Clay oven, modern materials
I'm building my first WFO, and would appreciate any advice that you may have.
So far, I've built a massive base out of river stones, (leaving a hollow middle with a ring around the outside), and filled the void with wine bottles and perlicrete (approx. 12-16" thick). It's a lot of vermicrete, but is less work than driving around looking for fill, and moving the heavy fill. Plus, the hearth will be extremely well insulated...
My plan is for an approx. 36" oven, 17" tall dome, made out of clay/sand mixture, 4" thick. I'm planning on catenary arches for inside and out. When fully dry, the dome will be covered with perlicrete, and finished shape will be like an old-fashioned beehive (also a catenary dome).
The oven will have an entry archway, with a flue built into the top.
So, a few questions:
1) Running the chimney straight up from the flue is not possible due to space limitations. So the plan is to have the top of the entry arch cut out, leaving an 8" hole. On top of this, an upside down flower pot with the bottom cut off (tapering from 8" to 6") From this, a flexible 6" aluminum duct running back to the middle of the oven dome and then straight up(but with as little angle as possible...) The flower pot/duct will be filled around with perlicrete, and the whole chimney will be buried in insulation. So, I know that the aluminium will degrade quickly - but will the remaining void work just as well as a chimney? I can't see any reason why not (they use a similar mix to re-line old house chimneys these days) but i thought I`d better ask here first.
2) As far as the clay mix goes, I was thinking of using (by volume) 1:1:3 powdered clay, rock dust, sand. In addition, I am planning on adding shredded rockwool fibers for more strength. Straw seems like a joke to me, for both insulation properties and for adding strength. I want strength in my thermal mass, not insulation. And I want insulation in the insulative layer, not more mass. So the dense thermal layer covered with perlicrete seems like the way to go. Has anyone done this before? How much rockwool did you use, more or less?
3) I'm a little concerned about thermal expansion of the oven walls, and them having nowhere to go if surrounded by hard perlicrete. How much does a clay oven expand, anyway? Would it be worthwhile to put on a thin layer of rockwool over the clay, then a chicken wire or fiberglass mesh, then a thin layer of perlicrete? Let that dry a bit, before adding the bulk of the perlicrete on top?
4) Do people really spend all day pounding clay into powder, and then even more time stomping on it to mix it up with sand and water? I was thinking of laying my dryed clay out on a 4ftx8ft board, covering it with another board, and driving back and forth over it. Should make some decent clay powder i believe. ( I got pure clay for free but it's wet...so i'm not starting from powder) I plan on the same technique for the actual mixing, but using heavy duty tarps instead of boards. Has any one done this before? Any tips?
5)One last thing - has anyone mixed waterglass into the clay mix from the start? I read on another forum that Red Devil TSP90 is waterglass powder, and is not very expensive. Unfortunately, I can't find it here in Switzerland.
Thanks for any advice you can give me, and for all the great info that has already helped me on this forum.
Looking forward to that first pizza!
Re: Clay oven, modern materials
Can't answer all your questions, but I'll have ago at a couple of them.
As far as building clay cob ovens go, I think most people don't put any fibres in the hot face/thermal mass layer. Most people who use rockwool as insulation put something with a higher melting point between the oven and the rockwool, like ceramic fibre or perlcrete. Rockwoool fibres in or against the hot layer might melt.
The layer of perlcrete over the oven shouldn't be a problem with regard to expansion IMO
Waterglass is one of the binders used in low cement refractories, so adding it to your cob probably won't d any harm, and might actually do some good.
Many ovens are built using a channel up over the top to direct the gases to a central chimney. That should work.
Last edited by wotavidone; 06-14-2014 at 04:21 PM.
Re: Clay oven, modern materials
2. I don't believe shredded rock wool fibres will give you more strength. Rockwool is an insulator. AR (alkaline resistant) glass fibres would be a better choice, try concrete counter top suppliers for these.
3. The expansion of a cob mix will be minimal so you shouldn't have to worry. a layer of rock wool over it will deal with that. You bigger problem will be the shrinkage that takes place as the cob mixture dries. If building over a sand castle, remove the sand as soon as you have completed the dome, to allow it to have somewhere to shrink to. Your other problem will be to dry the dome out. If it's say 3" thick, then don't even think about any fires for at least 3 weeks drying in the sun and wind. Also by the way allow a few weeks for your vermicrete/bottle layer to dry before covering over it or you will trap the moisture in and it will take months to eliminate.
4.I find the best way for this is to lay out a large sheet of plastic and mix the stuff with bare feet, you can grab a corner and roll the stuff periodically into the centre this way. Assistance from young women and beer is fun too.Peple making mud for mud brick houses sometimes use a tractor to drive back and forth, so your idea is feasible.
5. Not sure about the water glass, but some people add some Portland cement to the cob mix.
Last edited by david s; 06-14-2014 at 06:51 PM.
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