Embarrassing question from long time reader

[Bert]

Hi Folks

OK, I started my build but ran out of money for materials due to their expensive nature in OZ.

I?ll be picking up some more bricks and hearth bricks so I can continue and hopefully get this thing fired up by the end of winter.

I?m currently getting together my IT and I?m confused ?my oven is 1100mm diameter (internal). I thought I would just continue up to the apex at a 550mm off centre, but on reviewing a few threads, do I need to reduce the height?? If this is the case the IT needs adjusting but how do you know when to start altering?

As you can tell I?m no engineer.

HELP!


Kind regards Bert

[v12spirit]

Re: Embarrassing question from long time reader

This ain't an embarrassing question. I also was always wondering how the IT would be adjusted to achieve a low dome. I have an idea.
1- On a cardboard, draw a cross section of your low dome with the curve you see suitable for being "low".
2- Divide your drawing by the size of the bricks you will be using as in the drawing below
3- You get multiple line segments representing the change of the diameter i.e. the IT wrt brick layers.
4- Accurately measure these line segments to get how to alter the IT as long as you put on more layers.
Note: You MUST modify the IT for the purpose of the low dome so that you get a cross section like the “new IT” drawing, with the end of it able to rotate upwards and downwards with a fixing ability.
How to lay the bricks:
1- Lay the first layer with the diameter set to the hearth diameter
2- For the next layer, set the new IT to the length of the next line segment
3- Lay the first brick in the new layer with its LOWER inner edge identical with the upper inner edge of the previous layer and its UPPER inner edge identical with the new IT “fix-able joint”
4- Make the new IT flush with the inner face of the first brick
5- Fix the joint
6- You got the new IT adjusted for this layer. Use the new IT to finish the layer
7- For the rest of the layers repeat the adjusting process accordingly.
This is just a suggestion. I’m in no way an expert. I’m just a newbie.

A cross section of the low dome:


The original IT:


The modified new IT:


Regarding if you were compelled to reduce the height of your dome. It is a design preference; you can go with a fixed diameter so the dome is a hemisphere, or make the dome low. According to the plans, the first design is better at holding heat and is more fuel efficient, while the second is more appropriate for pizza making.

[Greenman]

Re: Embarrassing question from long time reader

Hey Bert - I built mine as a hemisphere and made no adjustments as I went up with the dome. Worked fine for me and I am happy with the oven. It is about what you want to do with it, personal preference and the rest.

You can flatten the dome by reducing the length of the IT with each chain once you get up to where you want to start flattening it and V12 has some legitimate thoughts about that.

I am sure that your plea will be answered in more detail by others with more experience and knowledge about such things. Good to hear that your are back in the building business.

[jeeppiper]

Re: Embarrassing question from long time reader

I really think that either way you will be happy with your choice.
I did a hemisphere and it works just fine. I don't understand why it is thought that the low-dome is "better" for pizzas. I realize the heat source is closer to the pizza, but my hemisphere can spit pizzas out in less than 30 seconds, depending on the size of the fire.

[wotavidone]

Re: Embarrassing question from long time reader

I reckon the decision on dome height depends on the thermal properties of the bricks somewhat.
My 750 diameter oven is a hemisphere, so the internal height is 375 mm.
I sometimes have to "dome" a pizza. There's a bit of a mismatch there, and the crust cooks before the top is as browned as I prefer it. Especially if I get the oven up to 90 second pizza temps.
I used two different sorts of brick, one for the floor, one for the dome and I suspect the transfer rate to the crust from the floor is a bit higher than the radiant heat rate from the dome. So I often think, for pizza, I could have made the dome lower.

Then I helped a mate build an oven. 850mm diameter, hemisphere shape, same bricks for floor and dome. His is better matched, and his pizzas cook more evenly.
It's for this reason I counselled my mate against using firebricks for the floor and ordinary brick for the dome, I reckon they have to match.

Anyway, if you want to lower the dome height a little bit, there are ways to do it without modifying the IT.

For example, imagine if you build your dome so that the floor is inside the dome, rather than the dome sitting on the floor bricks.
Get it all set up, with the floor cut and laid, and the first course of dome bricks laid.
Then take out the centre bricks and run your IT off the insulation layer.
After the dome is built, reinstall the floor bricks and then the dome to floor height is reduced by the thickness of the floor bricks.

Even simpler, but limited in how far you should go, is to offset the "hinge" bit of the IT from the centre pivot point.
Imagine using a T hinge instead of a butt hinge and you can offset the hinge point an inch or so from the centre pivot point, so the arm of the IT is shorter to reach your first row of dome bricks. Then when the arm swings towards the vertical it's shorter than it would be if the IT swivelled up from the centre point.
Note that the shorter you make the IT, the bigger the hole in the middle to fill with horizontal brick, and I assume there would be a point where that horizontal brick stops being a keystone makes the dome unstable.
However, I reckon you could achieve a couple inches lower dome this way.

You have to do a bit of thinking to get these methods right, but it can be done. Note also tha the flatter you make the roof, the more it will try to push outwards. At some point buttressing might be required.

On the other hand, my personal experience so far with a couple of ovens and oven owners is that we all, even those of us adamant that we wanted an oven mainly for pizza, graduate fairly quickly to all the other goodies that can be cooked in a WFO.

At which point a hemispherical dome is a mighty fine thing that does a more than merely acceptable job of pizza and has plenty of room for the aforementioned other goodies.

[kanoer54]

Re: Embarrassing question from long time reader

Bert,
Don't know if this will help you or not or even answers your question, but I built the IT on a lazy susan type system. My dome is 39 inches diameter but only 16 inches high. This system works until the IT is in a vertical position. The remainder of the dome needs to be built by hand. This is my first oven build and found this worked really well. Hope this helps.
Jon

[Bert]

Re: Embarrassing question from long time reader

Thanks to all for the very informative comments. As I am planning on cooking much more than just pizzas in this baby I thinks I'll go for the hemisphere, as said if required you can always lift the pizza to get extra heat into the top of the pizza at the end.

I must say I have seen several pizza restaurants where they do this.

Yet again the forum has come to the rescue.

Back to the soldier course...beautiful Autumn day @ 25 degrees!

[Maltpizza]

Re: Embarrassing question from long time reader

Hey Bert, my dome is a hemisphere and I have no issues cooking pizza on the floor of the oven and getting a nice looking cook on the top of the pizza. And yes you can raise it to the top to finish it off if you wish. In my humble opinion, I prefer the hemisphere as it a great all round oven, not just for pizza. Having said that, you can cook other things in a low dome oven, it?s just for roasting meats ect, I prefer a higher dome. Less burn on the top of the roast.
And yes, this forum is great thanks to the vast experience in oven builders and their willingness to share knowledge. I has saved my butt numerous times when I did my build, unfortunately I lost my password and could not reset it, so I have to start over again under a new username.
Good luck with your build and enjoy.