FINALLY!!! - Start Of Building My Oven

[david s]

Re: FINALLY!!! - Start Of Building My Oven

The best combustion chamber and the best form for radiating the heat back, is the hemisphere. Any deviation from this form is a compromise. If you want to bake a number of bread loaves for instance, then the hemisphere has its limitations. Straight sides may enable you to fit more in as will corners, but the downside is an oven with cool spots and uneven heating.

[Oven_Man]

Re: FINALLY!!! - Start Of Building My Oven

Hi David.
Thanks for you response.

You've been missing in action for a few days. Good to have you back.

So I guess since Dmun made a comment that my drawing looks like a Hemisphere and you said that the best combustion chamber is a hemisphere, I can safely say I am moving in the right direction.

My drawings are almost complete and I will start the creation of my mold shortly.

Wish me luck.

Regards
OM

[Oven_Man]

Re: FINALLY!!! - Start Of Building My Oven

Hi All

After much contemplation and advice from everyone, I have decided to take a different approach at this oven.

Initially, my plan was to make a mold and then pour my refractory cement into the mold to make my oven and then put all the pieces of the oven together.

After a friend knocked some sense into me, I realized I was being foolish.
I was getting ahead of myself by trying to make my oven as perfect and as "World Class" as possible. What I failed to realize was the fact that I was a beginner at this, never going at a project like this or even close to construction work before.

Now going forward, this is the new approach I wish to take.

Rather than a mold, I will start by making a sand dome.
I will then begin to plaster my refractory cement over the sand dome in a 1.5" thick layer. After the refractory cement dries, I will dig out the sand dome through the mouth opening of my oven.
I will then place my 4" thick ceramic blanket over my oven and then add another 1.5" thick layer of refractory cement.

Above is a summary of what I wish to do.
Feel free to correct me where I may be wrong.

Just a few questions if I may

1) To keep this oven as light weight as possible, how would you suggest I go about doing the vent? (I was considering a metal fabricated vent but how would it affect the heat of my oven and would larger heat loss occur with a metal vent compared to a concrete vent)

2) When plastering my refractory cement over the sand dome, what precautions should I take to avoid having the sand stick to my refractory cement and should I take other precautions or concern?

3) What tips do you'll have for mixing cement without hiring out heavy machinery? Maybe the use of some smaller home available equipment with a make shift attachment

4) With regards to the refractory cement, what should I ask the supplier in terms of the components (Alumina, Rating, Density etc) that the cement needs to have and how much of it?

5) How do I calculate how much of cement I would require based upon my oven dimensions and thickness

6) How do I calculate how much of Ceramic Blanket I would required based upon my oven size?

7) With regards to the concrete slab on which the FB Board & Cooking Floor sits, is the thickness important in terms of heat retention and heat loss. The reason I ask is I am trying as far as possible to reduce the total weight of the oven and plan on making it on 1" thick

8) Since I will be plastering my cement on the sand form, I guess I won't be using any mortar for the Dome. My concern here is how will the dome hold onto the oven base (Concrete Slab) and how do I make sure it is well secured to the base ensuring that it doesn't fall apart or move out of place and fall of my stand when transporting it around?

If there are any questions I may have missed and you'll think I should know, feel free to let me know.

Thanks
Regard
OM

[dmun]

Re: FINALLY!!! - Start Of Building My Oven

Two thoughts:

1) One and a half inches of refractory concrete is not nearly enough thermal mass. This shell is going to be thin and brittle.

2) There's no earthly reason for refractory concrete outside the insulation layer.

[Oven_Man]

Re: FINALLY!!! - Start Of Building My Oven

Two thoughts:

1) One and a half inches of refractory concrete is not nearly enough thermal mass. This shell is going to be thin and brittle.

2) There's no earthly reason for refractory concrete outside the insulation layer.

Point 1 has been noted. I would hate to poke a whole through my dome with just my finger. What thickness of refractory material do you suggest taking into consideration the fact that I want to keep this oven as light as possible.

