au naturale theme, lime mortar with vermiculite?