barrrel vault questions

[chickade]

I'm going to be building my hearth tomorrow and have a few questions. Keep in mind that I am planning a community bread oven.
There is growing interest among members of my town in using the oven for bread and pizza. It is a small town and members are closely knit.
Therefore I am adding additional mass to my hearth by setting the bricks on edge. I plan on using thick insulation instead of cladding on my walls.

1) How should I think about securing the transition between the ceramic fiber blanket layers and the oven base? I have 5 .5 inches of insulation under the hearth (2.5 Foamglas, 3 calsil). I'm planning to cut the insulation boards close to the hearth as I've seen others do with their Pompeii ovens. How does one make sure that there is no heat escaping from underneath the blanket layers? Does the layer of vermicrete that goes over the blanket and chicken wire take care of that issue?

2) Next is a question about the ash drop- I have one that is 2 inches wide and 10 inches long. I'm thinking I might need at least 3 inches width, which I could cut before the hearth goes in. Would you recommend that I widen it?

3)Finally, due to my inexperience, I did not build my base after completing a final plan of the oven. I'm designing the oven as I go and I'm running into size limitations in width (I can manage a 36 inch hearth space) with my 5 ft 2 inch wide base. I'm thinking I could manage a 60 inch hearth depth (I have 8 ft 2 inches length). My original oven design was relatively square and more similar to the Pompeii style (I have Alan Scott plans for his 36 x 48).

I'd like to be able to cook with a fire in the back of the oven. A deeper hearth would allow that. Could I make the oven substantially deeper than wide and still have good airflow?

Thanks so much for your expert advice.

Sarah

[Tscarborough]

Re: barrrel vault questions

1) Will the arch wall sit on top of the hearth bricks?

2) Personally I think it is a needless detail that complicates the design. You have an ash drop as wide as the door, make a bit of overhang on the entry and you can drop your ashes right into an ash can.

3) 36"x42" or 48" would be a big oven, and at 48" you will need long tools and gloves to work it. Airflow probably wouldn't be an issue at 60", but using 80" + tools might. Look at it like this: For pizza with the fire in the oven, you can have 24" wide plus 12" for a fire (I use about 4" for the fire), and unless the door is too tall, you will only be able to work 2 pizzas deep. For bread with no fire in the oven, a 36"x42" gives you 10.5 square foot of floor to work with, or better than 20 loaves per load. p.s. cooking with a fire in the back of the oven is hard to pull off, you will end up feeding pizzas to the flames for a while. Much better on the side where you can see what the fire is doing to the crust.

[chickade]

Re: barrrel vault questions

Thanks Tscarborough, you are the expert. I have only this forum to protect me from major errors. How can I know what choices to make? I will make it smaller, 36 x 42 it is.

The walls are going to sit on top of the bricks. Here is a thumbnail of the design I'm using.

My friends and I are working on the walls and vault tomorrow. It might be around 40 degrees out. Can we use the cement additive that allows it to set in colder temperatures?

Sarah

[Tscarborough]

Re: barrrel vault questions

You should not be using Portland cement, hence no additives. Refractory cement will set at low temps, and so so rapidly, so as long as it sets before freezing it will be fine. You should consider placing the hearth bricks at a 45 degree angle, either in a running bond or a herringbone to prevent catching the edge of the peel as well as for strength.

With the walls on top of the floor, you can allow the brick to run a bit wild, so cut accuracy is not important.

As you have it laid out, how will you lay brick in the corners of the arch?

[chickade]

Re: barrrel vault questions

Tscarborough.
What to do in the corners is a good question. We didn't think of that. Revision seems in order.

As for the Portland cement, I thought one should be using "homebrew" for the brickwork, which is 1 to 1 to 1 to 3 (Portland, hydrated lyme, fireclay, sand), right? I can get refractory cement. I thought the "homebrew" was the same as refractory cement.

We didn't do the herringbone design, didn't get your post in time. It is getting cold so we have to move ahead. You can see in the photo the layer of "homebrew" over the insulation. Is that what you were recommending?

Thanks so much for your interest. If it weren't for your advice I think I would have a disaster on my hands.

[Tscarborough]

Re: barrrel vault questions

Homebrew is fine, but it is not a refractory cement per se.