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armac 09-21-2006 11:01 AM

High mass Pompei oven
 
Does anyone have any experience with a higher mass Tuscan style oven somewhat more oriented toward bread baking rather than pizza?

What I’m considering is an artisanal baking enterprise with the oven being fired a minimum of 5 days a week, baking and roasting everything from pizza to large roasts. I’m thinking about a diameter of 60” and a lot of cladding and insulation. Does anyone think it makes sense to use a double layer of firebricks, maybe with the lower layer mortered in place, for the hearth or some other method of creating thermal mass above the insulating layer? What about creating more mass over the dome itself, does a Scott style mesh and concrete cladding make sense? Does the aluminum foil “slip plane” layer make sense or is this just overthinking the forces at work?

I like the round oven design and would like to pursue my idea of “radial” construction as previously posted, but want to be diligent about the details before proceeding. Thanks.

Rob

dmun 09-21-2006 04:28 PM

High mass oven
 
I don't have any experience with high mass ovens, but I don't think your additional mass needs to be refractory, that a layer of concrete will do just as well. I don't think the slip plane is needed, particularly if you do the stack-and-fill method of base construction, which might well have a little "give" built in.

I once built a fireplace, where a single layer of fire of firebricks was backfilled with concrete: there's been no cracking or other signs of thermal stress, but there is no intense heat like in a full firing oven, either.

jengineer 09-21-2006 04:38 PM

Calling Jim and James -

The Pompeii oven (a Tuscan style oven)as described in the how to section and on this forum has a low to medium thermal mass. If you have read through the past posting one thing you will realize is that we have flipped the order of the oven, bottom up

concrete structural floor resting on the hollow block walls
followed by an insulating layer of either vermiculite cement or Isol board
a firebrick floor and then the firebrick dome.
On top of the dome is a 1 to 2 inch layer of thermal mass (fireclay and cement)

this is then insulated to keep the heat in the oven the more insulation the better. Most of the bakers here are once a week or once a month bakers and are not in the business full time. as such one would bake pizzas and then throw in a roast the next day.

For your endeavour you will want something that has a lot more mass and definately more insulation to keep the heat localized in the oven. Look for threads with the term 'canuckjim' in it. Jim has built and is a local artisan bread maker using a Scott oven. He has noted and resolved a problem with the lower insulation - it was not thick enough and he was losing heat through it. So here is a non-builder engineers thoughts on what you would want to do - I feel so Walter Mitty.

Go back to the old style of Pompeii cooking floor but add some mods. Note this will be a slightly higher oven so you may have to modify either the block walls or make a landing to stand on.
4- 6 inches of insulating reinforced insulating vermiculite
2 inches of isol board
4 inches of thermal concrete
firebrick floor
firebrick dome
3 inches of thermal concrete
lots and lots of insulation

since you are going-pro you should add in thermal probe to
the middle of the insulating concrete floor
1 inch below the firebrick floor
1 inch below the face of a firebrick on one of the side walls
1 inch below th face of a firbirck on the dome
and one in the middle of the dome insulating concrete

I have not built my oven yet, but I did put a roof on the house finally. When I build mine I plan on making some changes that James has not gravitated to as not too many folks in the states have used the method, not a proven concept. 6 inches of insulating vermiculite, so we have been informed, has the same properites as 2 inches of super Isol board, so for me I am going to use Isol quicker maybe a tad more expensive but you get the same results and it lightens the oven. In the above scenario, and this is what I will do, I would use a tubular metal framework of steel, welded and then lay a 1/8 inch stee tray on top of it. Place down 4 inches of super isol and then pour the thermal concrete mass. add in the firebrick floor etc.

I do not think that even the Scott oven (Hey Jim verify this) has used a double layer of firebrick. That just sounds too industrial.

the reason you want more thermal mass is that you don't want your oven to cool down too much during your bakes and you don't want to have to keep adding massive amounts of wood to get it back up to cooking range after the night. Keep in mind that it may take 4 hours to get the oven at some level of equalibrium. Again go look at CanuckJim's posts.

If Alf is not too busy he will correct my statements and if the experts totally disagree with this then we can modify it so that you can use it.

All seplling and puncutation errors are due to the machine not the operator

je

CanuckJim 09-22-2006 02:42 AM

Double Layer
 
Armac, jengineer,

Joe Engineer is correct, as usual, except for the machine generated spelling errors. Mine is a high mass Scott design bread oven. I did not use a double layer of fire bricks, but I did install seven inches of refractory concrete over the dome. Also used a slip plane of foil. My plan was to use the oven for multiple bakes, and that's what I'm doing. It does take significantly longer to fire an oven like this, though. Recently, I've been firing overnight to lessen the amount of time it takes to bring it up to speed on bake day. Let me know specifics, and I'll answer best I can.

Jim

arevalo53anos 09-22-2006 03:54 AM

3 Attachment(s)
A little detail that could be missing there is that Alan Scott built his ovens with the firebricks laid on their side, even on the hearth as in the barrel.

