36 inch homebrew oven in SoCal

[Warhorn6]

Hey there fellow pizza aficionados. First and foremost i want to thank everyone here for the vast amounts of knowledge and inspiration. I don't even know how i got the bug to build a pizza oven, but i'm glad i did and i'm glad to have found this wonderful community.
So after lurking for some time i finally decided to cast my oven . It will be a 36" I.D . Although most would advice against using the yoga ball method i opted for that because i worked on it little by little (having small kids it's difficult to get much free time). I built a frame around the ball so that i would get 4 sections. That would not only help the problem with cracking but also make it a hell of a lot easier when it comes time to move it to its final spot.
Soon we will begin construction on our outoor kitchen.
To keep costs down and make it simpler i was thinking of placing the oven on top of the 3/4 inch granite slab we will be using for our countertops. I know everyone recomends a 4 inch reinforced concrete slab but i read somewhere that granite can support up to 1,000 lbs. I'm still not sure what course of action i will take so any advice is welcome.
For the yoga ball platform all i did was cut the hole out of the plywood and placed that ontop of my saw horses
( nothing fancy) i then cut out 2 pieces of plywood and locked them in a half lap joint to make the quadrants and act as thickness guides.
She came out a little thicker at the bottom
( its ok that's how i like em ) and she aint all that pretty but once i insulate and render you wont be able to tell. Now all i have left to cast is the flue gallery and decorative arch.
Still trying to figure how to attach a double walled flue pipe without the use of an anchor plate. I dont know if DavidS 3 tab method would work. Either that or a single wall to a double wall but just have to find the right pipes. Any way thanks for stopping by and im looking forward to finishing this project with all of you.

[david s]

A granite slab is not really a good choice as it does not like heat. I've done repairs on two ovens that both had cracks right through the granite slabs they were mounted on, despite the fact that both had insulation between the oven floor and the granite slab. Short of a total tear down I couldn't see any way to replace the granite. Once it has cracked it also can provide a pathway for water intrusion to the underfloor insulation. It would be far safer IMO to save your nice granite fro your countertops only.

Regarding the dome sections, any large refractory section can be vulnerable to cracking. The larger the section the greater the risk. Because heat rises by convection the tops of your 4 sections will get way hotter and have more thermal expansion than the bottoms of each section. This condition can lead to uneven expansion, stress and possible cracking. Don't be too alarmed as my own oven at home which is a one piece cast dome, around 15 years old, is still firing fine and in good condition, although it has a hairline crack running around the dome in almost exactly the same position as the revised multi section casts I now make. My own solution is explained in the pic. The lid section (not shown) can expand without placing stress on the other oven parts.

Out of interest, what mix did you use to create the castings?

[Warhorn6]

Thanks for the reply David, i figured about the granite. Only reason i considered it is because the contractor is making a fuss about the framing being lower for the concrete hearth than the rest of the kitchen , I was going to build a block stand myself but then the wife talked me into a whole outdoor kitchen. Do you think your method of attaching the flue pipe will work for a double walled pipe?
Thanks again.

[david s]

Yes certainly. The inner pipe can be fitted using the tab method and then the larger outer pipe slid over it. I did one like that recently because the flue is attached and supported by the timber of a pergola (see pic).in this case, because my ovens are small , two lengths each of stainless 5" inner and a 6" outer were used with some blanket stuffed between them to maintain an even space between the two.

[Warhorn6]

Yes certainly. The inner pipe can be fitted using the tab method and then the larger outer pipe slid over it. I did one like that recently because the flue is attached and supported by the timber of a pergola (see pic).in this case, because my ovens are small , two lengths each of stainless 5" inner and a 6" outer were used with some blanket stuffed between them to maintain an even space between the two.

Awesome, i was just a little concerned because all the pictures online of double walled pipes look like they are welded together at the ends.
I decided to demould today, there is some voids and even some valleys caused by the newspaper . The 2 newer sections are still fairly damp so i will fill those asap. The 2 older sections are dryer, don't know if i should try and fill those voids too. I'm afraid the homebrew won't bond and fall off on food. Could it have come out better? Yes of course, but still happy with the result . After all i'm not building this thing to look at it.