Point no 2 also noted. What do I use after wrapping the oven in FB Ceramic Blanket.

Any thoughts on the other points I've mentioned?

[dmun]

Re: FINALLY!!! - Start Of Building My Oven

Point 1 has been noted. I would hate to poke a whole through my dome with just my finger. What thickness of refractory material do you suggest taking into consideration the fact that I want to keep this oven as light as possible.

It's really my policy to keep my mouth shut on topics I know nothing about, and refractory castable concrete is certainly in that category. Maybe someone who has some experience will speak up. Thickness? I wouldn't worry about your finger, I'd worry about that log you're throwing in the oven with some force to keep your fingers from being burned. The modular pizza ovens are two and change inches thick, but they are made from fired refractory clay, an inherently tough material, not chalky castable.

Your other question deals with the stucco outer covering of the igloo. This is the other topic I generally keep my mouth shut on, because troweling stucco is a skill that utterly escapes me. There are builders that get these magnificently smooth hemispheres by hand but my efforts in that direction always look like children's mud pies.

Also, I'd rethink your one inch support slab. If you need a thin light support, you need to think metal. One inch of concrete is worse than useless.

[david s]

Re: FINALLY!!! - Start Of Building My Oven

Don't go less than 2" thick with the refractory concrete and add 2% by weight of stainless steel needles. With a single one piece cast the uneven and rapid heat rise will produce expansion forces that lead to hairline cracks, so reinforcing is advisable. Castable refractory sets quickly and does not give you much workable time. Suggest you on;y mix 25 Kg bag at a time. Easily mixed in a barrow with a spade, but mix it well and follow directions on water content. If the mix is too fluid it will not be strong enough and will not stand up against the vertical part of the mould.

[Oven_Man]

Re: FINALLY!!! - Start Of Building My Oven

It's really my policy to keep my mouth shut on topics I know nothing about, and refractory castable concrete is certainly in that category.

Dmun, thankfully you didn't keep your mouth shut. I would have been in for a muddy disaster. I'm actually glad you brought this to my attention.

David s: Thanks for responding. I've never seen what refractory material looks or feels like in reality so I don't really know what I am in store for. I did read a little on it and now fully understand why it would be difficult to plaster the refractory cement over my form which in my case is a Sand Dome. As someone mentioned on the forum, its like Pooh.
I would really like to cast this in several pieces but the problem is creating a mold. I've explored so many options but I always hit a Brick Wall when it comes to the curve of the dome. I just don't know what to use that can easily bend into that arch shape for the dome curve. I will be using a screeder fitted onto a spinner as my guide if I cast the dome in 1 piece. The screeder will help make the entire dome uniform.

I do have another idea which just occurred. I will go ahead with the sand dome idea but just 1/6 of the dome will be in a sand form. I will use plywood on the sides of the 1/6 piece of the sand dome and keep it at a height of the needed height for my dome which is 2". This would have now indirectly created a form in which I can pour my refractory cement into. I don't know if this idea makes much sense but right now I am actually out of ideas on what to do.

You mentioned I should use steel needles. How do I use them when casting the dome in either a form/mold or plastering the cement over my sand dome?

Lastly if I choose to go ahead and cast the dome in 1 piece, you mentioned it is advisable to re-inforce it. What suggestions do you have for reinforcements?

Thanks
Regards
OM

[david s]

Re: FINALLY!!! - Start Of Building My Oven

It is actually quite easy to build the dome in sand. What you do is cut a stick to the length of the height of your dome interior measurement, which will be half the oven diameter. build the sand around it until you have a hemisphere. You will be able to judge by eye this form quite accurately. The correct proportions are sand 80% water 20% but you can muck around with these proportions. It's just like being a kid on the beach again.Cover the sand castle with damp newspaper to give the castable a smooth surface.

