double chamber oven questions

I have a 55 gallon barrel of high fire stoneware clay used for making pottery.  I also have 6 quantity 55 gallon drums of vermiculite, unlimited wine/beer bottles and about 200 used solid bricks.  Can I use the clay mixed with ? as the interior domed liner for a pizza/bread oven?  Can I mix the vermiculite with the clay as an insulation layer over the interior domed liner?  I would like the interior to be 42"....what size should the pipe be for the chimney?

I thought I would box the entire stove in with sheetstock of durock, hardiboard, and densglas that has come off jobsites.  I could use vermiculite to fill the box and "super insulate" the oven itself.  I would stucco the exterior of all of it with PermaCrete to give it a weather barrier and to look pretty for the wife.

If the clay you have is stoneware clay, then it will probably not be great for the shell of a pizza oven, as it will get well above cone 6 during the heat-up phase.  What you need is FIRECLAY!  Fireclay is what they make fire bricks out of.  It is naturally more porous than stoneware clays (it is found at the bottom of coal mines - it is the soil that peat grew in millions of years ago, so it has fossilized plant remains that make it porous).  However, to make a shell out of just clay and then fire it will probably not work - you'd essentially have to build a temporary "kiln" around it to evenly heat the entire thing to vitrification.

If you had a ton of grog, it may be possible, since that is added to Raku clays to increase shock resistance.  But, you don't have grog, and I doubt this "ceramic" shell will last you very long with a stoneware clay anyhow.  Old "ceramic" baking ovens (before modern ceramic chemistry was understood) were usually made of low-fire natural terracotta clays - not a high-fire stoneware.  I used to do pottery, and improperly firing / uneven firing will cause an explosion with high-fire stoneware clays.

Based on the ingredients you do have, I would make a cob oven out of the clay by mixing in sand and hay into it (look up cob ovens).  Vermiculite is a perfect insulation material for outside the shell, and you can use the clay as a binder to hold it in a curved shape, but it may not harden to a waterproof stage.

If the bricks you have are regular bricks, not fire bricks, then they cannot be in direct contact with the fire because they will explode.  Use them as the base.  Although glass is an insulator, your glass bottles cannot come into direct contact with the fire either as they will explode.  But you can use them as accents in your surround for decoration.

It sounds like you want to build a “house” around the oven.  Durock (cement board) is a fine heat shield, but don’t use the Hardiboard, as it contains toxic resins and it will melt.  I don’t know what Densglas is, but if it is fiberglass it’s no good; rock and mineral wools are fine.

BTW:  Where did you get 55 gallon drums of the clay and vermiculite?  I’m looking for a vermiculite source, and my clay only comes in 50 pound bags or less.

Thanks for the great response!  I did not feel that I could use the clay "as is" to make the shell.  I assumed that I could use it with sand and straw as cob.  I thought that Paul Wheaton had made his without the straw for the interior dome?  The bricks would be purely decorative and the bottles would be used in a base as an insulator.  I thought I would make a slip out of the clay and mix it 50/50 with vermiculite to fill in around the bottles.  I would add 1" of sand over this and then firebrick ($1.80 locally) as the oven floor.

I am a craigslist junkie!  The vermiculite came in very large bags that had been sitting in a barn for years.  I got it free.  I bought food grade 55 gallon drums off CL for $5 that I stored both the vermiculite and clay in.  A pottery studio/class has their students buy a block of clay for their class.  At the end of the class.....most of the students don't take their unused clay home.  All the clay is the same firing just different colors.  I mix it all together and get a "brown" color.  The studio told me that they would have this same amount about every other month.  The bricks were also free off craigslist.  I think I have a 20# bag of grog also if that would be of use?

Would an 8" steel pipe be appropriate for the chimney?  I currently have 4" stainless steel pipe and was wondering if I could bundle 3 or 4 of these together as the chimney rather than go buy pipe?
Thanks again for the great response and any others would be appreciated!

hmm the clay is fine mix it well with a good sharp sand for the first layer. the vermiculite is going to make for a really thick insulation layer. the ss pipe would work better than mild steel.

you oven will look like this: 2~4 inches of thermal cob (no straw), 6~10 inches vermiculite insulation (i would lean toward the 10 since i have found vermiculite and clay is not that good an insulator compared to perlite (mostly what i use) ). 2~4 inches sculptural cob (with straw) and an inch of good plaster.

A bit large but it should give you a nice oven when done.

Thanks Ernie....I appreciate the detailed response.  Do you think that I could use the 4" stainless pipe bundled either 3-4 together for the chimney or would I need a solid 8" or larger pipe? 

I think that I am going to build a "house" around the entire oven with durock and fill it with the vermiculite.  My thought was:  4" of thermal clay + 6" of vermiculite clay+8" of loose vermiculite encapsulated with durock sides and roof.

