Newbee Questions on building 42" wood fired oven

[BenKeith]

For the past few months I've been reading a researching wood fired bread/pizza brick ovens and hope to get some guidance.
My plan is to build the 42" ID dome using 36.5% alumina refractory bricks cut in half to make the dome 4.5" thick and cover that with 2" of #8 inswool blanket and then stucco over that.
I plan to use the same bricks laid flat at 2.5" thick for the floor. I'm still working on plans for what to use to insulate under that.
I'm running into a couple of problems getting the refractory brick that's delaying my build for at least another month so I'm thinking about changing boats mid stream here and is where I'm looking for help.
I have 30, 55 pound bags for 43% alumina castable refractory and can easily get more if needed, so I'm thinking of just building a mold and building it with a 4" thick dome of castable with a 2" thick blanket over that. I already have the inswool blanket, and a 2" thick floor.
My concern with the castable is cracking. If I make a dome mold and lay over the top of it, when the stuff shrinks as it dries, it will crack. If I invert it like a big bowl and lay it so there will be no pressure when it shrinks, then I've got a 1,500 pound dome I've got to get flipped over. Doing this in my back yard might make that difficult.
I've also considered trying to do it in a couple of halves but not sure I want the seam to deal with.
Has anyone on here used castable and has experience with it? Sure would like some insight into what I might be getting into if I try going that route.

[UtahBeehiver]

Look at "Other type of ovens" in the Forum Topics. This is where most of the cast ovens preside. I would suggest you look at David S's threads and comments, he is our resident cast expert. I do believe he will say 4" is pretty thick and may not be needed. I would also suggest the 2"of insulation is the bare min. and you may want to supplement with a perlite/vermeculite-concrete insulation which also gives you a great stucco base. There is a nice cast oven going on right now where the builder used foam board in layers as his mould.

https://community.fornobravo.com/for...shelbyville-ky

[david s]

Most commercially cast ovens are only 2" thick, but a 42" is pretty big, you may want to go to 3".
if you cast it in one single piece you will probably get some cracks, usually a vertical one starting from the base at the back, plus a few more smaller ones. This is still ok because it's a dome and won't fall down. Most brick ovens also suffer some cracks too, because of the constant expansion and contraction with uneven heating and cooling.You can add stainless steel needle reinforcing for extra strength (most commercially cast ovens don't).
A cast with castable refractory laid over a mould should have no shrinkage, unlike a cob oven which will have some shrinkage. The easiest way is to make a sand form covered in thin strips of newspaper as a slip layer to prevent sand sticking to the castable and giving you a nice smooth surface on the inside of your dome. Done this way, your cast can be made in situ very easily.

[BenKeith]

Thanks for the inputs. Under the assumption it takes mass to hold heat was the reason for my idea of making the dome 4" thick. I've read some recommend as much as 6" using that home made fire clay stuff. So you say it would be better to make it 3" instead of four. What I have may be a little on the high side for and ideal refractory and might get a little hotter but since I have it, I figure why not use it if I go with the castable. According to the manufacture, this has about 1% shrinkage so I figured at 42", 1% might be enough to cause problems over a mold.

If my #1 Arch bricks were readily available, there would be no issues but those are what have me on hold and I'm not really interested in cutting over 150 bricks for a flush fit in the dome. I'm not into the idea of using wedges on each brick, having done some refractory in large boilers, I want my bricks to fit.

[UtahBeehiver]

There are trade offs with the amount of thermal mass, too little, the oven will not hold heat for extended use, but lighter, heats quicker, and uses less fuel. On the other end of the spectrum, thick thermal mass (typically used by high production bakers or commercial apps) are heavier, take longer to heat up and use more fuel (wood) to get to temp.

So are you deciding whether to cast or build a brick dome, I am not certain in you post. I was just merely pointing out how the brick face need to be at 90 degrees to the center point of the dome floor. With these type you will just need to aware of the alignment.

[BenKeith]

OK, All the research and plans up to this point have been with the intentions of building a brick oven. However, for me to do that without having to source another supplier of #1 Arch bricks at twice the cost of what I can get them for, it's going to be a minimum of a month and possibly longer. I know I've seen it done on this site where they just wedge flat bricks to make the dome, but my problem is I'm somewhat of an anal perfectionist and don't care for that design. I want my bricks to some a lot closer to fitting the contour and I'm not really wanting to cut everyone to do that. I do have some experience with doing refractory years ago and would have never dared try laying brick in that manor in a big boiler or something.
Now I'm giving serious thoughts to building a cast oven because I already have enough materials to pretty much build it. The only thing I'm missing to do that is the know how and something to insulate the floor with,
I figure if I don't like the cast or I screw it up, I'm only out the 20-30 bags castable, which didn't cost me anything anyway, and I can just knock it off and build the brick on top of the hearth the cast was on.
I started digging my foundation today, and my wife hit me with she wants me to lay a 12x25 slab for a patio to go along with the oven. The temps are hitting the 100's now and at 70 years old, the waiting on brick might not be a problem by the time I get the that big of a slab dug and poured.

[UtahBeehiver]

The No. !arch bricks will only reduce the number of taper or angle cuts in the dome not the bevel requirements that will be needed to eliminate the "inverted" V in the upper courses. I have attached a couple pics for your info, One showing the taper/bevel cuts required for tight nice fitting joints and two a picture of a build that has inverted Vs due to no bevels and some other issues as well.

[BenKeith]

I know the #1's won't eliminate all the angles that I need to cut but that's two cuts on every brick in the dome and that will help, Figure that's almost 300 cuts just to do those two sides. I have a sealed motor for my 10" Delta table saw and have made all electrical sections water proof and have set it up a couple of times to use as a wet saw with diamond blades. I also have a 12" dry cut blade for my DeWalt 12" compound miter saw I was planning on using on the end cuts. Not sure how hard these bricks are to cut with a dry blade, but if I go with bricks, I guess I will be finding out. I know a number of people are reading this and thinking I must be an idiot wet cutting on a table saw and don't think much of my equipment but when prepared for it, it works pretty darn good. I've done it several times in the past 30 years or so and it's still going strong.

[UtahBeehiver]

Silica from dry cutting or grinding is a health concern for the cutter as well as those around you so appropriate measures should be taken. Good luck.

[BenKeith]

It's wise to warn of the hazards. I don't have close neighbors and will be taking prercautions. I also do some sand blasting from time to time and the silica sand is a problem then also.. I will be out in the open, have a 42" shop fan behind me, wearing a Tyvek suit and a full respitator. When done I blow off the equipment and pressure wash the area.

[BenKeith]

Well, I got the first 12'x12' section of the slab dug out, formed in and sand base ready for the cement. A heat index of 107 degrees and in the sun most of the day, it's kicking my butt. Put some rebar in where I'm going to build the oven on it and get it poured and I will finally feel like I'm off to the races on getting this thing started. My wife is thinking here picnic table is going to fit on that slab with the oven, I haven't told here it's not, or I would be digging a much larger slab just to get started.