Questions re: cladding and insulation

[George_in_MA]

I built an Alan Scott oven over the summer, and have been thinking I'd like to try my hand at a Pompeii. I've been looking over the plans and have a few questions regarding concrete cladding and insulation. I'd appreciate it if anyone could shed some light for me.

With the Alan Scott oven, you cover the brick baking chamber or vault with a layer of aluminum foil, then cover the entire thing with several inches of concrete cladding (concrete reinforced with a sort of wire mesh cage over the vault). The main reason for the cladding is to build up the mass of the oven so it will store more heat, but the cladding also is supposed to support the sides of the vault and keep the walls of the oven from collapsing outward. The aluminum foil is there to create some slip between the oven vault and the cladding, so that the oven can expand a bit during firing without putting any major cracks in the concrete. Outside the cladding is insulated with vermiculite. You either mix the vermiculite with cement (about a 7:1 mix) and apply a coating iver the cladding, or you put it in loose between the cladding and the oven's enclosure.

The Pompeii plans mention little about using concrete or mortar to build up the oven's mass. I was thinking I'd like to build up the oven's mass just a bit to extend the baking time with retained heat. Can this be done? If it can, should you put a layer of aluminum foil between the brick dome and the cladding to create slip and prevent cracks. Also, should you reinforce the cladding with mesh? The Pompeii plans call for an insulation blanket over the dome. Is this necessary, or will vermiculite or a vermiculite-concrete mix be sufficient?

Lots of questions. Thanks in advance to anyone who can answer.

If anyone's interested, I do like my Alan Scott oven--I built the 36-loaf model with a 36-inch X 48-inch hearth--and I do cook pizza in it. But it takes several hours to fire properly, which is a substantial investment in time and wood to cook only a few pizzas.

George

[paulages]

But it takes several hours to fire properly, which is a substantial investment in time and wood to cook only a few pizzas.

you answered your own question here. the purpose of having less thermal mass but heavy insulation is so that it takes little fuel to reach and hold pizza cooking temperatures. i can have my pompeii to 900F and white walls in 45 min. to an hour. i think you'll also find that most people using this design, with 5" or so of thermal mass in the walls, feel that this is more than adequate heat storage for home bread baking, unless you really plan on doing many batches at once.

search around on here, and you'll find plenty of opinions on the subject. i will cut down my thermal mass even more on my next oven, particularly in the floor, unless it is for commercial use.

[james]

Hi George,

Welcome to the group. My guess is that you will really enjoy the round Italian design. I've been using an indoor Scott oven a great deal this winter (the rain is really coming down in Headlsburg), and I am constantly reminded of the differences. One thing that strikes me is that the "letterbox" shape doesn't bounce heat evenly down onto the oven floor.

The concept of "cladding" in unique to the Scott oven, where you cover the brick vault with 5" of concrete. Those two pieces of thermal mass move differently, which is why you use aluminum foil as a slip plan. A round brick oven uses mortar to hold the oven together, but doesn't need any additional mass -- a 4 1/2" half brick it is already overkill for most residential oven users. Image the fuel necessary to fully heat 9" of thermal mass. I read in Reinhart's "The Bread Makers Apprecentice" earlier today that somebody got 12 commercial loads of bread from one firing in a modified Scott oven. Great for a bakery, but not so good for you or me.

The professionally made Italian brick oven that Forno Bravo sells (the Artigiano made in Tuscany) has walls that are about 3.3", including the external mortars, while our precast ovens are about 2". The precast ovens cook great, and easily hold enough heat for baking bread, roasts, etc. Some owners build them up to 3" with additional mortar, but that is optional.

I hope this gives you some ideas on how a Pompeii oven (or other Italian oven) will behave.

Also, there are various postings on the relative merits of Insulfrax, loose insulation and castable insulation in the forum, with good ideas on the various trade-offs.

Enjoy your project.
James

[james]

Heat holding

This is fun. Right after I posted my reply, I just read that Carl (also in NorCal and sitting in the rain) has one of our precast ovens outdoors which is holding heat at 200F after 3 days. That's another good data point on mass (and another vote for good insulation).

James

[KiwiPete]

I concur with what James and Paulages are saying.

Learning from their experiences, I have deliberately gone for 3" wall thickness, rather than the standard 4.5". My oven heats up quite quickly (45- 60 minutes as well, to get the white walls etc).

In combination with that, I have gone overboard on insulation, to try and retain the heat as much as you can, once you have got the oven to working temperature. I have used a 75 mm (3") blanket rather than the standard 1" combined with a couple of inches of perlite. When the oven is white hot and roaring, the temperature on the outside of the dome (right at the top, which theoretically is the hottest point) is just the same as the ambient temperature.

This costs a bit more in the short term, but I think it will repay over the long term in terms of wood usage.

[George_in_MA]

Thanks

Thanks to all for your input.

I won't be building anything until spring, so I have plenty of time to think it over and search the forum for various subjects.

Just looking at the Pompeii, I'm thinking I can build it for significantly less money than what the Alan Scott oven cost (we paid as we went, and didn't keep records, but I figure around $1,000).

Don't get me wrong--I do like my Scott oven. But one of the main reasons I built it was because my wife had become a degenerate baker who would use our indoor convection oven in the dead of summer when the temps hit 100 degrees. Also, she was daydreaming about running a bakery. I told her I'd build an oven that could turn out marketable quantites of bread and we could sell them at farmer's markets or deliver them to neighbors. Then, if she couldn't get her fill of baking that way, we'd look into renting a retail space.

Now that I've got the oven finished, it looks like her daydream is over. And my bread baking skills are not at a point where I can prepare 30 or so loaves to bake at one time. Add to this that I'm determined to bake with natural leavens--which I've found a bit trickier. So I'm sitting with an oven that is beyond my abilities. But I'm determined to forge on. I've got this killer bread oven, and with all the work and all the money it took to construct it, it would be a shame not to try to make use of it.

I have no trouble believing someone got 12 loads of bread from a Scott oven between firings. I've heard of someone getting 15. For my longest firing so far, which I guess was about 6 hours with cord wood, I raked the coals out at about 4 P.M. to bake. I baked a load of bread--about 11 1-pound loaves of sourdough--cooked our dinner, and was done by about 7 P.M. The temperature at that point was about 450 degrees. Twelve hours later, at 7 A.M., the temperature was 350 degrees, and was over 200 on the third day. This was in December in Massachusetts.

Still, it would be nice to have an oven I could bring to temperature in an hour with a minimum of wood, cook a few pizzas, bake a few loaves of bread, and be done with it.

As much fun as I've had cooking with the oven, I think I had even more fun building it.

Thanks again.

George

[dmun]

Oven thickness

Since you have a fully functional bread oven, you don't have to compromise on your pizza oven. I would suggest making it as thin as possible, as I outlined on another post.

http://fornobravo.com/forum/showthread.php?t=215

Using the smallest, thinnest oven possible, with a lot of insulation, will mean fast, economical firing. You can save your Scott oven for all day baking marathons and thanksgiving feasts.

David

[George_in_MA]

Thanks, and oven costs

David,

Thanks for your suggestion. I read your thread.

I've been doing all of my cuts with a brickset, even on facade bricks. (Perfect cuts just don't look right to me. Hand-cut bricks look more rustic, seem to give the work more character.) However, I realize I'd have to use a saw to cut firebrick into thirds. I'll have to give that some thought.

As I mentioned, I figure my Alan Scott oven cost me about $1,000 in materials. Does anyone have a good idea what their Pompeii oven cost them...?

George