Thanks again for the replies! 

its a simple chimney so the bundle should work (at least till you want a longer one)

good plan on the loose fill; vermiculite works well when its loose. the board stuff i have no experience with in that application so i dont have a comment on it.

Wow!  I can't believe they'd give away recycled clay for free - lucky you!  My old studio recycled all of ours with a pug mill.

If you have free grog, I imagine it can't hurt.  Grog is high-fired clay that is pulverized into sand-like particles.  It is added to clay to increase its resistance to shock.  In ceramics sand is not (normally) used because it is denser than grog and usually has a different vitrification point than the clay itself, which can cause cracking.  Building an oven is different than firing a bowl or sculpture, so sand should be fine.  Follow Ernie's instructions, just be sure to let it dry slowly and start with small fires to prevent excessive cracking.

Good idea on the small fires.  I thought I would put fans both on the inside and outside to help dry it out for about a week before I started the first small fire.  I also was not going to superinsulate it with the loose vermiculite until it had time to "cure" fully.  The durock structure around it and a small tarp should keep it from getting wet from rains.

In the video a steel door with 2 small vents at the bottom is used to help pull air into the oven.  Do you think that a door made from durock would hold up to the temperature?  He seemed to pull it out with his bare hand leading me to believe that the door does not get very hot?

I dont know anything about the durarock so i cant comment on it.
however; the air intakes need to match the cross sectional area of the chimney for the cleanest burn.  I should probably say folks need to take that up as a rule of thumb for all wood burning devices that you want a clean burn from.

Does the metal door get very hot?  I have some relatively thin sheet metal stock that I could bend around 3/4" plywood or durock.  If the door does not get very hot then I think this would last and be simple to make.

I don't understand what you mean by:
"the air intakes need to match the cross sectional area of the chimney for the cleanest burn"

Could you give me an example?  I am assuming that the air intake are the 2 cutouts on the metal door piece.  Is there an equation for sizing these in correlation to the "cross sectional area" of the chimney?  I was amazed at how "roaring" your fire got and how high it was shooting out of the chimney pipe.  I would like to put a chimney cap over my pipe to keep rain from getting in when not in use.  Considering how high the fire was coming out of your pipe.......I figure that I will have to get some fairly heavy gauge steel or the chimney will just burn through it in a short time.

If anyone is interested, I will take pictures of my progress and post them here. 

I really do appreciate the great responses and all the help!

the air intakes need to have the same area as the exhaust.

pi R 2 (exhaust) (circler chimney) 

Length X Width (rectangle) square

some one probably has the notation stuff that can clear up the math for you. but thats the best i can do.

oh yes the burn door gets very very hot. again i dont know anything about the board stuff you are using so i cant speak to it.

Perfect answer for the air intake/chimney size.  I understand fully.  I found some heavy steel shelf's last night in the garage that I should be able to cut and bend like you did for your door.  This same stock could be used as a chimney cap.

I have some 4" thick oak stock that I thought I would make the insulated oven door out of.  This came from a downed tree on my neighbors property.

Is there a calculation for the size/depth of the chimney chamber in correlation to the baking chamber?  In the video it seems to be about 20-25% the size of the baking chamber.  The heights seem to be about the same.

25% of baking chamber is about right.
this is the heats your oven will be burning to. last weekend we took temps on our oven and where hitting 3000 F in the chamber and 2500 on the door (it dissipates fast). What ever you use keep those temps in mind. If the substance cant stand up to the heat its probably not a good thing to insulate. A nice chunk of pretty tile might do a better job (ya so i like tile).

Great thread! I'm very interested in building a double-chamber oven like this as well.
I'm curious though, does anybody know how the wood consumption and heat retention compares to a traditional cob oven?

Does the heat escaping from the chimney mean that this uses more wood to to reach sufficient temps for baking and then looses heat more quickly?
Or does the heat retained by the cob and higher initial temperatures from the more efficient burn negate whatever heat is lost through the chimney?

Thanks.

Johnzilla wrote:
Great thread! I'm very interested in building a double-chamber oven like this as well.
I'm curious though, does anybody know how the wood consumption and heat retention compares to a traditional cob oven?

Does the heat escaping from the chimney mean that this uses more wood to to reach sufficient temps for baking and then looses heat more quickly?
Or does the heat retained by the cob and higher initial temperatures from the more efficient burn negate whatever heat is lost through the chimney?

Thanks.

My understanding (i could be wrong) is that the second chamber uses flue gas that would normally be wasted anyway. If the second chamber can be close to the first then less wood might be used. In nay case it should not be more.

Yes, Id love to know the answer to this one too

does anybody know how the wood consumption and heat retention compares to a traditional cob oven?

I have been asked to make a cob oven for someone and we are just now trying to decide on which design to use. Ive always made the traditional single chamber dome ovens, but I could switch to the double chamber oven depending on how more efficient it is.