With this arrange, the mass of the firebrick is the mass being equivalent to four and half inches.

Since he added another four and half inches (or more) of cladding, it is there the 9 inches thermal mass.

In the vast majority of the home builder ovens, these brick were laid plain, resulting in a 2 inches mass.

Mine has plain refractory bricks plus two inches of (island) cladding in hearth and half bricks with less than half inches cladding over the dome, resulting in a whole oven that goes from ambient temperature to 400°C (750°) in one and half hours and maintaining very well (it was not built to lot of batches, anyway)



Luis

james 09-22-2006 04:22 AM

If you are attracted to the round cooking floor and traditional round dome, there is no reason why you cannot add further mass to a Pompeii oven design. I think the easiest way to do it would be to encase the dome with either standard concrete or refractroy concrete. You could build it up with a form, that would everything in place while it was drying.

The choice between standard concrete and refractory concrete is one of performance/longevity vs. cost. A true refractory will do a better job of conducting and holding heat, and will last longer if you are doing to do some serious baking. But, calcium aluminate is hard to find and harder to work with. Also, your sand and gravel aggregate isn't a true refractory, so you are in the middle somewhere.

If you are going to do serious baking, I would recommend using refractory concrete. There was an Alan Scott oven in a bakery in Sonoma county in CA that fell in after a couple of years, and they had to completely rebuild it -- and try to not lose all their customrs. It all came out OK, but it's a good lesson for commercial builders.

My understanding from my Scott ovens is that the aluminum flashing between the brick and concrete layers is a slip plane. The brick and concrete expand and contract at different rates, so the aluminum reduces friction and promotes longevity.

Is that right?

One side note. The original brick ovens in Pompeii were bread ovens, and they had thick brick on end domes, and concrete cladding. So, if you built one, you would be in great company.

Here are the photos. There are some examples of how they did it:

http://fornobravo.com/pompeii_oven/p.../ventarch.html

James

arevalo53anos 09-22-2006 06:37 AM

"My understanding from my Scott ovens is that the aluminum flashing between the brick and concrete layers is a slip plane. The brick and concrete expand and contract at different rates, so the aluminum reduces friction and promotes longevity"


This is right!
No such things as reflected heat and so on...
The only inconvenient that I see in the aluminum sheet between brick and concrete is that this foil stop the humidity from the embers go up and out.
This could be not a great problem, because the high temperatures in the dome and the slow rating to return to ambient, too.

Luis

Alf 09-24-2006 04:07 AM

Glad we have some high mass posts, I can have some fun.

The main difference from a building point between a high mass oven and say the Forno Bravo pre cast or the Pompeii oven plans on the website are weight. To get higher mass you have to add weight be it firebrick, concrete or in the older ovens sand. Once you add weight you add stress on the ovens arch. One way round it is to build an oven with a high “self supporting” arch like the original Pompeii ovens @ http://www.fornobravo.com/pompeii_oven/history.html or the newer square industrial type “scotch” coke / wood burning ovens. The arch being high can support the extra weight like a stone built road or Rail Bridge.

Problems associated with these designs are the distance from the cooking product to the ceiling of the oven, the amount of hot air circulating in the oven and fuel consumption. The high ceiling of the oven prevents the radiated heat from the ovens ceiling cooking the product so you lose the crisp brown top of the bread. The extra hot air within the oven can escape (to be replaced with cold air) when the door is open during oven operations thus increasing fuel consumption.

So, oven design moves forward and we get the lower arched Italian pizza ovens and the French / German low arched Vienna type ovens. To cope with the stress that a lower arch exerts these ovens are built with the bricks / stones on their ends mainly to increase the arches / ovens stability and also to increase mass and cooking times. Interesting example http://mha-net.org/docs/v8n2/wildac03b.htm

Once the ovens cooking area gets above about 1.4 m (55") the sidewalls of the oven need to be supported to prevent the ovens arch “pushing” the ovens walls outward. This can be achieved by enclosing the oven with thick stone walls like the old French ovens are, using steel to provide the structural support, concrete like a Scott oven or a combination. The reason the Scott oven collapsed as mentioned by James is that the concrete failed. No structural support from the concrete to counter the outward forces of the low arch plus the weight of the concrete would force the arch to fail.

If you are going to do commercial baking and you like the round oven go with JE’s suggestions but build your oven with full firebricks on their ends. Add 75 - 100 mm (3 - 4") thermal mass to the outside of the oven. Don’t use concrete as suggested but render the oven with several coats using a course sand and a few layers of thick chicken wire or mesh to add support and stability. Then insulate it with a thermal blanket and as much vermiculite / perlite weak concrete mix as possible.

Have fun

Alf

james 09-24-2006 10:52 AM

Excellent posting
 
Alf,

Excellent. Thanks for that posting. Informative and helpful -- this one is in your domain.
James


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