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okn 07-25-2011 03:25 PM

Why is the hearth so thin? Or Walls so thick?
 
Quick design question: Why do the plans call for a 2.5" thickness of fire brick (i.e. a standard fire brick on its side) on the floor, but call for a 4 1/2" thickness (i.e. half a brick length) for the dome? Would it be better to have a thinner dome, or a thicker floor?

brickie in oz 07-25-2011 03:48 PM

Re: Why is the hearth so thin? Or Walls so thick?
 
Most heat retention will be in the roof/dome.
You could make it all the same thickness/thinness it really wouldnt make that much difference apart from fuel and time use.

Edit: My oven floor and walls are all the same thickness.

aceves 07-25-2011 04:41 PM

Re: Why is the hearth so thin? Or Walls so thick?
 
okn,

Heat rises, so the more thickness on top the better. Also, the heat is reflected back down to the floor via the dome... Or so I think...

This is a tried and true method. You can't go wrong...

aceves

azpizzanut 07-25-2011 06:00 PM

Re: Why is the hearth so thin? Or Walls so thick?
 
Hello OKN,

Look at the Pompeii oven plans for insight on oven design and materials choice. Of interest is paragraph 1 on page 7, and the last paragraph on page 35.

Cheers,

okn 07-25-2011 10:10 PM

Re: Why is the hearth so thin? Or Walls so thick?
 
"Look at the Pompeii oven plans for insight on oven design and materials choice. Of interest is paragraph 1 on page 7, and the last paragraph on page 35."

Paragraph 1 on page 7 says 2-4 inches is fine, 95% of the people on the forum build to 4 1/2, which seems fine too (and what the plans call for). The ovens FB and other sell are much thinner. Why not 3" or 2 1/2"? Or use a split brick on the floor and make it even thinner. From what I've read about people who have inserted thermocouples into their builds it seems that the outside (facing the insulation) of the firebrick, doesn't even come close to the face of the firebrick. That's even after hours of having a fire than hours of not having a fire... So I guess my thinking is, while the brick does store a significant amount of thermal energy, even if not totally saturated, would it be better to try and make a thinner dome? Would that not speed heat up time, and reduce amount of wood burned?

Am I missing something? Am I totally off base? Give it to me straight even if it hurts!

All the best,

Kevin

david s 07-26-2011 02:46 AM

Re: Why is the hearth so thin? Or Walls so thick?
 
It's a bit like a battery, thicker walls and floor equals more thermal mass, which means more heat storage. This is an advantage for baking and roasting, however if you are doing pizza and maintain a fire on the side you are continually recharging your battery. Another possibility is to reduce the thickness of the inner oven walls and floor, but compensate by increasing the thickness of insulation.

dmun 07-26-2011 05:06 AM

Re: Why is the hearth so thin? Or Walls so thick?
 
Quote:

Why not 3" or 2 1/2"?
I built a two and a half inch thick dome, and it's full of cracks. I wouldn't do it again. Anyone planning to build an arch or a dome should be aware of this page about the structure of domes:

Auroville Earth Institute

In brief, the strongest shape of dome is a catenary, the shape a chain assumes when suspended between two points. A round dome is going to be less than stable unless it is thick enough to contain the imaginary catenary shape.

And as far as heat up time, my thin dome oven seems to take the same 1.5 - 2 hrs to burn white as everyone else's.

As far as the thickness of the floor, 2 1/2 is all you need unless you are building an oven for a commercial bakery where you are going to be doing repeated retained heat bakes.

david s 07-26-2011 07:46 AM

Re: Why is the hearth so thin? Or Walls so thick?
 
The strongest catenery arch is one which has it's base approximately equal to it's height. Departure from this produces a progressively weaker structure. Unfortunately for us what we require is an arch with a height approximately half of it's width because a higher dome results in having a heat source too far away from the food. A low catenery results in a dome with it's base fairly severely angled in producing a shape with little room at its base and it's strength compromised. I believe that the traditional hemisphere in this case would be a stronger structure.


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