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Old 07-07-2006, 07:44 AM
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Default 3 Days Later: Still 160 F.

(M) I'm never quite sure where to post a new thread so I posted over my old "Sink Height" post.

(M) There has been a lot of discussion about the order of concrete layers, island hearths, insulation sheets, etc. This post is only in support of the original (traditional) design that was built by ColCorn (Jim) well over a year ago. I used essentially his same design with a few modifications such as the orientation of the first course of bricks. The Hearth Slab was the same order.

(M) Three days ago I built a quick fire, perhaps 45 MIn. duration and only up to 500 degrees as I wanted to cook a brisket on low heat. That was still way too hot and "Streetcar" totally dried out the brisket. But this is not about cooking, but materials used and the order of their application.

(M) Day two showed my oven still at 250, and today it is at 160. This fire was purposely restrained for my hoped for slow cooking. I'm impressed with it's heat retention ability. I think that only an Allen Scott design could retain heat better and it would take a lot longer heating up and need a lot more wood.

(M) Here's the order and materials of my oven:

(M) On top of the concrete block stand I have a 2.5" layer of Perlcrete. On top of that, a 3.5" layer of steel reinforced refractory concrete.

(M) Once dry, I used a notched trowel to spread a thin moist (not wet) layer of leveling mix of sand and fire clay on the Hearth Slab. I placed my bricks in a true Herring Bone pattern.

(M) I built the dome of medium duty, heavy refractory bricks. The refractory mortar (8 parts mortar sand, 3 parts Portland cement, 2 parts fire clay) was also used as a 1/2 inch cladding layer once the dome was completed.

(M) On top of the refractory cladding layer I placed 12" wide ribbon sheets of heavy duty aluminum foil; starting from the bottom, and working my way to the top.

(M) Once the foil covered the dome, I placed 1" mesh chicken wire on top of the foil and covered both foil and wire with about an inch of Perlcrete. This layer was too thin to provide significant insulation. It's purpose was to protect the foil. But I subsequently poured 28 cubic feet of dry perlite inside the housing. If you decide against a gabled roof housing, I would strongly suggest as much Perlcrete as you can afford.

(M) I made many design changes, e.g. to the manifold, vent, and chimney but the only significant difference between the order and choice of materials to ColCorn's was the inclusion of the aluminum foil layer. I don't know to what degree it helped so this is not a plug for foil, but I can't see that it hurt.

(M) If you are a Newbie and want to follow the process more closely, you could look on this Forum for my postings but it would be simpler to go to PhotoBucket where the entire process is documented with about 89 images:

http://s14.photobucket.com/albums/a318/marceld/

Ciao,

Marcel

P.S. I'm sure that my oven would have retained heat even better had the 2 layer air space in my door not separated, and had the seal been tighter. The separation was caused by "Streetcar" having used screws that were a tad too long and their tips put a separating pressure between the layers.
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Old 07-08-2006, 06:41 AM
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Default

Marcel:


I am truly happy seeing the pictures of your excellent oven work.
Nice housing and an oven that deserves an A+.
I hope you will have the great pizzas and breads that your are waiting for.

Good baking!
Luis
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Old 07-08-2006, 08:02 AM
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Question Aluminum foil; did it help reflect heat?

(M) Thanks Luis, for your support.

(M) I had written earlier that the foil was installed to:

1- Keep the dome dry during construction of the housing.

2- Reduce smoke emmisions through any small cracks.

3- Provide a slip plane between the bricks and the cladding

==================================================

4-I postulated that radiant heat might be reflected off the foil, back to the dome, but that I would need the input of a thermal engineer to support that theory.

(M) So, Luis, (or any other engineer familiar with thermal properties) from an engineer's perspective) would you say that the reflective property of the aluminum foil could bounce radiant heat back to the bricks even though the aluminum is buried? ___

Obrigado,

Marcel
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Old 07-08-2006, 08:56 AM
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Default Foil

Streetkar,

I don't think the foil adds that much in reflectivity, but it does have a tendency to even things out and prevent hot spots, or at least it has on my oven.

I've been slow cooking meats over the last while, but I don't put them in until the hearth brick is at about 250F. This is plenty. Also, I use a very heavy French oval roaster (enamel over cast iron) right on the brick. This pot has a very heavy lid and a good seal. However, I do make sure that I add more liquid for the braise than I would in a kitchen oven. I've left a ten pound beef roast in overnight at this temp, and it works just fine. However, I left the oven door off overnight, because the hearth temp was a tad too high. Significantly, it did not drop all that much, even with the door off, and read 210 the next morning.

Maybe try it that way.

Jim
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Old 07-19-2006, 10:57 AM
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Default Aluminum foil; did it help reflect heat?

Marcel:



Sorry by taking so much time to answer your question about reflected heat by aluminum foil. Since I am not an expert in this matter, I expect that any other of this group readers could correct my thinking.



The aluminum shows low emissivity and high reflectance to thermal radiation. This means that it obstruct the heat transmission by radiation and reflects a big portion of the incident heat. If we think in polite aluminum, we could be speaking about the brilliant side facing the incident heat.

This is the good answer.

The bad one is that aluminum foil has a low emissivity, then the heat that reach the surface is reflected, no emitted. If the oven has an aluminum foil between the refractory bricks and the cladding, the temperature gradient will have a break in this surface, taking more time to radiate heat to this cladding.



By other point of view, is the mass of the aluminum foil big enough to make any difference in the temperature gradient (as by meaning something for the baker)? Here is where my mind is in conflict with my physical knowledge. I do not think so.



In reference to the three points of your message, I agree with them all.



However, there is another negative fact by using the aluminum foil.

The advantage in baking with a brick oven, besides the high temperature, is that the moisture of the wood gives to the food a differential taste. This moisture slowly penetrates the bricks, cladding and isolation. It is why it is recommendable to left a little hole in the oven casing.

The aluminum foil will encapsulate this moisture in the bricks. Nothing superb, IMHO, since during the next 2 hours of warm out firing the moisture could disappear. However, we are speaking about engineering, in this message.



I hope this answer your question.



Otherwise, do not worry, your oven is looking great and, I promise you, you will have lot of funny with it!

Please, do not let to try the empanadas recipe and the strawberry chocolate pizza. Funny and Yummy…



Luis
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