[Oven_Man]

Re: FINALLY!!! - Start Of Building My Oven

Hi David
Ok, I may be able to get my hands on a mold but I am also still considering the sand dome. My concern is what you mentioned in a post whereby you said that " A dome cast in 1 piece may get cracks due to expansion as a result of high heat"

David if it's OK can I ask you to answer these few questions.

1) With regards to the refractory cement, what should I ask the supplier in terms of the components (Alumina, Rating, Density etc) that the cement needs to have and how much of it?

2) How do I calculate how much of cement I would require based upon my oven dimensions and thickness

3) How do I calculate how much of Ceramic Blanket I would required based upon my oven size?

4) I am going to use aerated concrete as my hearth. It will be 2" thick. Will this be enough to support everything (dome, floor, etc) that will be placed on top of it?

Thanks
Regards
OM

[david s]

Re: FINALLY!!! - Start Of Building My Oven

Ask your supplier for a castable refractory rated at least to 1000 C I use one that is rated to 1450 C but that is probably overkill. I would say you'll need 5 or 6 25 Kg bags The stuff is not cheap. Use maths to calculate surface area of the hemisphere. 50 mm of blanket should do. I don't use blanket, but use vermiculite instead.Use 3" of the aerated concrete and try to get stuff that has reinforcing already cast into it. This is what I used and it has cracked in the middle (didn't like speed bumps) it doesn't matter because it sits in a steel cradle. The trde name of this stuff in Australia is Hebel.

[Oven_Man]

Re: FINALLY!!! - Start Of Building My Oven

Hi David

I will check with my supplier on what refractory castables they have with the required rating of 1000C or above.

As for re-inforcing my dome, I tried googling a bit on Needles Used For Re-Inforcing the dome but nothing has come up that in simple lay mans terms. Maybe my google keywords need to change. Is there anywhere that has info on Steel Needles and how it should be used on re-inforcements of concrete/cement/refractory material. I assume that they are just thrown into my Mould together with the cement/refractory material?

Other than that, all else should be fine.

Thanks
Regards
OM

[dmun]

Re: FINALLY!!! - Start Of Building My Oven

Anyone who sells the refractory concrete mix will sell the needles: you can't make refractory concrete without them:

Refractory Reinforcing 1" Stainless Steel Needles 10# - eBay (item 130337734449 end time Feb-13-10 06:04:54 PST)

Be careful: They're called needles for a reason.

[Johnny the oven man]

Re: FINALLY!!! - Start Of Building My Oven

Re what to do about covering the sand dome, most people just cover it with plastic sheet then add the castable over that, stops sand sticking to the dome. Also, 2% is the minimum rate for needle addition, go a little higher. The correct terminology for them is "melt extact fibres". Google is your friend.

[Oven_Man]

Re: FINALLY!!! - Start Of Building My Oven

Hi Guys
Thanks so much for all the info on the Steel Needles

I am about to locate a supplier for them and will surely use them as I see it quite important in the construction of a sturdy oven especially one on wheels.

Guys, I know I've asked questions like the ones to follow a thousand times and my sincere apologies for it once again but I need to be sure that I am doing the right thing.

I am planning on an oven with the cooking floor size of 24", similar to what David S has. With that floor size and a dome thickness of 3" followed by a ceramic blanket of 2" and lastly stucco of 1" together with a layer of paint.

Questions with regards to my above plan

1) Would I be able to bake a 12" pie comfortably on this 24" cooking floor?
2) After the 2" Ceramic blanket, can I add the stucco directly over or must something else go before it?
3) Based upon the above thicknesses and dimensions, my oven trolley will be around 36" X 36" (give or take). Is this calculation correct as I would need to advise the company that's fabricating the trolley for me.
4) Is my 2" ceramic blanket (Supplier said it is 96 density) sufficient as the refractory material I am getting is rated at 1600 Degrees Celsius?
5) If my dimensions are correct and my trolley comes in at 36" X 36", how much of refractory cement would I require to build the dome to 3" thick and 13" high.

Thanks guys.
Really, really appreciate all you guys patience and assistance along thus far.

Regards
OM