[david s]

That’s looking great. Once you’ve tidied the voids you should throw some wet hessian bags over the oven and cover it up to hold in the moisture for at least a week to enhance strength.

[Warhorn6]

Noooooo ! My leg broke! I knew I should of gone with a 16 inch opening instead of 18 inches. It would of given me a little more meat in that corner. I just wanted better access, guess i'm going to have to recast my entrance. So bummed now, i was so excited too.

[Warhorn6]

Getting ready to cast my entrance piece again after the first one broke. Originally i was going to cast the flue gallery seperately , now i'm thinking of doing it all in one piece with the last quadrant.
I was also thinking about perhaps adding a small thin rod in that corner as it still seems like it will be a weak point. Would that be advisable or would it be counter productive do to the expansion of said rod? I have edited the photos to show where i want to add it. I have some brass about the diameter of a coat hanger that i was going to use. Any advice, criticism , and opinions welcome. Thanks for stopping by.

[david s]

Brass is very conductive, around 8 times more than stainless steel, so maybe not the best solution as a reinforcing material, although it does have the advantage of not rusting in a hot and moist environment.. The recommended reinforcing for refractory material is stainless needles. Using a material with high conductivity creates problems if it expands more than the material that surrounds it.The needles present a greater surface area which enables them to conduct their heat more readily to the refractory while the thicker material cannot. This is not a problem with thick steel rebar in concrete because the change in temperature is so slow. But a different matter when an oven is pushed to 300+C/hr rise.
I had a flue gallery casting break in exactly the same position as yours. My solution was to add buttresses to strengthen that area. Of course you can just make the whole casting thicker io obtain more strength, but you don't really want too much thermal mass in the gallery because it will act as a heat sink.
For the temperatures we fire to AR fibreglass fibres are also an alternative, they don't begin to melt until around 900C and have the advantage of not increasing thermal mass and not producing incompatible thermal expansion.

[Warhorn6]

Hi David thank you for the response, after my post yesterday i did a little research and concluded it would not be a good idea to use the brass. Do you have a side view picture of that gallery? Do you think making the gallery and the entrance quadrant in one piece is wise, Due to the heat sink issue? Or is it better to cast seperately as was my original idea. Thanks again.

[david s]

I'm not sure. There will be a big difference between the temperature at the top of the dome casting and the bottom, but an even bigger difference between the top of the dome and the flue gallery. As thermal expansion is proportional to temperature it follows that separate castings might be a better approach. But being a big fan of pushing the envelope, you do what you can get away with, so an integrated dome/gallery casting might be just fine, try it and see, then redesign for your next build if necessary. Furnace builders do exactly this, by placing the expansion joints where cracks developed in the previous build.There is a further problem with a one piece casting in that creating the door rebate with a mould will be difficult. Doing it separately eliminates this extra mould making problem. You also need to consider how to make it easy to de-mould.

My gallery weighs only 12 kg and is around 25mm thick because my priority was to reduce thermal mass. Hence the need for the strengthening buttresses and fibre reinforcement.
Don't over-worry cracking as it's just the structure deciding where it would like to place the expansion joints, they will have no perceptible difference in the oven's performance.

[Warhorn6]

Thanks again David. I mainly want to avoid it completely braking off like it did before. I have just enough SS needles to add to the mix but i'm gonna try to find some AR fibres as well.

[david s]

This is the little tool I made to add the buttresses soon as I've laid the mix over the mould.It works a charm, results in providing strength where needed and contributes little to the overall mass.

[Warhorn6]

Looks fairly easy to make, just need a piece of tin. I'm also thinking of re designing the gallery to make it a little more shallow.

[Warhorn6]

I was finally able to cast the flue gallery and remaining dome quadrant. Its been damp curing for a little over a week now. I casted them both at the same time but if my plan worked they should come apart because i put some cling wrap in between them. Also went ahead and followed davidS advice and added some buttresses to the gallery. Now im just debating on wether to make a 5:1 perlite/ cement under floor insulation or bite the bullet and opt for the CalSil board. I know it will be alot faster and easier to buy the board but i dont know if i want to spend that much . Also i have some leftover dry mix with ss needles a pp fibers, is that ok to use for the decorative arch? Can't wait to move on to the next step.
Anyways thanks for stopping by.